SITES IMPORTANT FOR PLC-BETA(2) ACTIVATION BY THE G-PROTEIN BETA-GAMMA SUBUNIT MAP TO THE SIDES OF THE BETA-PROPELLER STRUCTURE

The beta gamma 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 seve ral types of effecters, including phospholipase C beta and adenylyl cy clase. The beta subunit is made up of two structurally different regio ns: an N-terminal alpha helix followed by a toroidal structure made up of 7 blades, each of which is a twisted beta sheet composed of four a nti-parallel beta strands (Wall, M. A., Coleman, D. E., Lee, E., Inigu ez-Lluhi, J. A, Posner, B. A, Oilman, A G., and Sprang, S. R. (1995) C ell 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 PLC beta(2), PLC beta(3 ), and adenylyl cyclase II overlap on the ''top'' surface of the prope ller, where G alpha also binds (Li, Y., Sternweis, P. M., Charnecki, S ., Smith, T. F., Oilman, A. G., Neer, E. J., and Kozasa, T. (1998) J. Biol. Chem. 273, 16265-16272). The present study was undertaken to ide ntify the regions on the side of the torus that might be important for effector interactions. We made mutations in each of the outer beta st rands of the G protein beta(1) propeller, as well as mutations in the loops that connect the outer strands to the adjacent beta strands. Our results suggest that activation of PLC beta(2) involves residues in t he outer strands of blades 2, 6, and 7 of the propeller. We tested thr ee of the mutations that most severely affected PLC beta(2) activity a gainst two forms of adenylyl cyclase (ACI and ACII). Both inhibition o f ACI and activation of ACII were unaffected by these mutations, sugge sting that if ACI and ACII contact the outer strands, the sites of con tact are different from those for PLC beta(2). We propose that distinc t sets of contacts along the sides of the propeller will define the sp ecificity of the interaction of beta gamma with effecters.