CLASS-I HISTOCOMPATIBILITY MOLECULE ASSOCIATION WITH PHOSPHORYLATED CALNEXIN - IMPLICATIONS FOR RATES OF INTRACELLULAR-TRANSPORT

Recent studies have shown that the endoplasmic reticulum (ER)-resident protein, calnexin, associates with class I major histocompatibility c omplex (MHC) molecules early in their biosynthesis. It has been sugges ted that calnexin participates in the assembly of class I MHC molecule s or in the retention within the ER of unassembled class I molecules. We have examined the role of phosphorylation of calnexin in its associ ation with mouse class I MHC molecules. We show that phosphocalnexin a ssociates with H-2L(d) and H-2D(b) molecules but not with H-2K(b) and H-2D(d) molecules, although calnexin-H2K(b) association can be demonst rated. These observations are interesting in light of the fact that H- 2K(b) and H-2D(d) molecules are transported out of the ER more rapidly than are H-2L(d) and H-2D(b) molecules. H-2L(d) and H-2D(b) molecules differ in amino acid sequence only in their membrane-distal alpha1 an d alpha2 domains. Nevertheless, the affinity of phosphocalnexin for H- 2L(d) is greater than its affinity for H-2D(b). Furthermore, H-2D(b) b ecomes endoglycosidase H-resistant more slowly in cells in which it as sociates with phosphocalnexin than in cells in which it does not. Ca2 ionophore A23187 prevents association of phosphocalnexin with H-2L(d) molecules in vivo but does not cause the disruption of phosphocalnexi n-H-2L(d) complexes after they have formed. A23187 does not prevent as sembly of H-2L(d)-beta2-microglobulin (beta2-m) heterodimers. Furtherm ore, phosphocalnexin is found associated with H-2L(d) molecules regard less of their state of assembly with beta2-m and antigenic peptide. Th ese results suggest that phosphocalnexin association with class I MHC molecules does not play a role in assembly of the class I MHC-beta2-m- peptide complex nor in preventing release of unassembled class I molec ules from the ER but may otherwise influence their rate of transport t hrough the ER.