IDENTIFICATION OF A MOUSE PROTEIN HOMOLOGOUS TO THE HUMAN CD6 T-CELL SURFACE PROTEIN AND SEQUENCE OF THE CORRESPONDING CDNA

CD6 is a 105/130 kDa monomeric T cell surface glycoprotein that has be en shown to play a role in human T cell activation. Recently a partial mouse CD6 cDNA sequence was described. We have isolated full-length c DNA clones including the initiation codon and sequence encoding the fu ll signal peptide, as well as an additional 39 amino acids within the cytoplasmic domain as compared to the previously reported clone. The p redicted full-length mouse CD6 protein contains 665 amino acids and ha s the features of a type I integral membrane protein. The extracellula r domain of mouse CD6 is composed of three repeated cysteine-rich doma ins similar to those in human CD6, mouse and human CD5, and other memb ers of a family of proteins whose prototype is the type I macrophage s cavenger receptor. In marked contrast to the previously published huma n CD6 sequence, the mouse sequence predicts a long cytoplasmic tail th at is not closely related to other proteins and possesses two proline- rich motifs containing the SH3-domain binding consensus sequence, thre e protein kinase C phosphorylation site motifs, nine casein kinase-2 p hosphorylation site motifs, and a serine-threonine-rich motif repeated three times. Northern blot analysis revealed that mouse CD6 mRNA is e xpressed predominantly in thymus, lymph node, and spleen. A polyclonal antiserum was raised against mouse CD6 by gene gun plasmid DNA immuni zation of rabbits with the mouse CD6 cDNA in an expression vector. In immunofluorescence analysis this polyclonal antiserum positively stain ed the surface of cells transfected with the mouse CD6 cDNA in an expr ession vector, as well as most normal mouse thymocytes and peripheral T cells. CD6 protein is expressed on most CD4(+)CD8(+) double-positive and CD4(+) or CD8(+) single-positive thymocytes, and is expressed at highest levels on mature CD3(high) thymocytes. The expression of mouse CD6 in thymocytes and peripheral T cells correlates closely with the expression of the related CD5 molecule. The polyclonal rabbit anti-mou se CD6 Abs immunoprecipitated a major polypeptide of 128 kDa from rest ing and 130 kDa from PMA- and FCS-activated mouse thymocytes and lymph node cells; it is likely that this increase in size upon activation i s due to phosphorylation of mouse CD6 as has been described for human CD6. These data demonstrate that mouse thymocytes and T cells express a 130-kDa cell surface protein homologous to human CD6.