Proteins regulating receptor desensitization

Agonist occupancy of GPCRs initiates activation of downstream signalling cascades that can often be followed by rapid attenuation of receptor responsiveness. This process of desensitization can result from a complex series of cellular protein interactions that mediate both acute and chronic receptor down-regulation. Desensitization can be either homologous where only the response of the activated receptor is desensitized or heterologous where activation of one receptor causes desensitization of others which stimulate/inhibit similar effector cascades. While the former involves decreased receptor G protein coupling the latter can also involve changes in the activity of effector molecules downstream of the G protein. Desensitization can occur rapidly within seconds to minutes where phosphorylation of the receptor and/or internalization of the receptor to an intracellular compartment occurs. Alternatively, desens-itization can be long-term (down-regulation) involving changes in expression and stability of mRNA for GPCR cascades and changes in receptor/G protein levels. Indeed such long-term down-regulation can account for tolerance as a consequence of chronic drug treatment or progressive pathological state.

Classically, the p2 adrenergic receptor has been used to study both homologous and heterologous desensitization. While the latter occurs through PKA phosphorylation of serine residues in intracellular loop 3 and the C-terminal tail, homologous desensitization occurs through a separate mechanism in which G protein-coupled receptor serine/threonine kinases (GRKs) phosphorylate the receptor. Depending on the receptor studied GRKs generally phosphorylate one or more serine and/or threonine residues in either the third intracellular loop or the C-terminal tail domains of the GPCR (Bunemann and Hosey 1999). No obvious consensus sequences are known to dictate which specific residues are preferred by GRK as phosphorylation sites in vivo. However, GRKs bind directly to GPCRs and following GRK phosphorylation of the GPCR, hydrophilic soluble protein arrestins are recruited from the cytosol to bind the phosphorylated sites within the third intracellular loop or the C-terminal tail. These arrestins form a stable complex with the receptor that prevents further G protein binding and promotes targeting of the GPCR/arrestin complex to specialized microdomains of the plasma membrane including clathrin coated pits and lipid rafts/caveolae thereby aiding internalization (endocytosis). Thus, GRKs work coordinately with arrestins to regulate the strength and duration of GPCR signalling events (Ferguson 2001). This regulation is complex in that many isoforms of GRK and arrestins exist. Thus, seven distinct isoforms of GRK (GRK1-7) have been identified ranging in size from 62-80 kDa. Whereas GRK1, 4, and 7 are limited in their tissue expression patterns to retinal rods and cones (GRK1 and 7, respectively) and testis (GRK4), GRK2, 3, 5, and 6 are found in many tissues (Pitcher et al. 1998a; Bunemann and Hosey 1999). GRKs share a conserved structure over the first 450 amino acids and within this region all family members contain an RGS-like domain and a catalytic domain. Outside of these domains, GRKs display highly divergent carboxy (C) termini that confer the capacity for different mechanisms of regulated membrane association (Penn et al. 2000). These include consensus sites for lipid acylation including farnesylation (GRK1), gerenyl gerenylation (GRK7), and palmitoylation (GRK4 and 6); a pleckstrin homology (PH) domain that promotes G^y binding (GRK2 and 3); phospholipid phosphatidyl inositol bisphosphate (PIP2) binding (GRK2, 3, and 5); and a polycationic binding domain (GRK1, 5, and 6). Cellular mechanisms that regulate GRK recruitment from the cytosol to the plasma membrane to interact directly with GPCRs is described in detail elsewhere (Penn et al. 2000; Ferguson 2001). With respect to arrestins, four mammalian isoforms exist including arrestin-1, arrestin-2, P-arrestin-1, and P-arrestin-2 (Miller and Lefkowitz 2001). The visual arrestins are limited in their expression to rods and cones and block signalling by rhodopsin. In contrast, the ^-arrestins are expressed in many cells and tissues and modulate the signalling and trafficking of numerous GPCRs (Miller et al. 2000). Given the many isoforms of arrestins and GRKs it is possible to encode specific interactions with different receptors. Thus, visual arrestin exhibits greatest affinity for phosphorylated rhodopsin with little affinity for ^-adrenergic or muscarinic receptors. Furthermore, the level and rate of desensitization of a given receptor type can vary from cell type to cell type possibly because of differential expression of GRK and arrestin isoforms.

While the role of GRKs and arrestins in GPCR desensitization have been extensively studied and reviewed in greater detail elsewhere (Penn et al. 2000; Ferguson 2001; Miller and Lefkowitz 2001) it should also be pointed out that these proteins are multifunctional engaging additional binding partners in many more unsuspected GPCR signalling functions far beyond just receptor desensitization. These newly appreciated roles for GRKs and arrestins as adaptors or scaffolds that link GPCRs to signalling proteins and pathways are described in more detail below.

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