G Protein Signaling in Plants: Characterization of Alpha and Gamma Subunits

Abstract

In plants heterotrimeric complexes of G proteins (consisting of alpha, beta, and gamma subunits) regulate several signaling pathways including seed germination, seedling development, organ shape and size determination. The alpha subunit has GTP binding and hydrolysis activity and the beta- gamma subunits interact with downstream effectors as a heterodimer. Some structural homology among the plant and mammalian subunits have led to early assumptions about similarities in the activation and transduction mechanisms in the two systems. However, recent evidence on the lack of membrane receptors in plants, the constitutively active state of the plant alpha subunit and the existence of a large family of gamma subunits indicate that the mechanisms involving the plant proteins may be significantly different from those in their mammalian counterparts. In our group the alpha subunit from A. thaliana (AtGPA1), an N-terminal mutant (GPA1t), the gamma subunits (AGG1, AGG2), and the rice gamma subunits (RGG1 and RGG2) were expressed in yeast and bacteria. Absorbance spectroscopy, circular dichroism spectropolarimetry, and dynamic light scattering (DLS) analyses show the structural and stability differences between AtGPA1 and GPA1t as well as among all gamma subunits. DLS, native-PAGE and small angle X-ray scattering measurements reveal the stable oligomeric forms of the proteins in solution indicating possible functional roles for the oligomers. Results also demonstrate the high level of the flexibility in the structures of all subunits. Models for possible roles of different subunits in G protein signaling in plants will be presented. Supported by Turkish Atomic Energy Commission and Instruct, a Landmark ESFRI project.