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 βγ 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 acid-rich 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 α(q), 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.