Cytosolic N‐Glycans: Triggers for Ubiquitination Directing Proteasomal and Autophagic Degradation

Proteins on the cell surface and secreted proteins are modified with sugar chains that generate and modulate biological complexity and diversity. Sugar chains not only contribute physically to the conformation and solubility of proteins, but also exert various functions via sugar-binding proteins that reside on the cell surface or in organelles of the secretory pathway. However, some glycosidases and lectins are found in the cytosol or nucleus. Recent studies of cytosolic sugar–related molecules have revealed that sugar chains on proteins in the cytosol act as signals of adverse cellular conditions. In this review, we summarize recent reports that cytosolic sugar chains can trigger ubiquitination, followed by proteasomal and autophagic degradation to maintain cellular homeostasis. In addition, we discuss the functions of sugar-binding proteins revealed to date, along with possibilities not yet explored. The cytosolic N-glycans that normally reside on the cell surface or in organelles serves as a signal for the presence of unwanted proteins or organelles. Here, we review the monitoring system for cytosolic glycans including glycosidases and lectin in the cytosol.