Residue 3 of β2-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 (Aotus) possesses at least two variants of the β2-microglobulin (β2m) protein. These two variants have different isoelectric points, and exhibit differential reactivity with the monoclonal antibody W6/32. We report cDNA sequences of the B2m gene, from W6/32-positive and W6/32-negative Aotus cell lines. The two β2m variants we identified exhibit a single amino acid difference at position three. An arginine residue at position 3 was correlated with W6/32 reactivity, whereas histidine was associated with non-reactivity. W6/32 reactivity was conferred to a W6/32-negative Aotus cell line when it was transfected with the B2m from the W6/32-positive cell line. Residue 3 of β2m 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 previously been shown to influence W6/32 antibody binding. We conclude that W6/32 binds a compact epitope on the class I molecule that includes both residue 3 of β2m and residue 121 of the heavy chain. We examined the distribution of the two β2m motifs in a sample Aotus population using an allele-specific polymerase chain reaction assay. The pattern of β2m segregation we observed matches that which was defined previously by serology. Additionally, we identified laboratory-born hybrid animals who possess both variants of β2m.