CALNEXIN FAILS TO ASSOCIATE WITH SUBSTRATE PROTEINS IN GLUCOSIDASE-DEFICIENT CELL-LINES

Increasing evidence shows that calnexin, a membrane-bound chaperone in the endoplasmic reticulum, is a lectin that binds to newly synthesize d glycoproteins that have partially trimmed N-linked oligosaccharides. It specifically attaches to core glycans from which two glucoses have been removed by glucosidases I and II. Several recent reports suggest , however, that it can also bind to proteins devoid of N-linked glycan s. To investigate the extent of glycan-independent binding, we have an alyzed two mutant cell lines (Lee 23 and Pha(R) 2.7) that are unable t o process the core glycans because they lack glucosidase I or glucosid ase II, respectively, In contrast to parental cell lines, calnexin bin ding of substrate proteins was found to be virtually nonexistent in th ese cells. Neither cellular nor viral proteins associated with the cha perone, It was concluded that glycans are crucial for calnexin associa tion and that the vast majority of substrate proteins are therefore gl ycoproteins.