Differential regulation of phospholipase C-β₂ activity and membrane interaction by Gα_q, Gβ₁γ ₂, and Rac2

We combined fluorescence recovery after photobleaching (FRAP) beam-size analysis with biochemical assays to investigate the mechanisms of membrane recruitment and activation of phospholipase C-β₂ (PLCβ₂) by G protein α_q and βγ dimers. We show that activation by α_q and βγ differ from activation by Rac2 and from each other. Stimulation by α_q enhanced the plasma membrane association of PLCβ₂, but not of PLCβ₂Δ, which lacks the α_q-interacting region. Although α_q resembled Rac2 in increasing the contribution of exchange to the FRAP of PLCβ₂ and in enhancing its membrane association, the latter effect was weaker than with Rac2. Moreover, the membrane recruitment of PLCβ₂ by α_q occurred by enhancing PLCβ₂ association with fast-diffusing (lipid-like) membrane components, whereas stimulation by Rac2 led to interactions with slow diffusing membrane sites. On the other hand, activation by βγ shifted the FRAP of PLCβ₂ and PLCβ₂Δ to pure lateral diffusion 3- to 5-fold faster than lipids, suggesting surfing-like diffusion along the membrane. We propose that these different modes of PLCβ₂ membrane recruitment may accommodate contrasting functional needs to hydrolyze phosphatidylinositol 4,5-bisphosphate (PtdInsP₂) in localized versus dispersed populations. PLCβ₂ activation by Rac2, which leads to slow lateral diffusion and much faster exchange, recruits PLCβ₂ to act locally on PtdInsP₂ at specific domains. Activation by α_q leads to lipid-like diffusion of PLCβ₂ accompanied by exchange, enabling the sampling of larger, yet limited, areas prior to dissociation. Finally, activation by βγ recruits PLCβ₂ to the membrane by transient interactions, leading to fast "surfing" diffusion along the membrane, sampling large regions for dispersed PtdInsP₂ populations.