Arrestin

Arrestins are a small family of proteins important for regulating signal transduction.

Function
Arrestins were first discovered as a part of a conserved two-step mechanism for regulating the activity of G protein-coupled receptors (GPCRs) in the visual rhodopsin system by Hermann Kühn and co-workers and in the β-adrenergic system by Martin J. Lohse and co-workers. In response to a stimulus, GPCRs activate heterotrimeric G proteins. In order to turn off this response, or adapt to a persistent stimulus, activated receptors need to be silenced. The first step is phosphorylation by a class of serine/threonine kinases called G protein coupled receptor kinases (GRKs). GRK phosphorylation specifically prepares the activated receptor for arrestin binding. Arrestin binding to the receptor blocks further G protein-mediated signaling, targets receptor for internalization, and redirects signaling to alternative G protein-independent pathways. In addition to GPCRs, arrestins bind to other classes of cell surface receptors and a variety of other signaling proteins.

Subtypes
Mammals express four arrestin subtypes and each arrestin subtype is known by multiple aliases. The systematic arrestin name (1-4) plus the most widely used aliases for each arrestin subtype are listed in bold below:


 * Arrestin-1 was originally identified as the S-antigen (SAG) causing uveitis (autoimmune eye disease), then independently described as a 48 kDa protein that binds light-activated phosphorylated rhodopsin before it became clear that both are one and the same. It was later renamed visual arrestin, but when another cone-specific visual subtype was cloned the term rod arrestin was coined. This also turned out to be a misnomer: arrestin-1 expresses at comparable very high levels in both rod and cone photoreceptor cells.


 * Arrestin-2 was the first non-visual arrestin cloned. It was first named β-arrestin simply because between two GPCRs available in purified form at the time, rhodopsin and β2-adrenergic receptor, it showed preference for the latter.


 * Arrestin-3. The second non-visual arrestin cloned was first termed β-arrestin-2 (retroactively changing the name of β-arrestin into β-arrestin-1), even though by that time it was clear that non-visual arrestins interact with hundreds of different GPCRs, not just with β2-adrenergic receptor. Systematic names, arrestin-2 and arrestin-3, respectively, were proposed soon after that.


 * Arrestin-4 was cloned by two groups and termed cone arrestin, after photoreceptor type that expresses it, and X-arrestin, after the chromosome where its gene resides. In the HUGO database its gene is called arrestin-3.

Fish and other vertebrates appear to have only three arrestins: no equivalent of arrestin-2, which is the most abundant non-visual subtype in mammals, was cloned so far. The proto-chordate C. intestinalis (sea squirt) has only one arrestin, which serves as visual in its mobile larva with highly developed eyes, and becomes generic non-visual in the blind sessile adult. Conserved positions of multiple introns in its gene and those of our arrestin subtypes suggest that they all evolved from this ancestral arrestin. Lower invertebrates, such as roundworm C. elegans, also have only one arrestin. Insects have arr1 and arr2, originally termed “visual arrestins” because they are expressed in photoreceptors, and one non-visual subtype (kurtz in Drosophila). Later arr1 and arr2 were found to play an important role in olfactory neurons and renamed “sensory”. Fungi have distant arrestin relatives involved in pH sensing.

Tissue distribution
One or more arrestin is expressed virtually in every eukaryotic cell. In mammals, arrestin-1 and arrestin-4 are largely confined to photoreceptors, whereas arrestin-2 and arrestin-3 are ubiquitous. Neurons have the highest expression level of both non-visual subtypes. In neuronal precursors both are expressed at comparable levels, whereas in mature neurons arrestin-2 is present at 10-20 fold higher levels than arrestin-3.

Mechanism
Arrestins block GPCR coupling to G proteins via two mechanisms. First, arrestin binding to the cytoplasmic tip of the receptor occludes the binding site for the heterotrimeric G-protein, preventing its activation (desensitization). Second, arrestins link the receptor to elements of the internalization machinery, clathrin and clathrin adaptor AP2, which promotes receptor internalization via coated pits and subsequent transport to an internal compartment, called endosome. Subsequently, the receptor could be either targeted to degradation compartments (lysosomes) or recycled back to the plasma membrane where it can signal again. The strength of arrestin-receptor interaction plays a role in this choice: tighter complexes tend to increase the probability of receptor degradation, whereas more transient complexes favor recycling, although this “rule” is far from absolute.

Structure
Arrestins are elongated molecules, in which several intra-molecular interactions hold the relative orientation of the two domains. In unstimulated cell arrestins are localized in the cytoplasm in this basal “inactive” conformation. Active phosphorylated GPCRs recruit arrestin to the plasma membrane. Receptor binding induces a global conformational change that involves the movement of the two arrestin domains and the release of its C-terminal tail that contains clathrin and AP2 binding sites. Increased accessibility of these sites in receptor-bound arrestin targets the arrestin-receptor complex to the coated pit. Arrestins also bind microtubules (part of the cellular “skeleton”), where they assume yet another conformation, different from both free and receptor-bound form. Microtubule-bound arrestins recruit certain proteins to the cytoskeleton, which affects their activity and/or redirects it to microtubule-associated proteins.

Arrestins shuttle between cell nucleus and cytoplasm. Their nuclear functions are not fully understood, but it was shown that all four mammalian arrestin subtypes remove some of their partners, such as protein kinase JNK3 or the ubiquitin ligase Mdm2, from the nucleus. Arrestins also modify gene expression by enhancing transcription of certain genes.