Identification of a Region at the N-Terminus of Phospholipase C-β3 That Interacts with G Protein βγ Subunits^†

Members of the phospholipase C-eta (PLC-beta) family of proteins are activated either by G alpha or G beta gamma subunits of heterotrimeric G proteins. To define specific regions of PLC-beta 3 that are involved in binding and activation by G beta gamma, a series of fragments of PLC-beta 3 as glutathione-S-transferase (GST) fusion proteins were produced. A fragment encompassing the N-terminal pleckstrin homology (PH) domain and downstream sequence (GST-N) bound to G protein beta(1)gamma(2) in an in vitro binding assay, and binding was inhibited by G protein alpha subunit, G alpha(il). This PLC-beta 3 fragment also inhibited G beta gamma-stimulated PLC-beta activity in a reconstitution system, while having no significant effect on G alpha q-stimulated PLC-beta 3 activity. The N-terminal G beta gamma binding region was delineated further to the first 180 amino-aids, and the sequence Asn(180)-Ser(180), just distal to the PH domain, was found to be required for the interaction. Mutation of basic residues (154)Arg, (155)Lys, (159)Lys, and (161)Lys to Glu within this region reduced G beta gamma binding affinity and specifically reduced the EC50 for G beta gamma-dependent activation of the mutant enzyme 3-fold. Basal activity and G alpha q-dependent activation of the enzyme were unaffected by the mutations. While these basic residues may not directly mediate the interaction with G beta gamma, the data provide evidence for an N-terminal G beta gamma binding region of PLC-beta 3 that is involved in activation of the enzyme.