Binding of Regulator of G Protein Signaling (RGS) Proteins to

Regulator of G protein signaling (RGS) proteins must bind membranes in an orientation that permits the pro-tein-protein interactions necessary for regulatory activ-ity. RGS4 binds to phospholipid surfaces in a slow, mul-tistep process that leads to maximal GTPase-activating protein (GAP) activity. When RGS4 is added to phospho-lipid vesicles that contain m2 or m1 muscarinic receptor and Gi, Gz, or Gq, GAP activity increases;3-fold over 4 h at 30 °C and more slowly at 20 °C. This increase in GAP activity is preceded by several other events that suggest that, after binding, optimal interaction with G protein and receptor requires reorientation of RGS4 on the membrane surface, a conformational change, or both. Binding of RGS4 is initially reversible but becomes irre-versible within 5 min. Onset of irreversibility parallel