G protein-coupled receptor kinase 2 (ADRBK1, GRK2, beta-ARK), ADRBK2 (GRK3, beta-ARK2), GRK5 and GRK6 can phosphorylate activated beta-adrenergic receptors (Benovic et al. 1986a, Richardson et al. 1993, Menard et al. 1996, Violin et al. 2006). GRK2, GRK5 and GRK6 can bind and phosphorylate the beta-2 adrenergic receptor (ADRB2) in the carboxy-terminal tail region (Premont et al. 1994, 1995, Violin et al. 2006). Following this, phosphorylated ADRB2 dissociates from the ADRB2:GRK complex. GRK2 and GRK6 are thought to be the predominant GRKs that mediate ADRB2 desensitization (Violin et al. 2006), phosphorylating distinct serine residues (Nobles et al. 2011).
GRK phosphorylation follwed by arrestin binding and internalization is the classical model for GPCR desensitization. Many GPCRs have been demonstrated to require phosphorylation before they can bind arrestin, but other receptors do not appear to require phosphorylation in order to bind arrestin (see refs included in Gurevich & Gurevich 2006). In these receptors, spatially close acidic amino acids are thought to provide sites that can bind the arrestin phosphate sensing region. In GPCRs that require phosphorylation, the region most commonly involved in arrestin binding is the C-terminus, but many GPCRs have phosphorylation sites in the 3rd cytoplasmic loop, while in some cases phosphorylation sites are found in the first (i1) or second (i2) cytoplasmic loops (Gurevich & Gurevich 2006).