Mutant ras gene induces elevated levels of inositol tris- and hexakisphosphates in Dictyostelium

Abstract

Previous studies of Europe-Finner & Newell indicated that in amoebae of Dictyostelium discoideum, signal transduction used for chemotaxis to cyclic AMP involved transient formation of inositol tris- and polyphosphates. Evidence was also presented for the involvement of a GTP-binding G-protein. Here we report evidence for the involvement of a ras gene product in the D. discoideum inositol phosphate pathway. Use was made of strains of Dictyostelium transformed with a wild-type D. discoideum ras gene (ras-Gly12) or a mutant form of the gene (ras-Thr12). Experiments using separation of soluble inositol phosphates by Dowex anion-exchange resin chromatography indicated that cells transformed with the wild-type ras-Gly12 gene were unaffected in their basal levels of inositol polyphosphates and in the inositol phosphates formed in response to stimulation with the chemotactic agent cyclic AMP. In contrast, cells transformed with the mutant ras-Thr12 gene showed a basal level of inositol polyphosphate that was several-fold elevated over the controls and stimulation of these cells with cyclic AMP produced only a small further elevation. When the inositol phosphates were analysed by h.p.l.c. it was found that the basal level of inositol 1,4,5-trisphosphate was raised three- to fivefold in the ras-Thr12 strain compared to the strain transformed with ras-Gly12, and that inositol hexakisphosphate (which was found to be present in large amounts relative to other inositol phosphates in D. discoideum cells) was also raised to a similar extent in the ras-Thr12-transformed cells. We propose that the Dictyostelium ras gene product codes for a regulatory protein involved in the inositol phosphate chemotactic signal-transduction pathway.