Glycan-dependent and -independent association of vesicular stomatitis virus G protein with calnexin.

Calnexin (CNX) is a membrane-bound molecular chaperone that associates with newly synthesized proteins in the endoplasmic reticulum. Although several studies have indicated that it interacts exclusively with glycoproteins that carry monoglucosylated N-linked oligosaccharides, others have reported that it can bind to proteins that have no glycans. To address this discrepancy, we translated wild-type vesicular stomatitis virus G protein and nonglycosylated mutant forms in the presence of microsomes and examined their association with CNX. Individual G protein molecules were found to efficiently associate with CNX when both glycans were present and less efficiently if there was only a single glycan. Nonglycosylated G protein also interacted with CNX, but only when misfolded and present in high molecular weight aggregates. The results indicated that CNX can interact with G protein in two ways: through an oligosaccharide-dependent mechanism that involves individual substrate proteins; and in an oligosaccharide-independent association with large aggregates.