 Because there are only seven GRKs (only 4 of which are widely expressed throughout the body) but over 800 human GPCRs, GRKs appear to have limited phosphorylation site selectivity and are regulated primarily by the GPCR active state.G protein-coupled receptor kinases phosphorylate activated G protein-coupled receptors, which promotes the binding of an arrestin protein to the receptor.Phosphorylated serine and threonine residues in GPCRs act as binding sites for and activators of arrestin proteins.