Gγ in Dictyostelium:: Its role in localization of Gβγ to the membrane is required for chemotaxis in shallow gradients

G-protein-mediated signal transduction pathways play an essential role in the developmental program of the simple eukaryotic organism Dictyostelium discoideum. Database searches have yielded 11 G alpha -subunits, a single G beta -subunit, but no G gamma -subunits. We report here the purification, cDNA isolation, and functional analysis of a G gamma -subunit. Like G beta, the G gamma appears to be unique and hybridization studies show that G gamma and G beta are expressed in parallel during development. Species-wide sequence comparisons of G gamma -subunits and T-like domains of RGS proteins reveal short stretches of highly conserved residues as well as the common CXXL motif at the COOH-terminal of G beta gamma that target G beta gammas to plasma membrane. Overexpression of a CSVL-deleted G gamma (G gamma Delta) in wild-type cells shifts G beta gamma to the cytosol and selectively impairs certain G gamma -protein-mediated signal transduction pathways. These cells are able to respond to increments in the stimulus, but are unable to sense chemoattractant gradients. They neither move directionally nor recruit PH-domains to their leading edge. Thus, a full complement of membrane-tethered G beta gamma is required for sensing shallow gradients, but is not essential for responses to increments in extracellular stimuli.