The Small G Protein Rac1 Activates Phospholipase Cδ1 through Phospholipase Cβ2

Rac1, which is associated with cytoskeletal pathways, can activate phospholipase Cβ2 (PLCβ2) to increase intracellular Ca2+ levels. This increased Ca2+ can in turn activate the very robust PLCδ1 to synergize Ca2+ signals. We have previously found that PLCβ2 will bind to and inhibit PLCδ1 in solution by an unknown mechanism and that PLCβ2·PLCδ1 complexes can be disrupted by Gβγ subunits. However, because the major populations of PLCβ2 and PLCδ1 are cytosolic, their regulation by Gβγ subunits is not clear. Here, we have found that the pleckstrin homology (PH) domains of PLCβ2 and PLCβ3 are the regions that result in PLCδ1 binding and inhibition. In cells, PLCβ2·PLCδ1 form complexes as seen by Förster resonance energy transfer and co-immunoprecipitation, and microinjection of PHβ2 dissociates the complex. Using PHβ2 as a tool to assess the contribution of PLCβ inhibition of PLCδ1 to Ca2+ release, we found that, although PHβ2 only results in a 25% inhibition of PLCδ1 in solution, in cells the presence of PHβ2 appears to eliminates Ca2+ release suggesting a large threshold effect. We found that the small plasma membrane population of PLCβ2·PLCδ1 is disrupted by activation of heterotrimeric G proteins, and that the major cytosolic population of the complexes are disrupted by Rac1 activation. Thus, the activity of PLCδ1 is controlled by the amount of bound PLCβ2 that changes with displacement of the enzyme by heterotrimeric or small G proteins. Through PLCβ2, PLCδ1 activation is linked to surface receptors as well as signals that mediate cytoskeletal pathways.