Glycoprotein trafficking in cells and organisms

This chapter explores lectins that function in the trafficking of glycoproteins by recognizing endogenous glycans. It first looks at the intracellular recognition events that involve glycans common to all proteins that are N-glycosylated. Glycans of this type tag many different glycoproteins that serve a variety of different functions at the cell surface. Similarly, the asialoglycoprotein and mannose receptor clearance systems can handle a multitude of different glycoproteins as long as they bear the correct glycans. However, two of the systems, the mannose 6-phosphate receptor and the binding of glycans containing 4-O-sulphated GalNAc residues to the mannose receptor, involve unusual glycans that are attached to specific subsets of glycoproteins. These special glycans allow targeting of selected glycoproteins in ways that are different from the bulk of glycoproteins. Attachment of special glycans to the selected subset of glycoprotein targets is directed by information in the amino acid sequence of the glycoprotein.