 The non-visual GRKs are inhibited instead by the calcium-binding protein calmodulin.GRK2 and GRK3 share a carboxyl terminal pleckstrin homology (PH) domain that binds to G protein beta/gamma subunits, and GPCR activation of heterotrimeric G proteins releases this free beta/gamma complex that binds to GRK2/3 to recruit these kinases to the cell membrane precisely at the location of the activated receptor, augmenting GRK activity to regulate the activated receptor.GRK2 activity can be modulated by its phosphorylation by protein kinase A or protein kinase C, and by post-translational modification of cysteines by S-nitrosylation.