Folding, unfolding, and refolding of the vesicular stomatitis virus glycoprotein

Folding and refolding of the vesicular stomatitis virus (VSV) glycoprotein (G protein), New Jersey serotype, were studied both in infected cells and after urea denaturation and reduction of isolated protein in vitro. To assess the contribution of disulfide bonds to the conformation of this type I membrane glycoprotein, reduced and alkylated forms were compared with unreduced G proteins by their mobility on SDS-polyacrylamide gels and by their reactivity with conformation-dependent monoclonal antibodies (MAbs). Pulse-chase experiments showed that G protein folding in the endoplasmic reticulum (ER) of infected cells occurred rapidly (estimated half-time of 1-2 min) and involved transient association with the ER chaperone calnexin. Inhibition of glycosylation by tunicamycin slowed the folding process and emergence from the ER but did not prevent the appearance of a conformationally mature transport-competent G protein. For in vitro refolding studies, native G protein isolated from virus particles was denatured and reduced with urea and beta-mercaptoethanol. When rapidly diluted into a denaturant-free buffer containing oxidized glutathione and the nonionic detergent octyl glucoside, the G protein regained considerable native structure, as determined by reactivity with five monoclonal antibodies specific for different conformation-dependent epitopes. Whereas the refolding process was slow and inefficient in vitro relative to folding in the cell, this observation nonetheless demonstrated that an integral fully glycosylated membrane protein can be refolded to form a structure similar to that of the original protein processed during in vivo synthesis. If, however, unfolded nonglycosylated G protein was the starting material, refolding in vitro failed. In summary, we have shown that VSV G protein folding can be analyzed both in vivo and in vitro and that folding in the cell involves at least one chaperone and can occur in vivo even if not glycosylated.