Peroxisomal matrix proteins are synthesized on cytosolic ribosomes and rapidly transported into the organelle by a complex machinery. The data gathered in recent years suggest that this machinery operates through a syringe-like mechanism, in which the shuttling receptor PEX5 − the “plunger” − pushes a newly synthesized protein all the way through a peroxisomal transmembrane protein complex − the “barrel” − into the matrix of the organelle. Notably, insertion of cargo-loaded receptor into the “barrel” is an ATP-independent process, (...) whereas extraction of the receptor back into the cytosol requires its monoubiquitination and the action of ATP-dependent mechanoenzymes. Here, we review the main data behind this model. The peroxin PEX5 is the peroxisomal matrix protein receptor. It shuttles between the cytosol and the organelle membrane, where it gets inserted into the docking/translocation module, thus pushing the cargo-protein into the peroxisome lumen. Resetting the system involves PEX5 monoubiquitination and its extraction from the membrane by the AAA-ATPases, PEX1/PEX6. (shrink)

The cellular machinery responsible for conveying proteins between the endoplasmic reticulum and the Golgi is being investigated using genetics and biochemistry. A role for vesicles in mediating protein traffic between the ER and the Golgi has been established by characterizing yeast mutants defective in this process, and by using recently developed cell‐free assays that measure ER to Golgi transport. These tools have also allowed the identification of several proteins crucial to intracellular protein trafficking. The characterization and possible (...) functions of several GTP‐binding proteins, peripheral membrane proteins, and an integral membrane protein during ER to Golgi transport are discussed here. (shrink)

The essential biological processes that sustain life are catalyzed by protein nano-engines, which maintain living systems in far-from-equilibrium ordered states. To investigate energetic processes in proteins, we have analyzed the system of generalized Davydov equations that govern the quantum dynamics of multiple amide I exciton quanta propagating along the hydrogen-bonded peptide groups in α-helices. Computational simulations have confirmed the generation of moving Davydov solitons by applied pulses of amide I energy for protein α-helices of varying length. The stability (...) and mobility of these solitons depended on the uniformity of dipole-dipole coupling between amide I oscillators, and the isotropy of the exciton-phonon interaction. Davydov solitons were also able to quantum tunnel through massive barriers, or to quantum interfere at collision sites. The results presented here support a nontrivial role of quantum effects in biological systems that lies beyond the mechanistic support of covalent bonds as binding agents of macromolecular structures. Quantum tunneling and interference of Davydov solitons provide catalytically active macromolecular protein complexes with a physical mechanism allowing highly efficient transport, delivery, and utilization of free energy, besides the evolutionary mandate of biological order that supports the existence of such genuine quantum phenomena, and may indeed demarcate the quantum boundaries of life. (shrink)

A stereospecific binding site is not the only determinant governing the selectivity of transport proteins. An understanding of transport across cellular membranes requires a description of the different compartments within a transmembrane channel; evidence for the existence of these compartments comes from the selectivity properties of genetically modified maltoporin. Such compartments may also be of significance in determining the specificity of other transport proteins.

Different families of transport proteins catalyze transmembrane solute translocation, employing different mechanisms and energy sources. Several of these functionally dissimilar proteins nevertheless exhibit similar strutural units, consisting of six tightly packed α‐helices which may comprise all or part of a transmembrane channel. It is now recognized that some of these families arose independently of each other by convergence, while others arose from common precursors by divergence. The former families apparently arose at different times in evolutionary history, in different groups (...) of organisms, employing different routes. (shrink)

As targeted proteins that move within the cell, the steroid receptors have become very useful probes for understanding the linked phenomena of protein folding and transport. From the study of steroid receptor‐associated proteins it has become clear over the past two years that these receptors are bound to a multiprotein complex containing at least two heat shock proteins, hsp90 and hsp56. Attachment of receptors to this complex in a cell‐free system appears to require the protein unfolding/folding activity (...) of a third heat shock protein, hsp70. Like the oncogenic tyrosine kinase pp60src, steroid receptors bind to this complex of chaperone proteins at the time of their translation. Binding of the receptor to the hsp90 component of the system occurs through the hormone binding domain and is under strict hormonal control. The hormone binding domain of the receptor acts as a transferable regulatory unit that confers both tight hormonal control and hsp90 binding onto chimaeric proteins. The model of folding and transport being developed for steroid receptors leads to some general suggestions regarding the folding and transport of targeted proteins in the cell. (shrink)

The Type VI secretion system is a multiprotein and mosaic apparatus that delivers protein effectors into prokaryotic or eukaryotic cells. Recent data on the enteroaggregative Escherichia coli T6SS have provided evidence that the TssA protein is a key component during T6SS biogenesis. The T6SS comprises a trans-envelope complex that docks the baseplate, a cytoplasmic complex that represents the assembly platform for the tail. The T6SS tail is structurally, evolutionarily and functionally similar to the contractile tails of bacteriophages. We (...) have shown that TssA docks to the membrane complex, recruits the baseplate complex and initiates and coordinates the polymerization of the inner tube with that of the sheath. Here, we review these recent findings, discuss the variations within TssA-like proteins, speculate on the role of EAEC TssA in T6SS biogenesis and propose future research perspectives. In the environment, bacteria have to cope with other microbial species and therefore have evolved anti-microbial mechanisms. The bacterial Type VI secretion system transports toxins into bacterial and/or eukaryotic cells using a contractile mechanism. At the architectural level, the T6SS could be viewed as a nano-speargun assembled in the bacterial cytoplasm and anchored to a trans-envelope complex. We describe and speculate on recent findings demonstrating that the TssA subunit is involved in the different stages of T6SS biogenesis. (shrink)

When cells are starved of their substrate, many nutrient transporters are induced. These undergo rapid endocytosis and redirection of their intracellular trafficking when their substrate becomes available again. The discovery that some of these transporters also act as receptors, or transceptors, suggests that at least part of the sophisticated controls governing the trafficking of these proteins has to do with their signaling function rather than with control of transport. In yeast, the general amino acid permease Gap1 mediates signaling to (...) the protein kinase A pathway. Its endocytic internalization and intracellular trafficking are subject to amino acid control. Other nutrient transceptors controlling this signal transduction pathway appear to be subject to similar trafficking regulation. Transporters with complex regulatory control have also been suggested to function as transceptors in other organisms. Hence, precise regulation of intracellular trafficking in nutrient transporters may be related to the need for tight control of nutrient‐induced signaling. (shrink)

Proteins of the exocytotic (secretory) pathway are initially targeted to the endoplasmic reticulum (ER) and then translocated across and/or inserted into the membrane of the ER. During their anterograde transport with the bulk of the membrane flow along the exocytotic pathway, some proteins are selectively retained in various intracellular compartments, while others are sorted to different branches of the pathway. The signals or structural motifs that are involved in these selective targeting processes are being revealed and investigations into the (...) mechanistic nature of these processes are actively underway. (shrink)

The transport of molecules across the nuclear envelope plays a central role in the metabolism of the cell. Significant advances hi three major areas highlight the limits of our current knowledge and point to the prospect of exciting future developments. Firstly, findings that ions and small proteins do not diffuse freely into the nucleus call into question the current views of nuclear envelope permeability. Secondly, indications that nuclear protein import can be regulated in conjunction with the cell cycle (...) and development have been confirmed by the clear demonstration of regulated nuclear entry of specific transcription factors and morphogens. Thirdly, identification and characterisation of candidate receptor proteins indicates that the recognition of the nuclear targeting sequence occurs in the cytoplasm, suggesting that a different recognition event occurs at the nuclear pore. (shrink)

The invariant left–right asymmetry of animal body plans raises fascinating questions in cell, developmental, evolutionary, and neuro‐biology. While intermediate mechanisms (e.g., asymmetric gene expression) have been well‐characterized, very early steps remain elusive. Recent studies suggested a candidate for the origins of asymmetry: rotary movement of extracellular morphogens by cilia during gastrulation. This model is intellectually satisfying, because it bootstraps asymmetry from the intrinsic biochemical chirality of cilia. However, conceptual and practical problems remain with this hypothesis, and the genetic data is (...) consistent with a different mechanism. Based on wide‐ranging data on ion fluxes and motor protein action in a number of species, a model is proposed whereby laterality is generated much earlier, by asymmetric transport of ions, which results in pH/voltage gradients across the midline. These asymmetries are in turn generated by a new candidate for “step 1”: asymmetric localization of electrogenic proteins by cytoplasmic motors. BioEssays 25:1002–1010, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

The invariant left–right asymmetry of animal body plans raises fascinating questions in cell, developmental, evolutionary, and neuro‐biology. While intermediate mechanisms (e.g., asymmetric gene expression) have been well‐characterized, very early steps remain elusive. Recent studies suggested a candidate for the origins of asymmetry: rotary movement of extracellular morphogens by cilia during gastrulation. This model is intellectually satisfying, because it bootstraps asymmetry from the intrinsic biochemical chirality of cilia. However, conceptual and practical problems remain with this hypothesis, and the genetic data is (...) consistent with a different mechanism. Based on wide‐ranging data on ion fluxes and motor protein action in a number of species, a model is proposed whereby laterality is generated much earlier, by asymmetric transport of ions, which results in pH/voltage gradients across the midline. These asymmetries are in turn generated by a new candidate for “step 1”: asymmetric localization of electrogenic proteins by cytoplasmic motors. BioEssays 25:1002–1010, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Coronins constitute an evolutionarily conserved family of WD‐repeat actin‐binding proteins, which can be clearly classified into two distinct groups based on their structural features. All coronins possess a conserved basic N‐terminal motif and three to ten WD repeats clustered in one or two core domains. Dictyostelium and mammalian coronins are important regulators of the actin cytoskeleton, while the fly Dpod1 and the yeast coronin proteins crosslink both actin and microtubules. Apart from that, several coronins have been shown to be involved (...) in vesicular transport. C. elegans POD‐1 and Drosophila coro regulate the actin cytoskeleton, but also govern vesicular trafficking as indicated by mutant phenotypes. In both organisms, defects in cytoskeleton and trafficking lead to severe developmental defects ranging from abnormal cell division to aberrant formation of morphogen gradients. Finally, mammalian coronin 7 appears not to execute any cytoskeleton‐related functions, but rather participates in regulating Golgi trafficking. Here, we review recent data providing more insight into molecular mechanisms underlying the regulation of F‐actin structures, cytoskeletal rearrangements and intracellular membrane transport by coronin proteins and the way that they might link cytoskeleton with trafficking in development and disease. BioEssays 27:625–632, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The cilium/flagellum is a sensory-motile organelle ancestrally present in eukaryotic cells. For assembly cilia universally rely on intraflagellar transport (IFT), a specialised bidirectional transport process mediated by the ancestral and conserved IFT complex. Based on the homology of IFT complex proteins to components of coat protein I (COPI) and clathrin-coated vesicles, we propose that the non- vesicular, membrane-bound IFT evolved as a specialised form of coated vesicle transport from a protocoatomer complex. IFT thus shares common ancestry (...) with all protocoatomer derivatives, including all vesicle coats and the nuclear pore complex (NPC). This has major implications for the evolutionary origin of the cilium. First, it reinforces the tenet that duplication and divergence of pre-existing structures, rather than symbiosis, were the major themes during cilium evolution. Second, it suggests that the initial step in the autogenous origin of the cilium was the establishment of a membrane patch with transmembrane proteins transported by the ancestral vesicle-coating IFT complex. We propose a scenario for how the initial membrane patch gradually protruded to enhance exposure to the environment, then started to move, and finally compartmentalised to render receptor signalling and ciliary beating more efficient. BioEssays 28: 191–198, 2006. © 2006 Wiley periodicals, Inc. (shrink)

Ribosome biogenesis includes the making and processing of ribosomal RNAs, the biosynthesis of ribosomal proteins from their mRNAs in the cytosol and their transport to the nucleolus to assemble pre‐ribosomal particles. Several stresses including cellular senescence reduce nucleolar rRNA synthesis and maturation increasing the availability of ribosome‐free ribosomal proteins. Several ribosomal proteins can activate the p53 tumor suppressor pathway but cells without p53 can still arrest their proliferation in response to an imbalance between ribosomal proteins and mature rRNA production. (...) Recent results on senescence‐associated ribogenesis defects (SARD) show that the ribosomal protein S14 (RPS14 or uS11) can act as a CDK4/6 inhibitor linking ribosome biogenesis defects to the main engine of cell cycle progression. This work offers new insights into the regulation of the cell cycle and suggests novel avenues to design anticancer drugs. (shrink)

In all domains of life, transmembrane proteins from the ATP‐binding cassette (ABC) transporter family drive the translocation of diverse substances across lipid bilayers. In pathogenic fungi, the ABC transporters of the pleiotropic drug resistance (PDR) subfamily confer antibiotic resistance and so are of interest as therapeutic targets. They also drive the quest for understanding how ABC transporters can generally accommodate such a wide range of substrates. The Pdr5 transporter from baker's yeast is representative of the PDR group and, ever since (...) its discovery more than 30 years ago, has been the subject of extensive functional analyses. A new perspective of these studies has been recently provided in the framework of the first electron cryo‐microscopy structures of Pdr5, as well as emergent applications of machine learning in the field. Taken together, the old and the new developments have been used to propose a mechanism for the transport process in PDR proteins. This mechanism involves a “flippase” step that moves the substrates from one leaflet of the bilayer to the other, as a central element of cellular efflux. (shrink)

The cellular processes of transport, division and, possibly, early development all involve microtubule‐based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin‐related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule‐motor proteins that share a 340‐amino‐acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization (...) and related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles. (shrink)

The mitochondrion is known as the “powerhouse” of eukaryotic cells since it is the main site of adenosine 5′‐triphosphate (ATP) production. Using a temperature‐sensitive fluorescent probe, it has recently been suggested that the stray free energy, not captured into ATP, is potentially sufficient to sustain mitochondrial temperatures higher than the cellular environment, possibly reaching up to 50 °C. By 50 °C, some DNA and mitochondrial proteins may reach their melting temperatures; how then do these biomolecules maintain their structure and function? (...) Further, the production of reactive oxygen species (ROS) accelerates with temperature, implying higher oxidative stresses in the mitochondrion than generally appreciated. Herein, it is proposed that mitochondrial heat shock proteins (particularly Hsp70), in addition to their roles in protein transport and folding, protect mitochondrial proteins and DNA from thermal and ROS damage. Other thermoprotectant mechanisms are also discussed. (shrink)

Physiological studies show that the driving force for long distance transport and the control of nutrient movement in plants resides largely in the regulated, membrane transport of a few carbohydrates, principally sucrose. The evidence is reviewed here and biochemical studies on sucrose carrier proteins are discussed.

Cilia are unique eukaryotic organelles and exhibit remarkable conservation across evolution. Nevertheless, very different types of configurations are encountered, raising the question of their evolution. Cilia are constructed by intraflagellar transport (IFT), the movement of large protein complexes or trains that deliver cilia components to the distal tip for assembly. Recent data revealed that IFT trains are restricted to some but not all nine doublet microtubules in the protist Trypanosoma brucei. Here, we propose that restricted positioning of IFT (...) trains could offer potent options for cilia to evolve towards more complex (addition of new structural elements like in spermatozoa) or simpler configuration (loss of some elements like in primary cilia), and therefore be a driver of cilia diversification. We present two hypotheses to explain how IFT trains could be restricted to some doublets, either by a triage process taking place at the basal body level or by the development of molecular differences between ciliary microtubules. (shrink)

PstS and DING proteins are members of a superfamily of secreted, high‐affinity phosphate‐binding proteins. Whereas microbial PstS have a well‐defined role in phosphate ABC transporters, the physiological function of DING proteins, named after their DINGGG N termini, still needs to be determined. PstS and DING proteins co‐exist in some Pseudomonas strains, to which they confer a highly adhesive and virulent phenotype. More than 30 DING proteins have now been purified, mostly from eukaryotes. They are often associated with infections or with (...) dysregulation of cell proliferation. Consequently, eukaryotic DING proteins could also be involved in cell–cell communication or adherence. The ubiquitous presence in eukaryotes of proteins structurally and functionally related to bacterial virulence factors is intriguing, as is the absence of eukaryotic genes encoding DING proteins in databases. DING proteins in eukaryotes could originate from unidentified commensal or symbiotic bacteria and could contribute to essential functions. Alternatively, DING proteins could be encoded by eukaryotic genes sharing special features that prevent their cloning. Both hypotheses are discussed. (shrink)

Biological order provided by α-helical secondary protein structures is an important resource exploitable by living organisms for increasing the efficiency of energy transport. In particular, self-trapping of amide I energy quanta by the induced phonon deformation of the hydrogen-bonded lattice of peptide groups is capable of generating either pinned or moving solitary waves following the Davydov quasiparticle/soliton model. The effect of applied in-phase Gaussian pulses of amide I energy, however, was found to be strongly dependent on the site (...) of application. Moving solitons were only launched when the amide I energy was applied at one of the α-helix ends, whereas pinned solitons were produced in the α-helix interior. In this paper, we describe a general mechanism that launches moving solitons in the interior of the α-helix through phase-modulated Gaussian pulses of amide I energy. We also compare the predicted soliton velocity based on effective soliton mass and the observed soliton velocity in computer simulations for different parameter values of the isotropy of the exciton-phonon interaction. The presented results demonstrate the capacity for explicit control of soliton velocity in protein α-helices, and further support the plausibility of gradual optimization of quantum dynamics for achieving specialized protein functions through natural selection. (shrink)

Organelles transported along microtubules are normally moved to precise locations within cells. For example, synaptic vesiceles are transported to the neruronal synapse, the Golgi apparatus is generally found in a perinuclear location, and the membranes of the endoplasmic reticulum are actively extended to the cell periphery. The correct positioning of these organelles depends on microtubules and microtubule motors. Melanophores provide an extreme example of organized organelle transport. These cells are specialized to transport pigment granules, which are coordinately moved (...) towards or away from the cell center, and result in the cell appearing alternately light or dark. Melanophores have proved to be an ideal system for studying the mechanisms by which the cell controls the direction of its organelle transport. Pigment granule dispersion (the movement away from the cell center) requires protein phosphorylation, while pigment aggregation (the movement towards the cell center) requires protein dephosphorylation. The target of this phosphorylation and dephosphorylation event is a protein that interacts with the microtubule motor protein, kinesin. Thus, the direction of organelle transport along microtubules may be regulated by controlling the activity of a microtubule motor. (shrink)

Protein α-helices provide an ordered biological environment that is conducive to soliton-assisted energy transport. The nonlinear interaction between amide I excitons and phonon deformations induced in the hydrogen-bonded lattice of peptide groups leads to self-trapping of the amide I energy, thereby creating a localized quasiparticle (soliton) that persists at zero temperature. The presence of thermal noise, however, could destabilize the protein soliton and dissipate its energy within a finite lifetime. In this work, we have computationally solved the (...) system of stochastic differential equations that govern the quantum dynamics of protein solitons at physiological temperature, T=310 K, for either a single isolated α-helix spine of hydrogen bonded peptide groups or the full protein α-helix comprised of three parallel α-helix spines. The simulated stochastic dynamics revealed that although the thermal noise is detrimental for the single isolated α-helix spine, the cooperative action of three amide I exciton quanta in the full protein α-helix ensures soliton lifetime of over 30 ps, during which the amide I energy could be transported along the entire extent of an 18-nm-long α-helix. Thus, macromolecular protein complexes, which are built up of protein α-helices could harness soliton-assisted energy transport at physiological temperature. Because the hydrolysis of a single adenosine triphosphate molecule is able to initiate three amide I exciton quanta, it is feasible that multiquantal protein solitons subserve a variety of specialized physiological functions in living systems. (shrink)

Although many aspects of the physiological and pathophysiological mechanisms remain unknown, recent advances in our knowledge suggest that the CREC proteins are promising disease biomarkers or targets for therapeutic intervention in a variety of diseases. The CREC family of low affinity, Ca2+‐binding, multiple EF‐hand proteins are encoded by five genes,RCN1,RCN2,RCN3,SDF4, andCALU, resulting in reticulocalbin, ER Ca2+‐binding protein of 55 kDa (ERC‐55), reticulocalbin‐3, Ca2+‐binding protein of 45 kDa (Cab45), and calumenin. Alternative splicing increases the number of gene products. The (...) proteins are localized in the cytosol, in various parts of the secretory pathway, secreted to the extracellular space or localized on the cell surface. The emerging functions appear to be highly diverse. The proteins interact with several different ligands. Rather well‐described functions are attached to calumenin with the inhibition of several proteins in the endoplasmic or sarcoplasmic reticulum membrane, the vitamin K12,3‐epoxide reductase, the γ‐carboxylase, the ryanodine receptor, and the Ca2+‐transporting ATPase. Other functions concern participation in the secretory process, chaperone activity, signal transduction as well as participation in a large variety of disease processes. (shrink)

The secretory pathway delivers proteins synthesized at the rough endoplasmic reticulum (RER) to various subcellular locations via the Golgi apparatus. Currently, efforts are focused on understanding the molecular machineries driving individual processes at the RER and Golgi that package, modify and transport proteins. However, studies are routinely performed using non‐dividing cells. This obscures the critical issue of how the secretory pathway is affected by cell division. Indeed, several studies have indicated that protein trafficking is down‐regulated during mitosis. Moreover, (...) the RER and Golgi apparatus exhibit gross reorganization in mitosis. Here I provide a relatively neglected perspective of how the mitotic cyclin‐dependent kinase (CDK1) could regulate various stages of the secretory pathway. I highlight several aspects of the mitotic control of protein trafficking that remain unresolved and suggest that further studies on how the mitotic CDK1 influences the secretory pathway are necessary to obtain a deeper understanding of protein transport. (shrink)

The Endosomal Sorting Complexes Required for Transport (ESCRTs) drive membrane remodeling in a variety of cellular processes that include the formation of endosomal intralumenal vesicles (ILVs) during multivesicular body (MVB) biogenesis. During MVB sorting, ESCRTs recognize ubiquitin (Ub) attached to membrane protein cargo and execute ILV formation by controlling the activities of ESCRT‐III polymers regulated by the AAA‐ATPase Vps4. Exactly how these events are coordinated to ensure proper cargo loading into ILVs remains unclear. Here we discuss recent work (...) documenting the ability of Bro1, an ESCRT‐associated Ub‐binding protein, to coordinate ESCRT‐III and Vps4‐dependent ILV biogenesis with upstream events such as cargo recognition. (shrink)

Parvalbumins (PVs) are acidic, intracellular Ca2+‐binding proteins of low molecular weight. They are associated with several Ca2+‐mediated cellular activities and physiological processes. It has been suggested that PV might function as a “Ca2+ shuttle” transporting Ca2+ from troponin‐C (TnC) to the sarcoplasmic reticulum (SR) Ca2+ pump during muscle relaxation. Thus, PV may contribute to the performance of rapid, phasic movements by accelerating the contraction–relaxation cycle of fast‐twitch muscle fibers. Interestingly, PVs promote the generation of power stroke in fish by speeding (...) up the rate of relaxation and thus provide impetus to attain maximal sustainable speeds. However, immunological monitoring of diverse tissues demonstrated that PVs are also present in non‐muscle cells. The axoplasmic transport and various intracellular secretory mechanisms including the endocrine secretions seem to be controlled by the Ca2+ regulation machinery. Any defect in the Ca2+ handling apparatus may cause several clinical problems; for instance, PV deficiency alters the neuronal activity, a key mechanism leading to epileptic seizures. Moreover, atypical relaxation of the heart results in diastolic dysfunction, which is a major cause of heart failure predominantly among the aged people. PV may offer a unique potential to correct defective relaxation in energetically compromised failing hearts through PV gene transfer. Consequently, PV gene transfer may present a new therapeutic approach to correct cellular disturbances in Ca2+ signaling pathways of diseased organs. Hence, PVs appear to be amazingly useful candidate proteins regulating a variety of cellular functions through action on Ca2+ flux management. (shrink)

Proteins and RNAs move between the nucleus and cytoplasm by translocation through nuclear pore complexes in the nuclear envelope. To do this, they require specific targeting signals, energy, and a cellular apparatus that catalyzes their transport. Several of the factors involved in nucleocytoplasmic trafficking of proteins have been identified and characterized in some detail. The emerging picture for nuclear transport proposes a central role for the small GTPase Ran and proteins with which it interacts. In particular, asymmetric distribution (...) of these proteins between nucleus and cytoplasm appears to be responsible for the vectorial nature of nucleocytoplasmic transport. Here, we summarize the role of Ran and Ran-binding proteins in nuclear trafficking of proteins with classical nuclear localisation signals. We also discuss examples of the growing number of alternative pathways that are involved in transport of proteins across the nuclear envelope. BioEssays 21:579–589, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

The nuclear envelope has recently become the object of intense scrutiny because it is the site of nuclear transport and is possibly involved in the organization of the interphase genome, thereby affecting gene expression. The major structural support for the nuclear envelope is the nuclear lamina, composed of the nuclear lamin proteins. They lie on the surface of the inner nuclear membrane and are in direct contact with the chromatin at the edge of the nucleus. The structure of the (...) nuclear lamin proteins has recently been deduced from their cDNAs and shown to have remarkable homologies to the family of cytoplasmic intermediate filaments. However, the lamin proteins have been found to depolymerize in response to metaphase‐specific phosphorylation events, and reassemble around daughter chromosomes at the completion of cell division. Little is known of the mechanisms of these dynamics, nor of other post‐translational modifications evident in these proteins. In addition, we have as yet no concrete idea of the function of these highly conserved proteins in the cell. This review will summarize our present knowledge of nuclear lamin structure and the new experimental approaches designed to elucidate their function. (shrink)

The multistep pathway of eukaryotic gene expression involves a series of highly regulated events in the nucleus and cytoplasm. In the nucleus, genes are transcribed into pre‐messenger RNAs which undergo a series of nuclear processing steps. Mature mRNAs are then transported to the cytoplasm, where they are translated into protein and degraded at a rate dictated by transcript‐ and cell‐type‐specific cues. Until recently, these individual nuclear and cytoplasmic events were thought to be primarily regulated by different RNA‐ and DNA‐binding (...) proteins that are localized either only in the nucleus or only the cytoplasm. Here, we describe multifunctional proteins that control both nuclear and cytoplasmic steps of gene expression. One such class of multifunctional proteins (e.g., Bicoid and Y‐box proteins) regulates both transcription and translation whereas another class (e.g., Sex‐lethal) regulates both nuclear RNA processing and translation. Other events controlled by multifunctional proteins include assembly of spliceosome components, spliceosome recycling, RNA editing, cytoplasmic mRNA localization, and cytoplasmic RNA stability. The existence of multifunctional proteins may explain the paradoxical involvement of the nucleus in an RNA surveillance pathway (nonsense‐mediated decay) that detects cytoplasmic signals (premature termination codons). We speculate that shuttling multifunctional proteins serve to efficiently link RNA metabolism in the cytoplasmic and nuclear compartments. BioEssays 23:775–787, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

At first glance the three eukaryotic protein translocation machineries – the ER‐associated degradation (ERAD) transport apparatus of the endoplasmic reticulum, the peroxisomal importomer and SELMA, the pre‐protein translocator of complex plastids – appear quite different. However, mechanistic comparisons and phylogenetic analyses presented here suggest that all three translocation machineries share a common ancestral origin, which highlights the recycling of pre‐existing components as an effective evolutionary driving force.Editor's suggested further reading in BioEssays ERAD ubiquitin ligases Abstract.

Many biologically active compounds including neurotransmitters, metabolic precursors, and certain drugs are accumulated intracellularly by transporters that are coupled to the transmembrane Na+ gradient. Amino acid neurotransmitter transporters play a key role in the regulation of extracellular amino acid concentrations and termination of neurotransmission in the CNSAbbreviations: CNS, central nervous system; GABA, γ‐aminobutyric acid; cDNA, complementary deoxyribonucleic acid; mRNA, messenger ribonucleic acid; NMDA, N‐methyl‐D‐aspartate; PKC, protein kinase C; PMA, phorbol 12‐myristate 13‐acetate; DAG, diacyl glycerol; R59022, DAG kinase inhibitor; AA, (...) arachidonic acid; ACHC, cis‐3‐aminocyclohexanecarboxylic acid; GAT‐A, ACHC‐sensitive GABA transporter; GAT‐B, β‐alanine‐sensitive GABA transporter; GLY‐1 and GLYT‐1, glycine transporters; PROT‐1, proline transporter; BGT‐1, betaine transporter.. Transporters for the major amino acid neurotransmitters glutamate, GABA, and glycine are found in both neurons and glial cells. Recent work has resulted in the identification of cDNAs encoding several amino acid neurotransmitter transport proteins, all of which belong to the Na+‐and Cl−‐dependent transporter gene family. The diversity of this family suggests a degree of transporter heterogeneity that is greater than that indicated by biochemical and pharmacological studies. (shrink)

The assembly of the major small nuclear ribonucleoprotein (snRNP)d̊ particles begins in the cytoplasm where large pools of common core proteins are preassembled in several RNA‐free intermediate particles. Newly synthesized snRNAs transiently enter the cytoplasm and complex with core particles to form pre‐snRNP particles. Subsequently, the cap structure at the 5/end of the snRNA is hypermethylated. The Resulting trimethylguanosine (TMG) cap is an integral part of the nuclear localization signal for snRNP particles and the pre‐snRNP particles are rapidly transported into (...) the nucleus. SnRNP particles mature when snRNA‐specific proteins complex with the particles, in some cases, just before or during nuclear transport, but in most instances after the particles are in the nucleus. In addition, U6 snRNA hybridizes with U4 snRNA to form a U4/U6 snRNP in the nucleus. The transport signals are retained on the snRNP particles and proteins since existing particles and proteins enter the reformed nucleus after mitosis. (shrink)

This review explores the hypothesis that regulation of nucleocytoplasmic shuttling is a means of driving differentiation, using spermatogenesis as a model. The transition from undifferentiated spermatogonial stem cell to terminally differentiated spermatozoon is, at its most basic, a change in the repertoire of expressed genes. To effect this, the complement of nuclear proteins, such as transcription factors and chromatin remodelling components must change. Current knowledge of the nuclear proteins and nucleocytoplasmic transport machinery relevant to spermatogenesis is consolidated in this (...) review, and their functional linkages are highlighted not only as a means of regulating nuclear protein composition, but also as a key mechanism regulating gene transcription and hence cell fate. Through this, we hypothesize that male germ cell differentiation is mediated through regulation of nuclear transport machinery components, and thereby of the access of critical factors to the nucleus. The importance of nucleocytoplasmic trafficking to male germ cell differentiation is discussed, using the sex-determining factors Sry and SOX9, cell cycle regulators, CREM and cofactors and the Smads as specific examples, together with the roles in gametogenesis for particular nuclear transport factors in Caenorhabditis elegans and Drosophila. BioEssays 27:1011–1025, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Cytoplasmic dynein is the major minus‐end‐directed motor protein in eukaryotes, and has functions ranging from organelle and vesicle transport to spindle positioning and orientation. The mode of regulation of dynein in the cell remains elusive, but a tantalising possibility is that dynein is maintained in an inhibited, non‐motile state until bound to cargo. In vivo, stable attachment of dynein to the cell membrane via anchor proteins enables dynein to produce force by pulling on microtubules and serves to organise (...) the nuclear material. Anchor proteins of dynein assume diverse structures and functions and differ in their interaction with the membrane. In yeast, the anchor protein has come to the fore as one of the key mediators of dynein activity. In other systems, much is yet to be discovered about the anchors, but future work in this area will prove invaluable in understanding dynein regulation in the cell. (shrink)

The tetratricopeptide repeat (TPR) motif is a protein-protein interaction module found in multiple copies in a number of functionally different proteins that facilitates specific interactions with a partner protein(s). Three-dimensional structural data have shown that a TPR motif contains two antiparallel α-helices such that tandem arrays of TPR motifs generate a right-handed helical structure with an amphipathic channel that might accommodate the complementary region of a target protein. Most TPR-containing proteins are associated with multiprotein complexes, and (...) there is extensive evidence indicating that TPR motifs are important to the functioning of chaperone, cell-cycle, transcription, and protein transport complexes. The TPR motif may represent an ancient protein-protein interaction module that has been recruited by different proteins and adapted for specific functions. BioEssays 1999;21:932–939. © 1999 John Wiley & Sons, Inc. (shrink)

The tetratricopeptide repeat (TPR) motif is a protein-protein interaction module found in multiple copies in a number of functionally different proteins that facilitates specific interactions with a partner protein(s). Three-dimensional structural data have shown that a TPR motif contains two antiparallel α-helices such that tandem arrays of TPR motifs generate a right-handed helical structure with an amphipathic channel that might accommodate the complementary region of a target protein. Most TPR-containing proteins are associated with multiprotein complexes, and (...) there is extensive evidence indicating that TPR motifs are important to the functioning of chaperone, cell-cycle, transcription, and protein transport complexes. The TPR motif may represent an ancient protein-protein interaction module that has been recruited by different proteins and adapted for specific functions. BioEssays 1999;21:932–939. © 1999 John Wiley & Sons, Inc. (shrink)

Endosomes shuttle select cargoes between cellular compartments and, in doing so, maintain intracellular homeostasis and enable interactions with the extracellular space. Directionality of endosomal transport critically impinges on cargo fate, as retrograde (microtubule minus‐end directed) traffic delivers vesicle contents to the lysosome for proteolysis, while the opposing anterograde (plus‐end directed) movement promotes recycling and secretion. Intriguingly, the endoplasmic reticulum (ER) is emerging as a key player in spatiotemporal control of late endosome and lysosome transport, through the establishment of (...) physical contacts with these organelles. Earlier studies have described how minus‐end‐directed motor proteins become discharged from vesicles engaged at such contact sites. Now, Raiborg et al. implicate ER‐mediated interactions, induced by protrudin, in loading plus‐end‐directed motor kinesin‐1 onto endosomes, thereby stimulating their transport toward the cell's periphery. In this review, we recast the prevailing concepts on bidirectional late endosome transport and discuss the emerging paradigm of inter‐compartmental regulation from the ER‐endosome interface viewpoint. (shrink)

Uteroferrin, a purple‐colored, iron‐containing acid phosphatase, with many of the properties of a lysosomal hydrolase, transports iron from the mother to the conceptus in pregnant pigs. Uteroferrin, however, is but one member of what may be a broad class of iron‐containing phosphatases with unusual spectral properties which result from a novel type of di‐iron active site. The biological function of uteroferrin is unknown. We argue here that the in vivo function of uteroferrin, despite its undoubted ability to act as a (...) potent acid phosphatase, is that of a transplacental iron transporter. (shrink)

All insects appear to have a transport lipoprotein in the hemolymph (blood) that is responsible for moving hydrophobic materials through aqueous compartments. This has been called lipophorin because it is believed to be a reversible transport shuttle. Since most insects undergo some degree of metamorphosis from larval stages to the adult, the need to transport hydrophobic materials or the nature of these materials may change in the course of the life span. This is especially marked in the (...) case of the holometabolous insects – those which undergo drastic change of form, e.g. caterpillar‐pupa‐adult moth. We have found that lipophorin undergoes a molecular metamorphosis paralleling that of the insect. (shrink)

Substantial evidence implicates fast axonal transport (FAT) defects in neurodegeneration. In Alzheimer's disease (AD), it is controversial whether transport defects cause or arise from amyloid‐β (Aβ)‐induced toxicity. Using a novel, unbiased genetic screen, Morihara et al. identified kinesin light chain‐1 splice variant E (KLC1vE) as a modifier of Aβ accumulation. Here, we propose three mechanisms to explain this causal role. First, KLC1vE reduces APP transport, leading to Aβ accumulation. Second, reduced transport of APP by KLC1vE triggers (...) an ER stress response that activates the amyloidogenic pathway. Third, KLC1vE impairs transport of other KLC1 cargos that regulate amyloidogenesis, promoting Aβ retention within the secretory pathway. Collectively, KLC1vE perpetuates a vicious cycle of Aβ generation, kinase dysregulation, and global FAT impairment that inevitably leads to cellular toxicity. These concepts implicate alternative splicing of KLC1 in AD and suggest that the reciprocal influence of transport mechanisms on disease states contributes to neurodegeneration. (shrink)

The Segregation Distorter (SD) system of Drosophila melanogaster is one the best‐characterized meiotic drive complexes known. SD gains an unfair transmission advantage through heterozygous SD/SD+ males by incapacitating SD+‐bearing spermatids so that virtually all progeny inherit SD. Segregation distorter (Sd), the primary distorting locus in the SD complex, is a truncated duplication of the RanGAP gene, a major regulator of the small GTPase Ran, which has several functions including the maintenance of the nucleocytoplasmic RanGTP concentration gradient that mediates nuclear (...) class='Hi'>transport. The truncated Sd‐RanGAP protein is enzymatically active but mislocalizes to the nucleus where it somehow causes distortion. Here I present data consistent with the idea that wild‐type RanGAP, and possibly other loci able to influence the RanGTP gradient, has been caught up in an ancient genetic conflict that predates the SD complex. The legacy of this conflict could include the unexpectedly rapid evolution of nuclear transport‐related proteins, the accumulation of chromosomal inversions, the recruitment of gene duplications, and the turnover of repetitive sequences in the centric heterochromatin. BioEssays 29:386–391, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

The recent discovery and structure determination of a novel ubiquitin‐like dimerization domain in protein kinase D (PKD) has significant implications for its activation. PKD is a serine/threonine kinase activated by the lipid second messenger diacylglycerol (DAG). It is an essential and highly conserved protein that is implicated in plasma membrane directed trafficking processes from the trans‐Golgi network. However, many open questions surround its mechanism of activation, its localization, and its role in the biogenesis of cargo transport carriers. (...) In reviewing this field, the focus is primarily on the mechanisms that control the activation of PKD at precise locations in the cell. In light of the new structural findings, the understanding of the mechanisms underlying PKD activation is critically evaluated, with particular emphasis on the role of dimerization in PKD autophosphorylation, and the provenance and recognition of the DAG that activates PKD. (shrink)

Cells can change their function by rapidly modulating the levels of certain proteins at the plasma membrane. This rapid modulation is achieved by using a specialised trafficking process called constitutive cycling. The constitutive cycling of a variety of transmembrane proteins such as receptors, channels and transporters has recently been directly demonstrated in a wide range of cell types. This regulation is thought to underlie important biological phenomena such as learning and memory, gastric acid secretion and water and blood glucose homeostasis. (...) This review discusses the molecular mechanisms of constitutive cycling, its regulation by extracellular agents such as hormones and its misregulation in disease states. BioEssays 25:39–46, 2003. © 2002 Wiley Periodicals, Inc. (shrink)

The rate‐limiting step in the uptake and metabolism of Dglucose by insulin target cells is thought to be glucose transport mediated by glucose transporters (primarily the GLUT4 isoform) localized to the plasma membrane. However, subcellular fractionation, photolabelling and immunocytochemical studies have shown that the pool of GLUT4 present in the plasma membrane is only one of many subcellular pools of this protein. GLUT4 has been found in occluded vesicles at the plasma membrane, clathrin‐coated pits and vesicles, early endosomes, (...) and tubulo‐vesicular structures; the latter are analogous to known specialized secretory compartments. Tracking the movement of GLUT4 through these compartments, and defining the mechanism and site of action of insulin in stimulating this subcellular trafficking, are major topics of current investigation. Recent evidence focuses attention on the exocytosis of GLUT4 as the major site of insulin action. Increased exocytosis may be due to decreased retention of glucose transporters in an intracellular pool, or possibly to increased assembly of a vesicle docking and fusion complex. Although details are unknown, the presence in GLUT4 vesicles of a synaptobrevin homologue leads us to propose that a process analogous to that occurring in synaptic vesicle trafficking is involved in the assembly of GLUT4 vesicles into a form suitable for fusion with the plasma membrane. Evidence that the pathways of signalling from the insulin receptor and of GLUT4 vesicle exocytosis may converge at the level of the key signalling enzyme, phosphatidylinositol 3‐kinase, is discussed. (shrink)