The Small G Protein Rac1 Activates Phospholipase Cδ1 through Phospholipase Cβ2

Rac1, which is associated with cytoskeletal pathways, can activate phospholipase C beta 2 (PLC beta 2) to increase intracellular Ca2+ levels. This increased Ca2+ can in turn activate the very robust PLC delta 1 to synergize Ca2+ signals. Wehave previously found that PLC beta 2 will bind to and inhibit PLC delta 1 in solution by an unknown mechanism and that PLC beta 2 center dot PLC delta 1 complexes can be disrupted by G beta gamma subunits. However, because the major populations of PLC beta 2 and PLC delta 1 are cytosolic, their regulation by G beta gamma subunits is not clear. Here, we have found that the pleckstrin homology (PH) domains of PLC beta 2 and PLC beta 3 are the regions that result in PLC delta 1 binding and inhibition. In cells, PLC beta 2 center dot PLC delta 1 form complexes as seen by Forster resonance energy transfer and co-immunoprecipitation, and microinjection of PH beta 2 dissociates the complex. Using PH beta 2 as a tool to assess the contribution of PLC delta inhibition of PLC delta 1 to Ca2+ release, we found that, although PH beta 2 only results in a 25% inhibition of PLC delta 1 in solution, in cells the presence of PH beta 2 appears to eliminates Ca2+ release suggesting a large threshold effect. Wefound that the small plasma membrane population of PLC beta 2 center dot PLC delta 1 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 PLC delta 1 is controlled by the amount of bound PLC beta 2 that changes with displacement of the enzyme by heterotrimeric or smallGproteins. Through PLC beta 2, PLC delta 1 activation is linked to surface receptors as well as signals that mediate cytoskeletal pathways.