Research Scholar University of North Carolina at Chapel Hill Carrboro, North Carolina
Body of Abstract: Heterotrimeric G proteins are a class of signaling proteins that play crucial roles in cellular processes such as hormone signaling, responses to light, development, and defense in plants. Unlike mammals, the activation/inactivation mechanism in plants is based partially on a spontaneous GTP/GDP exchange cycle of the Gα subunit, independent of G protein-coupled receptors, suggesting a distinct regulatory mechanism. In Arabidopsis thaliana, the regulation/activation of G proteins is closely associated with the Regulator of G protein Signaling 1 protein (AtRGS1). AtRGS1 consists of a 7-transmembrane domain at the N-terminal half, a cytoplasmic linker region, and a conserved RGS domain, which interacts with Gα subunits to stimulate their GTPase activity. In addition, located C-terminal to the RGS domain is a tail that contains phosphorylated serines. Phosphorylation modulates the strength, duration, and specificity of G protein signaling. Specifically, phosphorylation of AtRGS1 by certain receptor kinases and by With No Lysine kinases influence AtRGS1 stability, localization, and activity, providing a regulatory mechanism. This phospho-barcode controls downstream signaling, including sugar and pathogen response pathways. Our work provides insights into this mechanism by focusing on the activity and behavior of AtRGS1 phosphomimetics and phospho-null mutants and show how they modulate the subsequent downstream signaling and G protein complex composition. We generated different phospho-mutants for the linker region and tail and demonstrated that different phosphorylation states alter the interaction with the Gα subunit. Predicted structural differences among the wildtype and phospho-mimetic/phospho-null mutants of AtRGS1 revealed that phosphorylation of the linker region regulates the RGS interface by interacting with positively charged residues in the tail. We also demonstrated an essential role of linker phosphorylation in regulating the stability and internalization of AtRGS1.