We performed analyses of the molecular mechanisms involved in the regulation of phospholipase C␥2 (PLC␥2). We identified several regions in the PLC␥-specific array, ␥SA, that contribute to autoinhibition in the basal state by occlusion of the catalytic domain. While the activation of PLC␥2 by Rac2 requires stable translocation to the membrane, the removal of the domains required for membrane translocation in the context of an enzyme with impaired autoinhibition generated constitutive, highly active PLC in cells. We further tested the possibility that the interaction of PLC␥2 with its activator protein Rac2 was sufficient for activation through the release of autoinhibition. However, we found that Rac2 binding in the absence of lipid surfaces was not able to activate PLC␥2. Together with other observations, these data suggest that an important consequence of Rac2 binding and translocation to the membrane is that membrane proximity, on its own or together with Rac2, has a role in the release of autoinhibition, resulting in interfacial activation.Phosphoinositide-specific phospholipase C (PLC)-catalyzed formation of the second messengers inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG) from its substrate phosphatidylinositol 4,5-bisphosphate (PIP 2 ) constitutes one of the major cell signaling responses (3, 36). There are six families of PLC enzymes (PLC␤, ␥, ␦, ε, , and ) consisting of 13 isoforms in humans. Enzymes from each PLC family are uniquely integrated into complex signaling networks through diverse regulatory mechanisms and contribute to the regulation of a variety of biological functions. Despite this diversity, some common principles of their regulation at the molecular level have been proposed for several PLC families (16). However, such mechanistic concepts need to be tested further for each of the families and, in particular, for PLC␥ enzymes, which represent a branch separate from all other PLC families (22).One of the two members of the PLC␥ family, PLC␥2, is most highly expressed in cells of the hematopoietic system and plays a key role in the regulation of the immune response. The regulatory interactions that control PLC␥2 in B cells have been well characterized and involve phosphorylation on critical tyrosine residues by Src and Tec family kinases; similar types of regulatory interactions have been described for the regulation of PLC␥1 in T-cell responses (2). Both PLC␥ enzymes can be activated in response to growth factor stimulation (26, 37). A number of studies of the ubiquitously expressed PLC␥1 in different cell types support the critical importance of tyrosine phosphorylation for PLC␥ activation. In addition to the regulatory mechanisms that are shared by the two PLC␥ enzymes, the Rac GTPases Rac1, Rac2, and Rac3 have been specifically implicated in PLC␥2 regulation that is not dependent on tyrosine phosphorylation of this enzyme (29). However, like stimulation via tyrosine kinase-linked receptors, Rac2 also leads to membrane translocation (29). Several studies suggest signali...

In this issue of Structure, Bunney and colleagues use a combination of NMR, SAXS, crystallography, ITC, and biochemical methods to elucidate, in molecular detail, the sequence of events causing receptor-mediated activation of phospholipase C-γ(1) by protein tyrosine phosphorylation.