Stimulation of phospholipase C-β2by the Rho GTPases Cdc42Hs and Rac1

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Abstract
Neutrophils contain a soluble guanine‐nucleotidebinding protein, made up of two components with molecular masses of 23 and 26 kDa, that mediates stimulation of phospholipase C‐β2 (PLCβ2). We have identified the two components of the stimulatory heterodimer by amino acid sequencing as a Rho GTPase and the Rho guanine nucleotide dissociation inhibitor LyGDI. Using recombinant Rho GTPases and LyGDI, we demonstrate that PLCβ2 is stimulated by guanosine 5′‐O‐(3‐thiotriphosphate) (GTP[S])‐activated Cdc42Hs×LyGDI, but not by RhoA×LyGDI. Stimulation of PLCβ2, which was also observed for GTP[S]‐activated recombinant Rac1, was independent of LyGDI, but required C‐terminal processing of Cdc42Hs/Rac1. Cdc42Hs/Rac1 also stimulated PLCβ2 in a system made up of purified recombinant proteins, suggesting that this function is mediated by direct protein–protein interaction. The Cdc42Hs mutants F37A and Y40C failed to stimulate PLCβ2, indicating that the Cdc42Hs effector site is involved in this interaction. The results identify PLCβ2 as a novel effector of the Rho GTPases Cdc42Hs and Rac1, and as the first mammalian effector directly regulated by both heterotrimeric and low‐molecular‐mass GTP‐binding proteins.