Gg. Capps et Mc. Zuniga, CLASS-I HISTOCOMPATIBILITY MOLECULE ASSOCIATION WITH PHOSPHORYLATED CALNEXIN - IMPLICATIONS FOR RATES OF INTRACELLULAR-TRANSPORT, The Journal of biological chemistry, 269(15), 1994, pp. 11634-11639
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.