Selection of Gβ subunits with point mutations that fail to activate specific signaling pathways in vivo:: Dissecting cellular responses mediated by a heterotrimeric G protein in Dictyostelium discoideum

In Dictyostelium discoideum, a unique G beta subunit is required for a G protein-coupled receptor system that mediates a variety of cellular responses. Binding of cAMP to cAR1, the receptor linked to the G protein G2, triggers a cascade of responses, including activation of adenylyl cyclase, gene induction, actin polymerization, and chemotaxis. Null mutations of the cAR1, G alpha 2, and G beta genes completely impair all these responses. To dissect specificity in G beta gamma signaling to downstream effectors in living cells, we screened a randomly mutagenized library of G beta genes and isolated G beta alleles that lacked the capacity to activate some effecters but retained the ability to regulate others. These mutant G beta subunits were able to link cAR1 to G2, to support gene expression, and to mediate cAMP-induced actin polymerization, and some were able to mediate to chemotaxis toward cAMP. None was able to activate adenylyl cyclase, and some did not support chemotaxis. Thus, we separated in vivo functions of G beta gamma by making point mutations on G beta. Using the structure of the heterotrimeric G protein displayed in the computer program CHAIN, we examined the positions and the molecular interactions of the amino acids substituted in each of the mutant G beta s and analyzed the possible effects of each replacement. We identified several residues that are crucial for activation of the adenylyl cyclase. These residues formed an area that overlaps but is not identical to regions where bovine Gt beta gamma interacts with its regulators, G alpha and phosducin.