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Selection of G 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

Tian Jin,^* Mario Amzel, Peter N. Devreotes,^* and Lijun Wu^§

Departments of  ^*Biological Chemistry and  Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205; and  ^§LeukoSite, Inc., Cambridge, Massachusetts 02142

In Dictyostelium discoideum, a unique G 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, G2, and G genes completely impair all these responses. To dissect specificity in G signaling to downstream effectors in living cells, we screened a randomly mutagenized library of G genes and isolated G alleles that lacked the capacity to activate some effectors but retained the ability to regulate others. These mutant G 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 by making point mutations on G. 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 Gs 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 interacts with its regulators, G and phosducin.

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   Corresponding author. E-mail address: pnd{at}welchlink.welch.jhu.edu.

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