THE GLUT-1 GLUCOSE-TRANSPORTER INTERACTS WITH CALNEXIN AND CALRETICULIN

Calnexin is an integral membrane protein that acts as a chaperone duri ng glycoprotein folding in the endoplasmic reticulum. Cross-linking st udies were carried out with the aim of investigating the interactions of calnexin with glycoproteins in vitro, A truncated version of the in tegral membrane glycoprotein Glut 1 (GT(155)) was synthesized in a rab bit reticulocyte translation system in the presence of canine pancreat ic microsomes, Following immunoprecipitation with an anti-calnexin ant iserum, a cross-linker-independent association was observed between GT (155) and calnexin. In addition, the anti-calnexin antiserum immunopre cipitated a W-dependent cross-linking product consisting of GT(155) an d a protein of approximately 60 kDa designated CAP-60 (calnexin-associ ated protein of 60 kDa), Both the GT(155)-calnexin and the GT(155)-CAP -60 interactions were dependent on the presence of a correctly modifie d oligosaccharide group on GT(155), a characteristic of many calnexin interactions, A GT(155) mutant that was not glycosylated (AGGT(155)) d id not associate with calnexin or CAP-60, Calreticulin, the soluble ho mologue of calnexin, was also shown to interact with GT(155) only when the protein bore a correctly modified oligosaccharide group, Thus, ou r data show that both calnexin and calreticulin with Glut 1 in a glyco sylation-dependent manner.