RhoGTPases (Rho, Rac, and Cdc42) are known to regulate multiple functions, including cell motility, adhesion, and proliferation; however, the signaling pathways underlying these pleiotropic effects are far from fully understood. 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 cloning, 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 provisionally designated Tat1 (for testis anion transporter 1), is closely related to a sulfate permease family comprising three proteins in human (DRA, Pendrin, and DTD); it is predicted to be an integral membrane protein with 14 transmembrane helices and intracytoplasmic NH(2) and COOH termini. In situ hybridization studies demonstrate that Tat1 and MgcRacGAP genes are coexpressed in male germ cells at the spermatocyte stage. On testis sections, Tat1 protein can be immunodetected in spermatocytes and spermatids associated with plasma membrane. Two-hybrid and in vitro binding assays demonstrate that MgcRacGAP stably interacts through its NH(2)-terminal domain with the Tat1 COOH-terminal region. Expression of Tat1 protein in COS7 cells generates a 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene and chloride-sensitive sulfate transport. Therefore we conclude that Tat1 is a novel sulfate transporter specifically expressed in spermatocytes and spermatids and interacts with MgcRacGAP in these cells. These observations raise the possibility of a new regulatory pathway linking sulfate transport to Rho signaling in male germ cells.