Brief DeHnitive Report

The I region of the murine major histocompatibility complex (MHC) encodes a group of membrane alloantigens (Ia antigens) which appear intimately associated with immune response (Ir) genes mapped to this region (1, 2). Although five I subregions have been defined by serological and functional analysis (3), Ia antigens have been demonstrated by immunoprecipitation techniques for only the A and E / C ~ subregions. The A and E/C alloantigens are each composed of two nondisulfide bonded polypeptide chains, alpha (31-34,000 dahons) and beta (26-29,000 dahons) (4, 5). Both subregion and haplotype-specific electrophoretic variation in these Ia antigens have been shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE) (5) and two-dimensional gel analysis (2-D gels) (6). By partial NH2-terminal sequence analysis, the A and E / C subregion polypeptides are not homologous and only the fl-chains display haplotype associated sequence variation (7, 8). Among the E/C allelic products, comparative peptide mapping has demonstrated extensive haplotype associated structural variation (50%) in the fl-chains, but only limited (10%) variation in the a-chains (9). A conservative interpretation of the above data is that at least the A and E/C fl-polypeptides are/-region encoded. Further insight into which loci control the expression of the E/C alloantigen has recently been provided in an elegant study by Jones et al. (10) by using 2-D gel analysis. Specific anti-E/C immunoprecipitates examined by this technique revealed two electrophoretically distinct species of polypeptides. Subsequent analysis of antiE /C immunoprecipitates and whole splenic lysates from intra-I region recombinants and Fx hybrids demonstrated that the two E/C polypeptides were controlled by different genes. A locus in E/C encoded one polypeptide; the other polypeptide, designated as A~, was controlled by the A subregion. Thus, strains with the same haplotype in E/C, but a different haplotype in A, expressed E/C alloantigens with electrophoretically distinct Ae polypeptides; this variation could result from either post-translational modification or primary structural differences. In the present report, we have isolated the E / C orand fl-subunits from appropriate