For cells to respond effectively to changes in their environment, they must not only activate a signaling pathway, but also down-modulate or terminate the response once it is no longer needed; otherwise, signal transduction pathways would remain “on” too long, or at too high a level, and the cell would become overstimulated.

Earlier we saw that multiple mechanisms can rapidly terminate the rhodopsin signal transduction pathway, including GAP proteins that stimulate the hydrolysis of GTP bound to G_αt, Ca²⁺-sensing proteins that activate guanylate cyclase, and phosphorylation of active rhodopsin by rhodopsin kinase followed by binding of arrestin (see Figure 15-21). In fact, most G protein–coupled receptors are modulated by multiple mechanisms that down-regulate their activity, as is exemplified by β-adrenergic receptors and others coupled to G_αs that activate adenylyl cyclase.

Most GPCRs are also down-regulated by feedback repression, in which an end product of a signaling pathway blocks an early step in that pathway. For instance, when a G_αs protein–coupled receptor is exposed to hormonal stimulation for several hours, several serine and threonine residues in the cytosolic domain of the receptor become phosphorylated by PKA. The phosphorylated receptor can bind its ligand, but cannot efficiently activate G_αs; thus ligand bound to the phosphorylated receptor is less efficient in activating adenylyl cyclase then is ligand bound to the nonphosphorylated receptor. Because the activity of PKA is enhanced by the high cAMP level induced by any hormone that activates G_αs, prolonged exposure to one such hormone—say, epinephrine—desensitizes not only β-adrenergic receptors, but also other G_αs protein–coupled receptors that are phosphorylated by PKA, even though they bind different ligands (e.g., glucagon receptors in the liver). This cross-regulation is called heterologous desensitization.

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Several residues in the cytosolic domain of the β-adrenergic receptor, different from those phosphorylated by PKA, are phosphorylated by the enzyme β-adrenergic receptor kinase (BARK), but only when epinephrine or an agonist is bound to the receptor and thus the receptor is in its active conformation. BARK is a member of the same kinase family as rhodopsin kinase, and its action is similar to the phosphorylation and down-modulation of activated rhodopsin by rhodopsin kinase (see Figure 15-21). This process is termed homologous desensitization because only those receptors that are in their active conformations are subject to deactivation by phosphorylation.

We noted that binding of arrestin to extensively phosphorylated opsin completely inhibits activation of coupled G proteins by activated opsin (see Figure 15-21). A related protein, termed β-arrestin, plays a similar role in silencing other G protein–coupled receptors, including β-adrenergic receptors (Figure 15-32).

An additional function of β-arrestin in regulating cell-surface receptors was initially suggested by the observation that disappearance of β-adrenergic receptors from the cell surface in response to ligand binding is stimulated by overexpression of BARK and β-arrestin. Subsequent studies revealed that β-arrestin binds not only to phosphorylated GPCRs, but also to clathrin and an associated protein termed AP2, two key components of the coated vesicles that are involved in endocytosis from the plasma membrane (see Figure 15-32; see also Chapter 14). These interactions promote the formation of coated pits and endocytosis of the associated receptors, thereby decreasing the number of receptors exposed on the cell surface. Eventually some of the internalized receptors are degraded intracellularly, and some are de-phosphorylated in endosomes. Following dissociation of β-arrestin, the resensitized (de-phosphorylated) receptors are recycled to the cell surface in a manner similar to the recycling of the LDL receptor (see Chapter 14).

In addition to its role in regulating receptor activity, β-arrestin functions as an adapter protein in transducing signals from G protein–coupled receptors to the nucleus (see Chapter 16). The GPCR-arrestin complex acts as a scaffold for the binding and activation of several cytosolic kinases (see Figure 15-32), which we discuss in detail in subsequent chapters. These kinases include Src, a cytosolic protein tyrosine kinase that activates the MAP kinase pathway and other pathways leading to the transcription of genes needed for cell division (see Chapters 16 and 19). A complex of three arrestin-bound proteins, including a Jun N-terminal kinase (JNK-3), initiates a kinase cascade that ultimately activates the Jun transcription factor, which promotes expression of certain growth-promoting enzymes and other proteins that help cells respond to stresses. Thus the BARK–β-arrestin pathway, originally just thought to suppress signaling by GPCRs, actually functions as a switch, turning off signaling by G proteins and turning on other signaling pathways. The multiple functions of β-arrestin illustrate the importance of adapter proteins in both regulating signaling and transducing signals from cell-surface receptors.