Residue 3 of beta(2)-microglobulin affects binding of class I MHC molecules by the W6/32 antibody

Previous studies of class I MHC molecules have shown that the owl monkey (A otus) possesses at least two variants of the beta(2)-microglobulin (beta(2) m) protein. These two variants have different isoelectric points, and exhib it differential reactivity with the monoclonal antibody W6/32. We report cD NA sequences of the B2m gene, from W6/32-positive and W6/32-negative Aotus cell lines. The two beta(2)m variants we identified exhibit a single amino acid difference at position three. An arginine residue at position 3 was co rrelated with W6/32 reactivity, whereas histidine was associated with non-r eactivity. W6/32 reactivity was conferred to a W6/32-negative Aotus cell li ne when it was transfected with the B2m from the W6/32-positive cell line. Residue 3 of beta(2)m is located at the surface of the class I molecule. It is also close to position 121 of the MHC class I heavy chain, which has pr eviously been shown to influence W6/32 antibody binding. We conclude that W 6/32 binds a compact epitope on the class I molecule that includes both res idue 3 of beta(2)m and residue 121 of the heavy chain. We examined the dist ribution of the two beta(2)m motifs in a sample Aotus population using an a llele-specific polymerase chain reaction assay. The pattern of beta(2)m seg regation we observed matches that which was defined previously by serology. Additionally, we identified laboratory-born hybrid animals who possess bot h variants of beta(2)m.