HUMAN CD6 POSSESSES A LARGE, ALTERNATIVELY SPLICED CYTOPLASMIC DOMAIN

Human CD6 is a monomeric 105/130-kDa T cell surface glycoprotein that is involved in T cell activation. The apparent discrepancy between the size of the cytoplasmic domain in human (44 amino acids) and mouse (2 43 amino acids) CD6, led us to use reverse transcriptase-polymerase ch ain reaction of human peripheral blood lymphocyte mRNA to isolate cDNA clones that include the carboxyl-terminal coding region of human CD6. The nucleotide sequence of the longest human cDNA clone, CD6-PB1, pre dicts a protein of 668 amino acids with a 244-amino acid cytoplasmic d omain similar in size to and possessing 71.5% amino acid sequence iden tity with the cytoplasmic domain of mouse CD6. This previously unrecog nized 244-amino acid cytoplasmic domain does not have significant homo logy to any other known protein (except mouse CD6), but does possess t wo proline-rich motifs containing the SH3 domain-binding consensus seq uence, a serine-threonine-rich motif repeated three times, three prote in kinase C phosphorylation-site motifs, and 10 casein kinase-2 phosph orylation-site motifs. These sequences are likely to play a role in th e ability of CD6-specific monoclonal antibodies to stimulate T cell pr oliferation. Full-length CD6 cDNA containing this cytoplasmic domain s equence encodes a monomeric 105/130-kDa protein that can be immunoprec ipitated from the surface of transfected cells and comigrates upon SDS -PAGE with wild-type CD6 immunoprecipitated from PBL. We also isolated two alternatively spliced forms of human CD6 cDNA lacking sequences e ncoding membrane-proximal regions of the cytoplasmic domain which main tain the same reading frame as CD6-PB1. The short cytoplasmic domain o f the previously reported human CD6-15 cDNA clone results from a delet ion of a 20-bp segment through use of an alternative 3' splice site, r esulting in a frame shift and premature termination of translation rel ative to the clones we have isolated. These data demonstrate that huma n CD6 possesses a large cytoplasmic domain containing sequence motifs that are likely to be involved in signal transduction upon stimulation of T cells through CD6 ligation.