Involvement of heterotrimeric G protein in signal transduction of extracellular calmodulin in regulatingrbcS expression

Abstract

The role of heterotrimeric G protein in signal transduction pathway of extracellular calmodulin in regulatingrbcS expression was examined in suspension-cultured cells of transgenic tobacco. Pharmalogical experiments indicated that G protein agonist cholera toxin enhancedrbcS expression and heterotrimeric G protein antagonist pertussis toxin inhibitedrbcS expression in transgenic tobacco cells. Pertussis toxin also inhibited the enhancement effect caused by exogenous purified calmodulin onrbcS expression, whereas cholera toxin completely reversed the inhibitory effects caused by anti-calmodulin serum onrbcS expression. The right side-out vesicles from tobacco cell membrane were purified, which contained all of substrates for fluometric assay of GTPase activity. Exogenous purified calmodulin, when adding directly to the medium of plasma membrane vesicles, significantly activated GTPase activity in the right side-out plasma membrane vesicles, and this increase in GTPase activity was completely inhibited both by heterotrimeric G proteins antagonist pertussis toxin and nonhydrolyzable GTP analogs GMP-PCP. These results provided the evidence that heterotrimeric G proteins may be involved in signal transduction pathways of extracellular calmodulin to regulaterbcS gene expression.