Qa-2, H-2K, and H-2D alloantigens evolved from a common ancestral gene

The Tla region located on the murine 17th chromosome controls several serologically defined cell surface antigens. These antigens, referred to as Qa-1-5 and TL, are expressed on a variety of hematopoeitic cell populations. In the present studies we have immunoprecipitated isotopically labeled Qa-2 and H-2 molecules from mitogen-stimulated B6 spleen cells. Sequential immunoprecipitation experiments have shown that the determinants recognized by αQa-2, αH-2K⁶, and αH-2D⁶ alloantisera reside on separate molecular species. Comparative mapping of the arginine-labeled tryptic peptides from Qa-2, H-2K⁶, and H-2D⁶ molecules indicate that Qa-2 is structurally distinct but that there is considerable structual homology; 21-43% of the Qa-2 peptides co-chromatograph with peptides derived from H-2D^b and H-2K^b, respectively. Similar levels of homology are observed when Qa-2 is compared with H-2K(k) or H-2D(d). The results show that the Qa-2 alloantigen is encoded by a locus separate from the loci encoding H-2K or H-2D alloantigens, but that the Qa-2, H-2K, and H-2D alloantigens are sufficiently related at the primary structural level to indicate that they evolved from a common primordial gene.