Effect of tunicamycin and monensin on the transport to the cell surface and secretion of a viral membrane glycoprotein containing both N- and O-linked sugars.

Most membrane glycoproteins contain either N-linked or O-linked oligosaccharides, which play important roles in correct folding, stability, and intracellular transport. Some glycoproteins, however, contain both the N- and O-linked sugars. To study the roles of the two types of glycosylation in intracellular transport we have used as a model the glycoprotein gC-1 of herpes simplex virus type 1 (HSV-1), which contains both N- and O-linked oligosaccharides. Cloned gene of gC-1 was expressed constitutively in mammalian cells to produce the gC-1 glycoprotein containing both types of glycosylation. Only a fraction of the gC-1 glycoprotein was secreted into the medium. Addition of tunicamycin blocked N-glycosylation and the gC-1 protein of reduced size containing only O-linked sugars was formed. This O-glycosylated gC-1 protein was transported to the cell surface and secreted into the medium, indicating that glycoprotein transport to and across the cell surface occurs in the absence of N-glycans. The data suggest either that O-glycans may contribute to this process or that transport can occur in the absence of both N- and O-glycans.