A major role of the classical transplantation antigens (designated class I antigens) is the presentation of virus-infected cells to cytotoxic T cells, a process that leads to the destruction of the cell displaying the viral antigen1. Consistent with this function is the finding that these transplantation antigens (encoded by the H-2K, H-2D and H-2L genes in mice) are cell-surface glycoproteins with their ammo-termini protruding extracel-lularly and their carboxy-termini located inside the cell2,3. While the external domain is expected to provide biological specificity required for the associative presentation of viral antigens, the role of the cytoplasmic domain remains obscure.The recent observation that this latter region of the molecule is encoded by three separate DNA exons4-9 has suggested a complex role for this portion of the polypeptide chain. We have now obtained evidence for the use of alternative acceptor splice sites in the H-2K gene, resulting in two RNA transcripts that would encode H-2K antigens differing in their carboxy-termini. This is the first demonstration of the use of alternative splice acceptor sites in the same class I gene, and indicates the existence of different functional subsets of antigens encoded by the same gene.
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