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 (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 phosphorylation-site 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.
Robinson
W H
WH
Department of Medicine, Stanford University School of Medicine, CA 94305-5487, USA.
Neuman de Vegvar
H E
HE
Prohaska
S S
SS
Rhee
J W
JW
Parnes
J R
JR
D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D015703:Antigens, CD / Q000737:chemistry* / Q000235:genetics; D000945:Antigens, Differentiation, T-Lymphocyte / Q000737:chemistry* / Q000235:genetics; D001483:Base Sequence; D018076:DNA, Complementary / Q000235:genetics; D006801:Humans; D007739:L Cells; D008213:Lymphocyte Activation; D051379:Mice; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D017434:Protein Structure, Tertiary; D012326:RNA Splicing; D016415:Sequence Alignment; D015398:Signal Transduction; D013045:Species Specificity; D013601:T-Lymphocytes / Q000276:immunology; D014162:Transfection; D018909:src Homology Domains
D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D000945:Antigens, Differentiation, T-Lymphocyte; C062279:CD6 antigen; D018076:DNA, Complementary