Involvement of heterotrimeric G protein in signal transduction of extracellular calmodulin in regulating rbcS expression

The role of heterotrimeric G protein in signal transduction pathway of extr acellular calmodulin in regulating rbcS expression was examined in suspensi on-cultured cells of transgenic tobacco. Pharmalogical experiments indicate d that G protein agonist cholera toxin enhanced rbcS expression and heterot rimeric G protein antagonist pertussis toxin inhibited rbcS expression in t ransgenic tobacco cells. Pertussis toxin also inhibited the enhancement eff ect caused by exogenous purified calmodulin on rbcS expression, whereas cho lera toxin completely reversed the inhibitory effects caused by anti calmod ulin serum on rbcS expression. The right side-out vesicles from tobacco cel l membrane were purified, which contained all of substrates for fIuometric assay of GTPase activity. Exogenous purified calmodulin, when adding direct ly to the medium of plasma membrane vesicles, significantly activated GTPas e activity in the right side-out plasma membrane vesicles, and this increas e in GTPase activity was completely inhibited both by heterotrimeric G prot eins antagonist pertussis toxin and nonhydrolyzable GTP analogs GMP-PCP. Th ese results provided the evidence that heterotrimeric G proteins may be inv olved in signal transduction pathways of extracellular calmodulin to regula te rbcS gene expression.