| Segregation of G protein-stimulated cell signaling responses into those mediated by heterotrimeric G proteins versus those promoted by small GTPases of the Ras superfamily is no longer vogue. Significant cross-talk occurs between G protein signaling pathways, and this conversation is dramatically spoken in a recently identified isozyme of the phospholipase C family, phospholipase C-epsilon (PLC-&egr;). Our studies illustrate that PLC-&egr; shares with PLC-β isozymes a capacity to be activated by Gβγ-subunits of heterotrimeric G proteins. However, whereas PLC-β isozymes are activated by Gα q/Gα11, PLC-&egr; is activated by Gα12 and Gα13. The small GTPase H-Ras also activates PLC-&egr; by binding to a Ras Associating (RA) domain in the carboxy-terminus. The presence of a conserved guanine nucleotide exchange domain in the amino terminus of PLC-&egr; also confers a novel capacity of heterotrimeric G protein-coupled receptors to cast signals into additional small GTPase signaling pathways.; We observed that RhoA, RhoB, and RhoC all markedly stimulate inositol phosphate accumulation in PLC-&egr;-expressing cells whereas no effect was observed with the other Rho family members Rac1, Rac2, Rac3, and Cdc42. PLC-&egr; mutants lacking either the CDC25 and PH domains or both RA domains retained capacity to be activated by Rho, Gα12/13, and Gβγ. Analysis of the sequence of PLC-&egr; in its catalytic core revealed a unique 60–70 amino acid insert in PLC-&egr; that is not present in PLC-β, γ, or δ. A PLC-&egr; construct lacking this region was not activated by Rho or Gα12/13 but retained regulation by Gβγ and H-Ras. Our results are consistent with the conclusion that Rho GTPases regulate PLC-&egr; by a mechanism independent of the CDC25 or RA domains. Rather, a unique sequence in the catalytic core of PLC-&egr; is involved in the action of Rho, and Rho may signal downstream of Gα12/13 in the regulation of PLC-&egr; since activation by both Rho and Gα 12/13 is lost in the absence of this sequence. |
