Identification of the region on the class I histocompatibility molecule that interacts with the molecular chaperone, p88 (calnexin, IP90).

During early stages in their biogenesis, murine class I histocompatibility molecules interact transiently with a molecular chaperone of the endoplasmic reticulum designated p88. Using a series of mutant class I heavy chains we mapped the region of the heavy chain that interacts with p88. Domain deletion mutants of the H-2Db and H-2Kb molecules revealed that most of the extracellular portion of the heavy chain and the bulk of the cytoplasmic domain were not required for the association. However, replacement of the transmembrane segment and cytoplasmic domain with a glycosyl phosphatidylinositol anchor from Q7b resulted in a heavy chain that was incapable of interaction with p88. These results suggested that the primary site of interaction with p88 is within a region containing the transmembrane segment and several flanking amino acids of the class I heavy chain. This finding was supported by replacing the glycosyl phosphatidylinositol anchor of the noninteracting Q7b protein with segments of the Db heavy chain containing the putative interaction site and showing that the hybrids were capable of associating with p88. The apparent lack of interaction between segments of p88 and the class I heavy chain that are present within the lumen of the endoplasmic reticulum was also observed when the association between p88 and the alpha chain of the T cell receptor was examined. The full-length transmembrane alpha chain formed a complex with p88, whereas a soluble variant consisting of most of the luminal portion of the alpha chain exhibited only minimal interaction. Thus, p88 is capable of associating with nascent integral membrane proteins through transmembrane interactions that are unavailable to the major soluble chaperone of the endoplasmic reticulum, BiP (GRP78).