MgcRacGAP, A New Human GTPase-activating Protein for Rac and Cdc42 Similar to Drosophila rotundRacGAP Gene Product, Is Expressed in Male Germ Cells

Abstract

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.

Implantation of the human embryo requires Rac1-dependent endometrial stromal cell migration

“…RacGAP1 is a GTPase-activating protein that has been implicated in controlling cell growth and differentiation (35) and cytokinesis (25) (24,36), and this may depend on its phosphorylation status (37). In contrast, some studies suggest that RacGAP1 is able to mediate some of its functions independently of its GAP activity (38).…”

“…Previous microarray data demonstrated that Rac-GAP1 (also known as MgcRacGAP) is down-regulated in hESCs after exposure to trophoblast-conditioned medium (23). We analyzed RacGAP1 expression in hESCs to determine whether the embryo-induced increase in hESC Rac1 activity that we observed in vitro resulted from localized inhibition or down-regulation of RacGAP1, which, in other cell types, inactivates Rac1 (24). Western blot analysis showed that RacGAP1 is expressed in hESCs and in human endometrial epithelial cells, JEG3 choriocarcinoma cells, and placental tissue (Fig.…”

Rho GTPase activity zones and transient contractile arrays

“…However, in vitro the Rho GAP activity of MgcRacGAP is relatively limited. (50,51) It has been reported that phosphorylation of MgcRacGAP by Aurora B kinase stimulates its Rho GAP activity, (52) and given that this kinase is activated prior to the onset of cytokinesis and localizes in the same general region as MgcRacGAP, (53) this could contribute to GTPase flux. However, based on structural considerations, the relevance of this phosphorylation event has been questioned.…”

Cytokinesis in Animal Cells

“…Initial evidence indicated that this GAP is significantly more active in vitro toward Rac, another member of the Rho family of GTPases, rather than RhoA (Toure et al 1998;JantschPlunger et al 2000). Subsequently, genetic studies in Drosophila and C. elegans also suggested that RacGAP1 could inactivate Rac GTPases in vivo (D'Avino et al 2004;Canman et al 2008), and this would inhibit the formation of a branched actomyosin web and, therefore, reduce stiffness at the equatorial cortex ( Fig.…”