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

Summary Modulation of the active versus inactive forms of the Gα 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 ( RGS 1) protein and a lipid‐hydrolyzing enzyme, phospholipase Dα1 ( PLD α1), both act as GTP ase‐activity accelerating proteins ( GAP s) for the Gα protein to attenuate its activity. RGS 1 and PLD α1 interact with each other, and RGS 1 inhibits the activity of PLD α1 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 PLD α1, is a molecular target of RGS 1. PA binds and inhibits the GAP activity of RGS 1. A conserved lysine residue in RGS 1 (Lys 259 ) is directly involved in RGS 1– PA binding. Introduction of this RGS 1 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 Gα, consequently generating a highly controlled signal–response output.