Disulfide structure of murine Ia alloantigens

The tryptic (T) and T insoluble chymotryptic (TIC) peptide maps from 35S-cysteine (Cys) labeled, disulfide-linked I-Ak dimer were compared to those from 35S-Cys labeled I-Ak a and β chains which were not covalently linked. These comparisons indicated that the α and β chains found in the covalent I-Ak dimer were not a specialized subset of I-A α and β chains. Furthermore, these data, along with the knowledge that alkylation of spleen cells prior to and during detergent solubilization prevents the formation of disulfide-linked I-Ak dimer, indicate that covalent dimer formation is an inefficient and artifactual process. Comparison of the T and TIC peptide maps of reduced and nonreduced 35S-Cys labeled I-Ak α and β chains suggests that the I-Ak α chain contains one intrachain disulfide bond, whereas the I-Ak β chain contains two intrachain disulfide bonds. Examination of the T and TIC peptide maps of the reduced and nonreduced 35S-Cys labeled I-Ak dimer identifies the Cys-containing peptides which are involved in the formation of the artifactual I-Ak dimer interchain (α-β) disulfide bond. Comparison of 35S-Cys labeled I-Ak and I-Ek α and β chains by T and TIC peptide mapping reveals considerably more homology between the two α-chains and between the two β-chains than is observed using other 3H-amino acid precursors, thus indicating that the I-Ak and I-Ek alloantigens are homologous in their amino acid sequences adjacent to the Cys resides. The reasons for the inability to induce formation of interchain (α-β) disulfide bonds in I-Ekmolecules are discussed.