H-2K molecules have two different C-termini, one of which is K-region specific.

Amino acid sequences encoded by exon 8 of the H-2K and H-2D/L genes appear to be locus specific. The majority of H-2Kb molecules contain 10 amino acids that are derived from exon 8. In contrast, the H-2Db, -Dd and -Ld molecules contain only one amino acid which is encoded by exon 8, even though the genetic information exists to encode 10 amino acids analogous to those encoded by the majority of H-2Kb transcripts. We have produced a rabbit anti-peptide serum reactive with the exon 8 encoded sequence of H-2Kb (alpha K-C) that specifically immunoprecipitates a molecule of 45 K mol. wt from spleen cell lysates of b, d, p, q and k haplotype mice. Further analysis by Western blots indicated that virtually all mouse strains express a 45 K protein reactive with alpha K-C. In sequential immunoprecipitations of spleen cell lysates from b, q, p and d haplotype mice using alpha K-C followed by H-2K or H-2D private specificity alloantisera, the anti-peptide serum removed nearly all of the molecules reactive with the anti-H-2K alloantisera (except in the k haplotype) and none of the molecules reactive with the anti-H-2D serum. In addition, no D-region molecules possessing a long C-terminal sequence were detected with an antiserum directed against a representative D-region long C-terminal peptide. We conclude that even though the genetic information for an extended exon 8 exists in K, D and L locus genes, apparently only K-region molecules are expressed with such a C-terminus. Furthermore, in most haplotypes the great majority of H-2K molecules are produced using long exon 8; however, H-2Kk is produced mostly from short exon 8. The absence or presence of key adenosine residues is predicted to be responsible for the variability in class I exon 8 splicing.