Abstract

Motile eukaryotic cilia and flagella are hair-like organelles responsible for cell motility and mucociliary clearance. Using cryo-electron tomography, it has been shown that the doublet microtubule, the cytoskeleton core of the cilia and flagella, has microtubule inner protein structures binding periodically inside its lumen. More recently, single-particle cryo-electron microscopy analyses of isolated doublet microtubules have shown that microtubule inner proteins form a meshwork inside the doublet microtubule. High-resolution structures revealed new types of interactions between the microtubule inner proteins and the tubulin lattice. In addition, they offered insights into the potential roles of microtubule inner proteins in the stabilization and assembly of the doublet microtubule. Herein, we review our new insights into microtubule inner proteins from the doublet microtubule together with the current body of literature on microtubule inner proteins. High-resolution cryo-electron microscopy structure of the doublet microtubule from Tetrahymena reveals insights into the interactions between microtubule inner proteins with the doublet microtubule tubulin lattice and implication of their functions in the stability and assembly of the doublet microtubule.