Chemotaxis to cyclic AMP and folic acid is mediated by different G proteins in Dictyostelium discoideum

Abstract

Mutant Frigid A (fgdA) of Dictyostelium discoideum is defective in a functional Ga2 subunit of a G protein and is characterized by a complete blockade of the cyclic AMP-mediated sensory transduction steps, including cyclic AMP relay, chemotaxis and the cyclic GMP response. Folic acid-mediated transmembrane signal transduction was investigated in this mutant; the results show that: (1) cell surface folic acid receptors are present in fgdA mutants. (2) Folic acid induces intracellular responses, including activation of guanylate cyclase and chemotaxis. (3) The inhibitory effect of GTP on folic acid binding to membranes is present. (4) GTPγS binding and highaffinity GTPase are stimulated by folic acid. These data strongly suggest that folic acid receptors are coupled to guanylate cyclase and chemotaxis via a Ga protein that is different from Ga2. The results imply that surface receptors for cyclic AMP and folic acid are coupled to different G proteins.