Sites Important for PLCβ2 Activation by the G Protein βγ Subunit Map to the Sides of the β Propeller Structure

The βγ subunits of the heterotrimeric GTP-binding proteins (G proteins) that couple heptahelical, plasma membrane-bound receptors to intracellular effector enzymes or ion channels directly regulate several types of effectors, including phospholipase Cβ and adenylyl cyclase. The β subunit is made up of two structurally different regions: an N-terminal α helix followed by a toroidal structure made up of 7 blades, each of which is a twisted β sheet composed of four anti-parallel β strands (Wall, M. A., Coleman, D. E., Lee, E., Iniguez-Lluhi, J. A., Posner, B. A., Gilman, A. G., and Sprang, S. R. (1995)Cell 83, 1047–1058; Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311–319). We have previously shown that sites for activation of PLCβ2, PLCβ3, and adenylyl cyclase II overlap on the “top” surface of the propeller, where Gα also binds (Li, Y., Sternweis, P. M., Charnecki, S., Smith, T. F., Gilman, A. G., Neer, E. J., and Kozasa, T. (1998)J. Biol. Chem. 273, 16265–16272). The present study was undertaken to identify the regions on the side of the torus that might be important for effector interactions. We made mutations in each of the outer β strands of the G protein β1 propeller, as well as mutations in the loops that connect the outer strands to the adjacent β strands. Our results suggest that activation of PLCβ2 involves residues in the outer strands of blades 2, 6, and 7 of the propeller. We tested three of the mutations that most severely affected PLCβ2 activity against two forms of adenylyl cyclase (ACI and ACII). Both inhibition of ACI and activation of ACII were unaffected by these mutations, suggesting that if ACI and ACII contact the outer strands, the sites of contact are different from those for PLCβ2. We propose that distinct sets of contacts along the sides of the propeller will define the specificity of the interaction of βγ with effectors.