Expression of mutant H-2d proteins encoded by class I genes which alternatively process the 5' end of their...

Mutation of the 3' splice sites bordering exon 2 of the H-2Dd and H-2Kd genes generated alternatively spliced transcripts when the constructs were transfected into L cells (J. Immunol. 143:1018). The H-2Dd transcripts contained an additional 84 nucleotides derived from the first intervening sequence, whereas 60 extra bases were included in the H-2Kd mRNA. Proteins derived from these transcripts were recognized by mAb. Moreover, both Ag served as recognition elements for CTL, and the mutant H-2Kd molecule functioned as a restricting element for an Ag peptide. As a result of alternative splicing, the mutant proteins should have additional residues at their NH2 termini to increase their lengths by 28 (Dd) or 20 (Kd) amino acids. Immunoprecipitation and analysis on SDS-PAGE demonstrated that the mutant H-2Kd molecule was indeed larger than the normal H-2Kd protein, but the mutant and wild-type H-2Dd Ag were the same size. In addition, treatment of H-2Dd mutant and normal Ag with N-glycanase produced molecules of equal size, demonstrating that the mutant protein was completely glycosylated. Limited amino acid sequencing of this Ag indicated that it was normal H-2Dd. Therefore, before its transfer to the cell surface, post-translational modifications remove the additional NH2-terminal residues of the mutant Dd but not Kd protein.