An extensive region of an MHC class I α2 domain loop influences interaction with the assembly complex

Presentation of antigenic peptides to CTLs at the cell surface first requires assembly of MHC class I with peptide and β₂-microglobulin in the endoplasmic reticulum. This process involves an assembly complex of several proteins, including TAP, tapasin, and calreticulin, all of which associate specifically with the β₂-microglobulin-assembled, open form of the class I heavy chain. To better comprehend at a molecular level the regulation of class I assembly, we have assessed the influence of multiple individual amino acid substitutions in the MHC class I α2 domain on interaction with TAP, tapasin, and calreticulin. In this report, we present evidence indicating that many residues surrounding position 134 in H-2L(d) influence interaction with assembly complex components. Most mutations decreased association, but one (L(d)K131D) strongly increased it. The L(d) mutants, with the exception of L(d)K131D, exhibited characteristics suggesting suboptimal intracellular peptide loading, similar to the phenotype of L(d) expressed in a tapasin- deficient cell line. Notably, K131D was less peptide inducible than wild-type L(d), which is consistent with its unusually strong association with the endoplasmic reticulum assembly complex.