CALNEXIN ACTS AS A MOLECULAR CHAPERONE DURING THE FOLDING OF GLYCOPROTEIN-B OF HUMAN CYTOMEGALOVIRUS

Human cytomegalovirus glycoprotein B (gB) is synthesized as a 105-kDa nonglycosylated polypeptide and cotranslationally modified by addition of N-linked oligosaccharides to a 160-kDa precursor in the endoplasmi c reticulum (ER). It is then transported to the Golgi complex, where i t is endoproteolytically cleaved to form the disulfide-linked mature g p55-116 complex. Pulse-chase experiments demonstrate that the 160-kDa gB precursor was transiently associated with calnexin, a membrane-boun d chaperone, in the ER. The association was maximal immediately after synthesis, and they dissociated with a half-time of 15 min. Complete i nhibition of binding by tunicamycin or castanospermine indicates the i mportance of N-linked oligosaccharides for it. Nonreducing sodium dode cyl sulfate-polyacrylamide gel electrophoresis demonstrated that durin g an initial stage in the biogenesis, the 160-kDa gB precursor was fir st synthesized as a fully reduced form and rapidly converted to an oxi dized form, with a half-time of 18 min. Both forms of the gB precursor could bind to calnexin. The kinetics of the conversion from the fully reduced to the oxidized form coincided with that of dissociation of t he 160-kDa gB precursor from calnexin, suggesting that the two steps a re closely related.