Robinson W H, Neuman de Vegvar H E, Prohaska S S, Rhee J W, Parnes J R
Department of Medicine, Stanford University School of Medicine, CA 94305-5487, USA.
Eur J Immunol. 1995 Oct;25(10):2765-9. doi: 10.1002/eji.1830251008.
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.
人CD6是一种单体105/130 kDa的T细胞表面糖蛋白，参与T细胞活化。人（44个氨基酸）和小鼠（243个氨基酸）CD6胞质结构域大小的明显差异，促使我们利用人外周血淋巴细胞mRNA的逆转录聚合酶链反应来分离包含人CD6羧基末端编码区的cDNA克隆。最长的人cDNA克隆CD6-PB1的核苷酸序列预测其编码一个668个氨基酸的蛋白质，其244个氨基酸的胞质结构域大小与小鼠CD6的胞质结构域相似，氨基酸序列同一性为71.5%。这个以前未被识别的244个氨基酸的胞质结构域与任何其他已知蛋白质（除小鼠CD6外）没有显著同源性，但确实具有两个富含脯氨酸的基序，包含SH3结构域结合共有序列，一个富含丝氨酸-苏氨酸的基序重复三次，三个蛋白激酶C磷酸化位点基序，以及10个酪蛋白激酶2磷酸化位点基序。这些序列可能在CD6特异性单克隆抗体刺激T细胞增殖的能力中发挥作用。包含该胞质结构域序列的全长CD6 cDNA编码一个单体105/130 kDa的蛋白质，该蛋白质可从转染细胞表面免疫沉淀，并在SDS-PAGE上与从外周血淋巴细胞免疫沉淀的野生型CD6共迁移。我们还分离出了两种人CD6 cDNA的可变剪接形式，它们缺少编码胞质结构域膜近端区域的序列，这些序列与CD6-PB1保持相同的阅读框。先前报道的人CD6-15 cDNA克隆的短胞质结构域是由于使用了一个替代的3'剪接位点而缺失了一个20 bp的片段，导致相对于我们分离的克隆发生了移码和翻译提前终止。这些数据表明，人CD6具有一个大的胞质结构域，包含可能参与通过CD6连接刺激T细胞时信号转导的序列基序。
