Abstract

The alpha subunits of heterotrimeric G proteins and calcium binding proteins play important roles in signal transduction and cellular regulation in plants. However, the molecular function of GÜ proteins in plants is still poorly understood. In order to understand the role of GÜ proteins in signaling pathways, the downstream effectors and upstream regulators need to be investigated. Here we report the first study of interactions between a canonical GÜ subunit, GPA1, and calcium binding proteins in plants. Two of these proteins in Arabidopsis are caleosins, RD20/AtClo3 and AtClo7, which are small Ca 2+ binding proteins with single EF hand domains. Interactions between the two calcium binding proteins and On were studied using bimolecular fluorescence complementation (BiFC) in vivo . RD20/AtClo3 and AtClo7 localized to the ER and tonoplast, however interactions between RD20/Clo3 and Clo7 with GÜ were detected in the ER and PM respectively. A similar approach was used to determine the interaction between TaClo3, a homolog of the caleosins, and TaGA3, a GÜ from wheat. Similar to Arabidopsis caleosins, the TaClo3 protein also localized to the ER and tonoplast. Interactions between TaClo3 and TaGA3 were detected by BiFC in Nicotiana benthamiana , though their localization is not clear. Wheat is hexaploid and is expected to contain more than one GÜ. The detection of the five GÜ genes, some of which contain small insertions in the coding region, suggests a diversity of functions among GÜ wheat homologs. The role of proteins is discussed as a basis for future studies.