In a search for new partners of the activated form of Rac
GTPase, we have isolated through a two-hybrid cloning procedure a human
cDNA encoding a new
GTPase-activating protein (GAP) for Rho family
GTPases. A specific
mRNA of 3.2 kilobases was detected in low abundance in many cell types and found highly expressed in testis. A
protein of the predicted size 58 kDa, which we call
MgcRacGAP, was detected in human testis as well as in
germ cell tumor extracts by immunoblotting with
antibodies specific to
recombinant protein. In vitro, the GAP domain of
MgcRacGAP strongly stimulates Rac1 and Cdc42
GTPase activity but is almost inactive on RhoA. N-terminal to its GAP domain,
MgcRacGAP contains a
cysteine-rich zinc finger-like motif characteristic of the Chimaerin family of RhoGAPs. The closest homolog of
MgcRacGAP is RotundRacGAP, a product of the Drosophila rotund locus. In situ hybridization experiments in human testis demonstrate a specific expression of
mgcRacGAP mRNA in spermatocytes similar to that of rotundRacGAP in Drosophila testis. Therefore,
protein sequence similarity and analogous developmental and tissue specificities of gene expression support the hypothesis that RotundRacGAP and
MgcRacGAP have equivalent functions in insect and mammalian germ cells. Since rotundRacGAP deletion leads to
male sterility in the fruit fly, the
mgcRacGAP gene may prove likewise to play a key role in mammalian male fertility.