Substitution of class II alpha chain polymorphic residues defines location of A alpha k serologic epitopes and alters association between alpha beta and Ii polypeptides.

Structure-function studies of the MHC class II alpha chain have been performed by constructing a panel of A alpha k cDNA genes with one or more d allele substitutions at each polymorphic residue of the alpha 1 domain. The altered genes (A alpha k*) were transfected into a B lymphoma cell line (BKO), which is deficient in A alpha mRNA but retains constitutive wild-type A beta k mRNA expression. Cytofluorometric analysis of cell surface A alpha k* Ak beta molecules was used to map the polymorphic alpha chain residues comprising four serologic epitopes. A alpha k-reactive mAbs 1E9, 2A2, and 3F12 recognize an epitope that includes the polymorphic residue 44 of the A alpha k polypeptide, and the A alpha k-reactive antibody, K24-199, recognizes a conformationally determined epitope influenced by residues from all three polymorphic regions. In addition, we confirmed previous studies demonstrating that la.19 and la.2 mAbs bind to epitopes adjacent to residue 75 in A alpha k. A cell surface negative expression variant also was identified in the panel of mutant cell lines and biochemically characterized. Substitution of six d allele polymorphic residues at positions 11, 14, 28, 69, 70, and 75 in the A alpha k polypeptide (T.EG A alpha k* A beta k*) results in an A alpha k* polypeptide that associates with the A alpha k polypeptide but does not associate with the li polypeptide. This defect in li-A alpha k A beta k interaction is associated with a conformational change in the alpha 1 beta 1 domain that was identified by altered reactivity of the T.EG complex with conformationally dependent anti-alpha and anti-beta mAbs.(ABSTRACT TRUNCATED AT 250 WORDS)