MGCRACGAP, A NEW HUMAN GTPASE-ACTIVATING PROTEIN FOR RAC AND CDC42 SIMILAR TO DROSOPHILA ROTUNDRACGAP GENE-PRODUCT, IS EXPRESSED IN MALE GERM-CELLS

In a search for new partners of the activated form of Pac GTPase, we h ave isolated through a two-hybrid cloning procedure a human cDNA encod ing a new GTPase-activating protein (GAP) for Rho family GTPases, A sp ecific mRNA of 3.2 kilobases was detected in low abundance in many cel l types and found highly expressed in testis. A protein of the predict ed size 58 kDa, which we call MgcRacGAP, was detected in human testis as well. as in germ cell. tumor extracts by immunoblotting with antibo dies specific to recombinant protein, In vitro, the GAP domain of MgcR acGAP strongly stimulates Rad and Cdc42 GTPase activity but is almost inactive on RhoA N-terminal to its GAP domain, MgcRacGAP contains a cy steine-rich zinc finger-like motif characteristic of the Chimaerin fam ily of RhoGAPs, The closest homolog of MgcRacGAP is RotundRacGAP, a pr oduct of the Drosophila refund locus, In situ hybridization experiment s in human testis demonstrate a specific expression of mgcRacGAP mRNA in spermatocytes similar to that of retundRacGAP in Drosophila testis, Therefore, protein sequence similarity and analogous developmental an d tissue specificities of gene expression support the hypothesis that RotundRacGAP and MgcRacGAP have equivalent functions in insect and mam malian germ cells, Since refundRacGAP deletion leads to male sterility in the fruit fly, the mgcRacGAP gene may prove likewise to play a key role in mammalian male fertility.