Activation of phospholipase C-β2 mutants by G protein α_q and βγ subunits

The β- but not the γ- and δ-type isozymes of inositol phospholipid-specific phospholipase C (PLC) are activated by G protein α_q and β_γ subunits. The β-type PLC isozymes differ from other isozymes in that they contain a long carboxyl-terminal region downstream of the Y catalytic domain and a region rich in acidic amino acids between the two separated X and Y catalytic domains. To determine the sites on PLC-β2 that participate in the interaction of the enzyme with α_q and βγ subunits, we introduced specific truncations and substitutions in the PLC-β2 cDNA at positions corresponding to the carboxyl-terminal and acidic amino acidrich regions, respectively. After transient expression of these cDNA clones in CV-1 cells, the mutant enzymes were partially purified and their capacity to be activated by α_q and βγ subunits determined. Substitution of glutamine residues for three or all seven of a stretch of consecutive glutamic acids in the acidic domain of PLC-β2 affected neither α_q- nor βγ-dependent activation significantly. Carboxyl-terminal truncation to residue Gly-934 or to residue Ala-867 resulted in enzymes that were activated by βγ but not by α_q. This result suggests that the carboxyl-terminal region of PLC-β2 is required for activation by αq, and that βγ subunits interact with a different region of the enzyme. Thus, α_q and βγ subunits may independently modulate a single PLC-β2 molecule concurrently.