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 complex (MHC) molecules early in their biosynthesis. It has been suggested that calnexin participates in the assembly of class I MHC molecules or in the retention within the ER of unassembled class I molecules. We have examined the role of phosphorylation of calnexin in its association with mouse class I MHC molecules. We show that phosphocalnexin associates with H-2Ld and H-2Db molecules but not with H-2Kb and H-2Dd molecules, although calnexin-H-2Kb association can be demonstrated. These observations are interesting in light of the fact that H-2Kb and H-2Dd molecules are transported out of the ER more rapidly than are H-2Ld and H-2Db molecules. H-2Ld and H-2Db molecules differ in amino acid sequence only in their membrane-distal alpha 1 and alpha 2 domains. Nevertheless, the affinity of phosphocalnexin for H-2Ld is greater than its affinity for H-2Db. Furthermore, H-2Db becomes endoglycosidase H-resistant more slowly in cells in which it associates with phosphocalnexin than in cells in which it does not. Ca2+ ionophore A23187 prevents association of phosphocalnexin with H-2Ld molecules in vivo but does not cause the disruption of phosphocalnexin-H-2Ld complexes after they have formed. A23187 does not prevent assembly of H-2Ld-beta 2-microglobulin (beta 2-m) heterodimers. Furthermore, phosphocalnexin is found associated with H-2Ld molecules regardless of their state of assembly with beta 2-m and antigenic peptide. These results suggest that phosphocalnexin association with class I MHC molecules does not play a role in assembly of the class I MHC-beta 2-m-peptide complex nor in preventing release of unassembled class I molecules from the ER but may otherwise influence their rate of transport through the ER.
Capps
G G
GG
Department of Biology, Sinsheimer Laboratories, University of California, Santa Cruz 95064.
Zúñiga
M C
MC
D000818:Animals; D001345:Autoradiography; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D037281:Calnexin; D002460:Cell Line; D002499:Centrifugation, Density Gradient; D006183:H-2 Antigens / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D007739:L Cells; D008715:Methionine / Q000378:metabolism; D051379:Mice; D010710:Phosphates / Q000378:metabolism; D010750:Phosphoproteins / Q000302:isolation & purification / Q000378:metabolism; D010761:Phosphorus Radioisotopes; D010766:Phosphorylation; D013462:Sulfur Radioisotopes; D014407:Tumor Cells, Cultured
D002135:Calcium-Binding Proteins; D006183:H-2 Antigens; D010710:Phosphates; D010750:Phosphoproteins; D010761:Phosphorus Radioisotopes; D013462:Sulfur Radioisotopes; D037281:Calnexin; D000001:Calcimycin; D008715:Methionine; D002118:Calcium