Tat1, a novel sulfate transporter specifically expressed in human male germ cells and potentially linked to RhoGTPase signaling

RhoGTPases (Rho, Rac, and Cdc42) are known to regulate multiple functions, including cell motility, adhesion, and proliferation; however, the signalin g pathways underlying these pleiotropic effects are far from fully understo od. We have recently described a new RhoGAP (GTPase activating protein for RhoGTPases) gene, MgcRacGAP, primarily expressed in male germ cells, at the spermatocyte stage. We report here the isolation, through two-hybrid cloni ng, of a new partner of MgcRacGAP, very specifically expressed in the male germ line and showing structural features of anion transporters. This large protein (970 amino acids and a predicted size of 109 kDa), we provisionall y designated Tat1 (for testis anion transporter 1), is closely related to a sulfate permease family comprising three proteins in human (DRA Pendrin, a nd DTD); it is predicted to be an integral membrane protein with 14 transme mbrane helices and intracytoplasmic NH2 and COOH termini. In situ hybridiza tion studies demonstrate that Tat1 and MgcRacGAP genes are coexpressed in m ale germ cells at the spermatocyte stage. On testis sections, Tat1 protein can be immunodetected in spermatocytes and spermatids associated with plasm a membrane. Two-hybrid and in vitro binding assays demonstrate that MgcRacG AP stably interacts through its NH2-terminal domain with the Tat1 COOH-term inal region. Expression of Tat1 protein in COS7 cells generates a 4,4'-diis othiocyano-2,2'-disulfonic acid stilbene and chloride-sensitive sulfate tra nsport. Therefore we conclude that Tat1 is a novel sulfate transporter spec ifically expressed in spermatocytes and spermatids and interacts with MgcRa cGAP in these cells. These observations raise the possibility of a new regu latory pathway linking sulfate transport to Rho signaling in male germ cell s.