Human CD6 possesses a large, alternatively spliced cytoplasmic domain

Authors

  • William H. Robinson,

    1. Department of Medicine, Division of Immunology and Rheumatology and the Digestive Disease Center, Stanford University School of Medicine, Stanford, USA
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  • Henry E. Neuman de Vegvar,

    1. Department of Medicine, Division of Immunology and Rheumatology and the Digestive Disease Center, Stanford University School of Medicine, Stanford, USA
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  • Susan S. Prohaska,

    1. Department of Medicine, Division of Immunology and Rheumatology and the Digestive Disease Center, Stanford University School of Medicine, Stanford, USA
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  • Joon W. Rhee,

    1. Department of Medicine, Division of Immunology and Rheumatology and the Digestive Disease Center, Stanford University School of Medicine, Stanford, USA
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  • Jane R. Parnes

    Corresponding author
    1. Department of Medicine, Division of Immunology and Rheumatology and the Digestive Disease Center, Stanford University School of Medicine, Stanford, USA
    • Division of Immunology and Rheumatology, Medical School Lab. Surge Building, Room P306, Stanford University Medical Center, Stanford, CA 94305-5487, USA (Fax: 415-723-7509)
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Abstract

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 (243 amino acids) CD6, led us to use reverse transcriptase-polymerase chain 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, predicts a protein of 668 amino acids with a 244-amino acid cytoplasmic domain similar in size to and possessing 71.5% amino acid sequence identity with the cytoplasmic domain of mouse CD6. This previously unrecognized 244-amino acid cytoplasmic domain does not have significant homology to any other known protein (except mouse CD6), but does possess two proline-rich motifs containing the SH3 domain-binding consensus sequence, a serine-threonine-rich motif repeated three times, three protein kinase C phosphorylation-site motifs, and 10 casein kinase-2 phosphorylationsite motifs. These sequences are likely to play a role in the ability of CD6-specific monoclonal antibodies to stimulate T cell proliferation. Full-length CD6 cDNA containing this cytoplasmic domain sequence encodes a monomeric 105/130-kDa protein that can be immunoprecipitated 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 encoding membrane-proximal regions of the cytoplasmic domain which maintain the same reading frame as CD6-PB1. The short cytoplasmic domain of the previously reported human CD6–15 cDNA clone results from a deletion of a 20-bp segment through use of an alternative 3' splice site, resulting in a frame shift and premature termination of translation relative to the clones we have isolated. These data demonstrate that human 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.

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