PHYSICAL AND FUNCTIONAL ASSOCIATION OF THE MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I HEAVY-CHAIN ALPHA-3 DOMAIN WITH THE TRANSPORTER ASSOCIATED WITH ANTIGEN-PROCESSING

CD8(+) T lymphocytes recognize antigens as short, MHC class I-associat ed peptides derived by processing of cytoplasmic proteins. The transpo rter associated with antigen processing translocates peptides from the cytosol into the ER lumen, where they bind to the nascent class I mol ecules. To date, the precise location of the class I-TAP interaction s ite remains unclear. We provide evidence that this site is contained w ithin the heavy chain alpha 3 domain. Substitution of a 15 amino acid portion of the H-2D(b) alpha 3 domain (aa 219-233) with the analogous MHC class II (H-2IA(d)) beta 2 domain region (aa 133-147) results in l oss of surface expression which can be partially restored upon incubat ion at 26 degrees C in the presence of excess peptide and beta 2-micro globulin. Mutant H-2D(b) (D(b)219-233) associates poorly with the TAP complex, and cannot present endogenously-derived antigenic peptides re quiring TAP-dependent translocation to the ER. However, this presentat ion defect can be overcome through use of an ER targeting sequence whi ch bypasses TAP-dependent peptide translocation. Thus, the alpha 3 dom ain serves as an important site of interaction (directly or-indirectly ) with the TAP complex and is necessary for TAP-dependent peptide load ing and class I surface expression.