Molecular requirements for the interaction of class II major histocompatibility complex molecules and invariant chain with calnexin.

Molecular chaperones are believed to retain misfolded and incompletely assembled oligomeric proteins in the endoplasmic reticulum (ER). Here, we have further analyzed the association of one such chaperone, calnexin, with human major histocompatibility complex class II alpha and beta subunits and the invariant chain. Calnexin associates with transport-competent invariant chain trimers (p33 or p41), as well as ER-retained trimers (p35/33 or p43/41), suggesting that ER retention of the latter is not because of calnexin association. Neither the replacement of the transmembrane segment of the DR beta subunit with a glycosyl phosphatidylinositol anchor nor deglycosylation of any of these proteins with tunicamycin or endoglycosidase H treatment abolished calnexin association. Using a cell-permeabilization system, we were unable to observe association of newly synthesized glycopeptides with calnexin, arguing that calnexin may not act like a simple lectin for association with glycoproteins. The results indicate that neither transmembrane regions nor N-linked glycans are exclusively responsible for calnexin association. Based on our data and the observations of others, we suggest that these features may have varying significance for different glycoproteins in determining their interaction with calnexin.