Study of the structure and the function of heterotrimeric G proteins in higher plants

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Plant heterotrimeric G proteins have been proposed to be involved in the transduction of several extracellular signals such as plant hormones, elicitor and light. In the plant G proteins, the precise intracellular localization of the subunits have not yet been analyzed. In the present work, we analyzed the localazation of the subunits of the rice G protein.1.Western blot analysis showed that the α, β, γ1 and γ2 subunits were localized in rice plasma membrane fractions.2.Membrane-proteins solubilized with cholate were fractionated by gel filtration (Superdex 200 PC3.2/30, SMART System, Pharmacia). Two fractions of the G protein were obtained ; one seemed to be large complexes (400 kDa complexes) containing heterotrimer(s) and the other, βγ dimmer(s) (60 kDa). We propose that, in rice plants, a large amount of βγ dimmer is present in the plasma membrane in a form without the αsubunit.3.Constitutively active a subunit (QL) was localized in plasma membrane fractions. When membrane fractions containing QL were solubilized and fractionated by gel filtration, QL was present in 60 kDa fractions. This result shows that the active form of the α subunit may dissociate from the heterotrimer.