Microsequence analysis of Ia antigens from three strains of rats

Homologues of Ia antigens of the mouse are identified in three rat strains by partial N-terminal amino acid sequence analysis. Ia antigens of the rat were isolated by indirect immune precipitation using specific rat alloantisera. Rat Ia antigens consist of two components, alpha and beta, which have respective mol. wts. of approximately 35 000 and 28 000. Partial N-terminal sequence analysis of each of the alpha components of the H-1a, H-11 and H-1n haplotypes yields a single, apparently homogeneous sequence which is identical among the three haplotypes and is strikingly homologous to the alpha polypeptides of the I-E subregion of mouse and to the human polypeptide, p34. Partial N-terminal sequence analysis of the beta components shows that a mixture of polypeptides is present for each haplotype. There are differences in the beta sequences among the three haplotypes and potential homologies between the rat beta sequences and the sequences of the beta polypeptides of the I-A and I-E subregions of mouse. These obserations imply that the rat has at least two distinct groups of Ia molecules. The organization of genes encoding the Ia polypeptides in the major histocompatibility complex of the rat is discussed.