V-ALPHA-3.2 SELECTION IN MHC CLASS-I MUTANT MICE - EVIDENCE FOR AN ALTERNATE ORIENTATION OF TCR-MHC CLASS-I INTERACTION

TCR V alpha elements are expressed preferentially in CD4 or CD8 subset s in a manner that is largely independent of MHC haplotype. It is like ly that the V alpha s interact preferentially with conserved regions o f class I or class II molecules. To investigate the topology of bindin g of TCR to MHC-peptide complexes, we screened a panel of H-2K(bm) mut ants for differential V alpha expression, One strain, bm23, showed a c onsistent alteration in V alpha expression, with increased V alpha 3.2 expression in CD8 peripheral T cells, This overselection is manifest in CD8 single-positive thymocytes and appears to be due to enhanced po sitive selection on K-bm23. There is no apparent effect of V beta elem ents. The K-bm23 molecule is unique compared with K-b and the other K- bm molecules at residue 75 on the helix of the alpha 1 domain, suggest ing an interaction between V alpha 3.2 and the alpha 1 helix at this p oint, Such an interaction is inconsistent with the orientation of TCR and MHC defined in two crystal structures, but is consistent with an o rientation where the TCR is rotated by 180 degrees relative to MHC.