Phosphatidic acid binding inhibits RGS1 activity to affect specific signaling pathways in Arabidopsis

Modulation of the active versus inactive forms of the G protein is critical for the signaling processes mediated by the heterotrimeric G-protein complex. We have recently established that in Arabidopsis, the regulator of G-protein signaling (RGS1) protein and a lipid-hydrolyzing enzyme, phospholipase D1 (PLD1), both act as GTPase-activity accelerating proteins (GAPs) for the G protein to attenuate its activity. RGS1 and PLD1 interact with each other, and RGS1 inhibits the activity of PLD1 during regulation of a subset of responses. In this study, we present evidence that this regulation is bidirectional. Phosphatidic acid (PA), a second messenger typically derived from the lipid-hydrolyzing activity of PLD1, is a molecular target of RGS1. PA binds and inhibits the GAP activity of RGS1. A conserved lysine residue in RGS1 (Lys(259)) is directly involved in RGS1-PA binding. Introduction of this RGS1 protein variant in the rgs1 mutant background makes plants hypersensitive to a subset of abscisic acid-mediated responses. Our data point to the existence of negative feedback loops between these two regulatory proteins that precisely modulate the level of active G, consequently generating a highly controlled signal-response output.