Ubiquitination of Glycoproteins in the Cytosol

Abstract

The secretory proteins (membrane proteins, secretion proteins, and lysosomal enzymes) that are synthesized in the ribosome are translocated into the endoplasmic reticulum (ER). These nascent proteins are subjected to modification as the mature glycoproteins by the attachment of carbohydrates and move through the ER via the Golgi complex to their final destinations. The carbohydrate portions of glycoconjugates are implicated in many biological functions and are used as specific markers for cells. They are mainly expressed on the outside of cells. Carbohydrates, however, play an important role in the inside of cells (Helenius and Aebi 2001). N-linked glycoproteins are subjected to the quality control of glycoproteins through which aberrant proteins are distinguished from properly folded proteins and retained in the ER compartment. The quality control system includes the calnexin-calreticulin cycle, a unique chaperone system (Parodi 2000). Here, the author describes briefly the identification of N-glycan-binding proteins for E3 ubiquitin ligases and their perspectives in basic science as well as in clinical medicine.