GTPASES A New Turn-on for the Rho Family

Meller et al. identified a new protein that may be a member of a superfamily of proteins that activate the Rho guanosine triphosphatases (GTPases) through a novel domain. The Rho family of low-molecular-weight GTPases, which includes Rho, Rac, and Cdc42, plays a critical role in regulating organization of the actin cytoskeleton and has been implicated in gene transcription, membrane trafficking, and malignant transformation. Guanine nucleotide exchange factors (GEFs) activate these GTPases by catalyzing nucleotide exchange. Meller et al. exploited the preferential binding of GEFs to nucleotide-depleted Rho-family proteins to purify a 220-kD protein that bound to nucleotide-depleted Cdc42-coupled beads and that could be eluted with guanosine 5'-O-(thiotriphosphate) (GTP--S). The protein, which they named Zizimin1, activated Cdc42, as determined by Western analysis of Cdc2 binding to an effector protein domain, and induced filopodia when overexpressed in NIH 3T3 cells. Filopodia induced by Zizimin1 were blocked by inhibition of Cdc42 and by a Cdc42 dominant-negative mutant. Interestingly, amino acid analysis indicated that Zizimin1 lacked a binding domain found in all known eukaryotic GEFs for Rho GTPases. Using a series of Zizimin1 deletion mutants expressed in COS-7 cells, the authors identified a novel domain, which they named CZH2, that was required for binding to Cdc42. Database searches indicated that Zizimin1 is a member of a superfamily of proteins containing the CZH2 domain. The Zizimin superfamily contains a family of proteins previously known to activate Rac through an unknown mechanism suggesting that CZH2 may represent a new GEF domain for the Rho GTPases.

N. Meller, M. Irani-Tehrani, W. B. Klosses, M. A. Del Pozo, M. A. Schwartz, Zizimin1, a novel Cdc42 activator, reveals a new GEF domain for Rho proteins. Nat. Cell Biol. 4, 639-647 (2002). [Online Journal]