The presence of a heterotrimeric G protein and its role in signal transduction of extracellular calmodulin in pollen germination and tube growth

The role of heterotrimeric G proteins in pollen germination, tube growth, a nd signal transduction of extracellular calmodulin (CaM) was examined in li ly pollen. Two kinds of antibodies raised against animal Gz(alpha), one aga inst an internal sequence and the other against its N terminus, cross-react ed with the same 41-kD protein from lily pollen plasma membrane. This 41-kD protein was also specifically ADP ribosylated by pertussis toxin. Microinj ection of the membrane-impermeable G protein agonist GTP-gamma-S into a pol len tube increased its growth rate, whereas microinjection of the membrane- impermeable G protein antagonist GDP-beta-S and the anti-G(alpha) antibody decreased pollen tube growth. The membrane-permeable G protein agonist chol era toxin stimulated pollen germination and tube growth. Anti-CaM antiserum inhibited pollen germination and tube growth, and this inhibitory effect w as completely reversed by cholera toxin. The membrane-permeable heterotrime ric G protein antagonist pertussis toxin completely stopped pollen germinat ion and tube growth. Purified CaM, when added directly to the medium of pla sma membrane vesicles, significantly activated GTPase activity in plasma me mbrane vesicles, and this increase in GTPase activity was completely inhibi ted by pertussis toxin and the nonhydrolyzable GTP analogs GTP-gamma-S and guanylyl-5'-imidodiphosphate. The GTPase activity in plasma membrane vesicl es was also stimulated by cholera toxin. These data suggest that heterotrim eric G proteins may be present in the pollen system where they may be invol ved in the signal transduction of extracellular CaM and in pollen germinati on and tube growth.