Isolation of a novel G-protein gamma-subunit from Arabidopsis thaliana and its interaction with Gbeta

There is increasing evidence that heterotrimeric G-proteins (G-proteins) are involved in many plant processes including phytohormone response, pathogen defence and stomatal control. In animal systems, each of the three G-protein subunits belong to large multigene families; however, few subunits have been isolated from plants. Here we report the cloning of a second plant G-protein gamma-subunit (AGG2) from Arabidopsis thaliana. The predicted AGG2 protein sequence shows 48% identity to the first identified Arabidopsis Ggamma-subunit, AGG1. Furthermore, AGG2 contains all of the conserved characteristics of gamma-subunits including a small size (100 amino acids, 11.1 kDa), C-terminal CAAX box and a N-terminal alpha-helix region capable of forming a coiled-coil interaction with the beta-subunit. A strong interaction between AGG2 and both the tobacco (TGB1) and Arabidopsis (AGB1) beta-subunits was observed in vivo using the yeast two-hybrid system. The strong association between AGG2 and AGB1 was confirmed in vitro. Southern and Northern analyses showed that AGG2 is a single copy gene in Arabidopsis producing two transcripts that are present in all tissues tested. The isolation of a second gamma-subunit from A. thaliana indicates that plant G-proteins, like their mammalian counterparts, may form different heterotrimer combinations that presumably regulate multiple signal transduction pathways.