THE ROLE OF MHC CLASS-I MOLECULES IN THE GENERATION OF ENDOGENOUS PEPTIDE MHC COMPLEXES/

Cellular proteins undergo proteolysis to yield peptide/MHC class I com plexes for display on the APC surface. During this process it is not c lear whether MHC molecules bind to and stabilize independently generat ed peptides, or whether they are involved in the peptide cleavage even ts. In this study, we analyzed the role of MHC molecules in Ag process ing by characterizing the naturally processed peptide analogues of OVA (OVA257-264, SL8) in APC. DNA constructs encoding SL8 precursors were transfected into cells that varied in their MHC expression. By HPLC f ractionation of cell extracts and with sensitive T cell assays for bot h the processed SL8 and its minimal Met-SL8 (MSL8) precursor, we deter mined that expression of K-b MHC molecule was essential for detecting processed peptides in living cells. Curiously, although the translated MSL8 nonapeptide precursor itself could bind K-b as well as the SL8 o ctapeptide, and MSL8 was available to MHC, only the SL8 peptide was fo und in K-b cell extracts. The presence of naturally processed SL8, but not MSL8 peptide in K-b-expressing cells suggests that the precise id entity of endogenously processed peptides is also strongly influenced by the MHC molecules.