MODEL FOR THE IN-VIVO ASSEMBLY OF NASCENT L(D) CLASS-I MOLECULES AND FOR THE EXPRESSION OF UNFOLDED L(D) MOLECULES AT THE CELL-SURFACE

To characterize the process of class I assembly and maturation, we hav e studied the L(d) molecule of the mouse. Previous studies have shown that a significant proportion of intracellular and surface L(d) molecu les can be detected in an alternative conformation designated L(d)alt1 . Nascent L(d)alt molecules are non-peptide ligand associated and are weakly associated with beta2-microglobulin (beta2m). Unexpectedly, whe n monoclonal antibodies were added directly to the lysis buffer, signi ficant amounts of L(d)alt/beta2m heterodimer were detected, suggesting that beta2m association is not necessarily sufficient to induce L(d) conformation. By contrast, addition of peptide to cell lysates rapidly induced the folding of beta2m-associated L(d)alt to conformed L(d). F urthermore, the time course and dynamics of this conversion correlated precisely with peptide binding to L(d). The precursor-product relatio nship of L(d)alt and conformed L(d) was also visualized in vivo by pul se-chase analysis of BALB/c splenocytes. To investigate the factors th at regulate intracellular transport of class I molecules, expression o f L(d) was studied in the peptide transport-deficient cell line, RMA.S -L(d), and in beta2m-/- splenocytes. In contrast to wild-type cell lin es both L(d)alt and conformed L(d) are poorly expressed at the cell su rface of RMA.S-L(d) and beta2m-/- splenocytes. Therefore, surface expr ession of L(d)alt is dependent upon the concomitant expression of conf ormed L(d) molecules. To determine whether surface L(d)alt molecules c an result from melting of conformed L(d) molecules, surface L(d) molec ules were loaded with several different known L(d) peptide ligands. Co mplexes of L(d) with different ligands were found to have dramatically disparate surface half-lives. Importantly, the L(d) peptide complexes that turned over the most rapidly resulted in the most gain in surfac e L(d)alt, implying that peptide dissociation can induce the accumulat ion of nonconformed L(d) heavy chains at the cell surface.