Search

RT6 is a glycosyl phosphatidylinositol-anchored cell membrane protein, whose expression is restricted to peripheral T cells and intraepithelial lymphocytes. It has attracted interest as a T cell differentiation marker and activation antigen in rats. The only known protein to which RT6 shows significant homology is a recently cloned mono(ADP-ribosyl)transferase of rabbit skeletal muscle which is distantly related also to certain bacterial toxins. Intriguingly, whereas the rat carries a single copy RT6 gene with two known highly divergent alleles, the mouse carries two closely linked, functional RT6 genes that show approximately 85% sequence identity. We have now cloned and sequenced the homologues of the RT6 genes from humans of distinct ethnic backgrounds and of the chimpanzee. Surprisingly, in each case, three premature in-frame stop codons preclude expression of the single copy RT6 gene as a cell surface protein. Otherwise, the RT6 genes of human and chimpanzee exhibit high structural conservation to their rodent counterparts. RNA expression analyses indicate that the RT6 gene is not transcriptionally active in human T cells or any other human tissue analyzed so far. To our knowledge, RT6 represents the first mammalian membrane protein identified that has been lost universally in the human and chimpanzee species due to gene inactivation.

RT6 is a T cell membrane protein that has attracted interest because a defect in RT6 expression is associated with susceptibility to autoimmune type I diabetes in DP-BB rats and NOD mice. Using PCR screening of human/rodent somatic cell hybrids and fluorescence in situ hybridization, we have determined that the gene for the human RT6 homologue is located at 11q13, centromeric to the gene for tyrosinase (TYR, albino locus) and telomeric to that for fibroblast growth factor 4 (FGF4). The data suggest that the human RT6 gene constitutes a new linkage group with TYR and the gene for olfactory marker protein (OMP) on 11q, which has a counterpart in mouse chromosome 7. Thus, in the human, the RT6 locus is dissociated from the hemoglobin beta chain locus (HBB) and its neighboring conserved linkage group at 11p15, in contrast to the mouse, in which RT6 shows a tighter linkage to Hbb than to Tyr. The results support the conclusion that there has been considerable intrachromosomal reshuffling of linked genes since the divergence of primates and rodents.