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 Gα-subunits, a single Gβ-subunit, but no Gγ-subunits. We report here the purification, cDNA isolation, and functional analysis of a Gγ-subunit. Like Gβ, the Gγ appears to be unique and hybridization studies show that Gγ and Gβ are expressed in parallel during development. Species-wide sequence comparisons of Gγ-subunits and γ-like domains of RGS proteins reveal short stretches of highly conserved residues as well as the common CXXL motif at the COOH-terminal of Gγs that target Gβγs to plasma membrane. Overexpression of a CSVL-deleted Gγ (GγΔ) in wild-type cells shifts Gβγ to the cytosol and selectively impairs certain G-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 Gβγ is required for sensing shallow gradients, but is not essential for responses to increments in extracellular stimuli.