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Carboxyl-terminal Fragments of Phospholipase C-β1 with Intrinsic Gq GTPase-activating Protein (GAP) Activity*

Fragments of the ∼50 kDa COOH-terminal region of phospholipase C-β1 (PLC-β11), ranging in size from 14 to 38 kDa, were expressed in Escherichia coli, purified, and tested for their regulatory activities. As expected, none of the fragments had phospholipase activity. Several fragments, referred to as PLC tails, displayed GTPase-activating protein (GAP) activity for Gq, the G protein class that stimulates the PLC-βs in response to receptors. Gq GAP activity is characteristic of intact PLC-βs. In reconstituted phospholipid vesicles that contained purified Gq and m1 muscarinic cholinergic receptors, the most active tails increased agonist-stimulated, steady-state GTPase activity over 4-fold. Stimulation of steady-state GTPase by the tails depended on receptors for facilitation of GDP-GTP exchange, suggesting that the tails act by accelerating hydrolysis of bound GTP. In addition to intrinsic GAP activity, one tail with high GAP activity and others with low or minimal activity potentiated the GAP activity of intact PLC-β1. Other tails inhibited PLC-β1s GAP effect. Both intrinsic GAP activity and potentiation of the PLC-β1 GAP effect were often biphasic, with maxima as low as 100 nM tail and declining activities at higher concentrations. Several tails inhibited either the phospholipase activity of PLC-β1, its stimulation by Gq, or both. The tails thus define the region of PLC-β1 that has Gq GAP activity and suggest a mechanism of action in which the COOH terminus of PLC-βs can interact with Gq and with other PLC-β1 molecules.