Baboon and cotton-top tamarin B₂m cDNA sequences and the evolution of primate β₂-microglobulin

Nonhuman primates represent phylogenetic intermediates for studying the divergence of human and murine β₂Ms. We report the nucleotide sequences of B₂m cDNA clones from a baboon cell line, 26CB-1 (Papio hamadryas); primates: Cercopithecoida), and a cotton-top tamarin cell line, 1605L (Saguinus oedipus; primates: Ceboidea). The baboon and tamarin B₂m sequences indicate a very slow rate of B₂m evolution in primates relative to that in murid rodents. Phenotypic evolution of β₂M has also been very conservative in primates, with only 9-14 substitutions separating baboon or tamarin β₂Ms from those of humans or orangutans. Analyses of silent and amino-acid-altering nucleotide substitutions provide evidence that negative selection has acted to limit variability in β strands of primate β₂Ms, while positive selection has promoted diversity in non-β-strand regions of murine β₂Ms. No evidence for the action of selection upon β₂M residues that contact the class I heavy chain was found in primates or mice. The finding that different selective forces have operated upon primate and murine β₂Ms suggests that β₂M may have evolved to serve distinct functions in primates and mice.