Biogenesis of MHC class I antigens: involvement of multiple chaperone molecules

To analyze the early events occurring during the folding and assembly of major histocompatibility complex class I antigens, we used a panel of P815 mouse mastocytoma transfectants expressing wild-type or mutant human leukocyte antigen (HLA)-Cw3 proteins. We observed that newly synthesized unassembled HLA-Cw3 heavy chains (Cw3 alpha) specifically associate with three major long-lived proteins denoted p105, p88 and p78, according to their size. These proteins display different kinetics of interaction. The association of p105 is transient, while p78, which we identified as the immunoglobulin binding protein BiP, interacts permanently with Cw3 alpha chains. Furthermore, the binding of p88, a calnexin candidate, seems delayed compared to that of p105 and p78. As the great majority of newly synthesized Cw3 alpha proteins expressed in P815 cells can associate with cotransfected human beta 2-microglobulin (beta 2m), our observations suggest that multiple molecular chaperones cooperate in the folding of class I heavy chains. We were unable to coimmunoprecipitate detectable levels of these proteins with oligomerized Cw3 alpha chains. However, we could still detect p78/BiP in transient association with mutant HLA-Cw3 heterodimers which were delayed in the endoplasmic reticulum (ER) compared to their wild-type counterparts. In this case, the dissociation of BiP preceded the ER to Golgi transport of these proteins. These results suggest that BiP release is neither related to the process of class I oligomerization nor to the ER retention of class I assembly intermediates.