Gpcr signaling pdf

These players turned out to be arrestins Figure 1. Figure 1. This reduces G protein coupling of active GPCR three shorter arrows , but complete blockade of G protein-mediated signaling requires arrestin binding to the active phosphorylated GPCR, where arrestins outcompete G proteins. The arrestin-receptor complex acts as a scaffold facilitating different branches of signaling Raf-MEK-ERK cascade is shown as an example. Preferential binding of arrestins to their cognate receptors when they are active and phosphorylated at the same time was demonstrated directly in case of visual arrestin-1 Wilden et al.

The role of arrestin-1 called 48 kDa protein at the time of discovery in preventing the coupling of phosphorylated rhodopsin to its cognate G protein, transducin, was established in mids Wilden et al.

Later is shown independently by two labs that visual arrestin-1 does that by successfully competing with transducin for the light-activated phosphorylated rhodopsin Wilden, ; Krupnick et al. It turned out that while highly purified GRK2 phosphorylated the receptor better than partially purified preparation, it failed to significantly suppress its coupling to the cognate G protein, Gs Benovic et al.

The addition of purified visual arrestin arrestin-1 in current systematic nomenclature significantly enhanced the desensitizing effect of receptor phosphorylation by GRK2, which suggested that non-visual homolog of arrestin-1 might be required for homologous desensitization of the non-rhodopsin GPCRs Benovic et al.

Soon thereafter the first non-visual arrestin was cloned Lohse et al. When it was cloned for the third time, a systematic arrestin nomenclature, with the number indicating the order of cloning, was proposed, which made this member of the family arrestin-3 Sterne-Marr et al.

Interestingly, only one additional arrestin, cone photoreceptor-specific arrestin-4, was found in mammals Murakami et al. Recent structural data revealed the molecular basis of the competition between G proteins and arrestins: both engage the same inter-helical cavity on the cytoplasmic side of the receptor Rasmussen et al.

Bound G proteins readily dissociate from the receptor in the presence of GTP, whereas arrestins do not. Thus, in case of active phosphorylated GPCRs, which arrestins bind with high affinity Gurevich and Gurevich, , arrestins easily win in the competition with G proteins. However, in addition to the inter-helical cavity, which is a shared docking site of G protein and arrestins, the latter tightly bind receptor-attached phosphates that fit into positive patches on the arrestin surface Zhou et al.

This dual-site binding, predicted in based on arrestin mutagenesis Gurevich and Benovic, , creates a possibility that arrestin might engage the receptor via only one site. Recent study of different subtypes of neuropeptide Y receptors suggests that this mechanism might operate in case of at least some wild type GPCRs Wanka et al.

However, it appears that simultaneous arrestin interaction with both inter-helical cavity and phosphorylated parts of the receptor, which precludes G protein binding, is the rule, rather than an exception.

This mode of arrestin binding is the basis of homologous GPCR desensitization, ensuring direct competition of arrestins with G proteins Wilden, ; Krupnick et al. Both non-visual arrestin subtypes effectively bind clathrin Goodman et al. Thus, non-visual arrestins not only block receptor coupling to the G proteins, but also facilitate GPCR internalization via coated pits reviewed in Gurevich and Gurevich, , further reducing cell responsiveness. Interestingly, visual arrestin-1 does not have a clathrin-binding site Goodman et al.

It is likely a relic, as all arrestins apparently arose from an ancestral universal form, similar to a single arrestin in ascidian Ciona officinalis , which serves as visual in the eyes of its tadpole and as non-visual in the sessile blind adult Gurevich and Gurevich, , where it likely promotes GPCR internalization Nakagawa et al.

The number of non-receptor binding partners of non-visual arrestins kept increasing, culminating in a comprehensive proteomics study that described more than a hundred proteins that bind each of the non-visual subtypes, many of which are bona fide signaling proteins Xiao et al.

However, numerous pathways lead to the activation of ERKs Luttrell, , and many of them, such as receptor tyrosine kinase-dependent Marshall, , are not even GPCR-driven.

The major aspect overlooked in virtually all studies of arrestin-mediated signaling is signal initiation Gurevich and Gurevich, MAPKs ultimately phosphorylate various nuclear and cytoplasmic proteins to elicit cellular response.

Scaffold proteins, such as arrestins, bring the kinases close to each other, thereby facilitating signal transduction. However, the signaling only occurs when the upstream-most MAP3Ks are activated, and arrestins were never implicated in this event. It is entirely possible that before arrestin-mediated scaffolding has a chance to facilitate signaling, MAP3Ks must be activated by arrestin-independent mechanisms, which can be G protein-dependent in case of GPCRs, or G protein- and GPCR-independent in case of growth factor receptors Garrington and Johnson, and integrins Stupack and Cheresh, This aspect of arrestin-mediated scaffolding needs to be studied experimentally.

Although in most cases the functional role of regulation via GRK-dependent phosphorylation remains poorly understood, the mere number of targets suggests that, in addition to playing the key role in controlling the GPCR signaling, GRKs might play important roles in cell growth, attachment and motility, cell death, proliferation and survival, immunity, cancer, as well as other pathological conditions.

In addition to regulating multiple non-GPCR signaling pathways via phosphorylation, GRKs can control signaling in phosphorylation-independent manner via direct protein-protein interaction. For example, GRK5 biding to calmodulin assists in the nuclear translocation associated with the cardiac hypertrophy Gold et al. Furthermore, in addition to phosphorylating HDAC5 in the nucleus, GRK5 contributes to pathological cardiac hypertrophy by activating nuclear factor of activated T cells NFAT transcription factor in phosphorylation-independent manner via direct binding Hullmann et al.

Thus, GRK functions requiring the kinase activity and phosphorylation-independent actions go hand in hand in physiological and pathological processes. Some of these functions are mediated by GRK interacting proteins GITs , which are themselves large multidomain scaffolding proteins interacting with multiple partners and playing important role in numerous cellular processes Premont et al.

This was later confirmed by the use of arrestin mutants that do not bind receptors Song et al. A short arrestinderived peptide lacking most known receptor-binding elements was found to facilitate JNK3 activation in vitro and in living cells Zhan et al.

This reduction in binding affinity is the most striking in case of JNK3, suggesting that activated doubly phosphorylated JNK3 likely dissociates, freeing the place for another molecule of inactive JNK3 to bind Perry et al. It would be interesting to test whether other MAP kinase scaffolds also employ similar amplification mechanism. The critical role of caspase-cleaved arrestin-2 in programmed cell death also did not appear to depend on receptor binding Kook et al.

Caspase cleavage of the other non-visual subtype, arrestin-3, which generates anti-apoptotic arrestin fragment Kook et al. The function of both non-visual arrestins in focal adhesion disassembly Cleghorn et al. Thus, widespread belief that arrestin-mediated signaling is always GPCR-driven appears to be wrong. These include E3 ubiquitin ligases Mdm2 Song et al.

Elucidation of every molecular mechanism of cellular functions paves the way to devising tools that affect this mechanism for therapeutic purposes. Arguably, congestive heart failure is the most studied condition where GPCR desensitization plays a role in the pathology reviewed in Lymperopoulos et al. Heart failure manifests itself as the loss of heart responsiveness to pro-contractile stimuli.

Both appear to undergo excessive desensitization mediated by GRK2. This does not appear to cause many side effects, in contrast to inhibitors of ubiquitously expressed GRK2 even if selective ones were available , that would likely indiscriminately affect the desensitization of many GPCRs in various cell types.

At least one of these mutants was shown to directly compete with GRK2 for the receptor and greatly facilitate receptor recycling back to the plasma membrane Pan et al. This approach was so far tested only in the visual system, where enhanced arrestin-1 improved photoreceptor performance and survival in mouse models with defective rhodopsin phosphorylation Song et al. However, for this purpose enhanced mutants of non-visual arrestins must be rendered specific for particular GPCRs to avoid their effect on the signaling of perfectly normal GPCRs co-expressed in the same cell Gurevich and Gurevich, Manipulation of the receptor-binding surface of arrestins was also shown to change their selectivity for particular functional forms of the receptor Prokop et al.

However, the ability of these receptor subtype-specific forms to selectively suppress signaling by certain GPCRs without affecting others co-expressed in the same cell still remains to be tested. Another therapeutic approach that needs to be tested is the use of signaling-biased arrestins. WT non-visual arrestins have a lot of functions: they bind hundreds of different GPCRs and dozens of non-receptor signaling proteins, affecting multiple branches of cellular signaling Hanson et al.

Thus, an increase or decrease of WT arrestin expression in cells cannot serve a specific function: too many things would change. However, quite a few arrestin mutants where individual functions are suppressed or destroyed were constructed: forms that do not bind GPCRs [KNC mutants Breitman et al.

One drawback is that in some cases mutations affect not only the intended function, but others, as well. It is hardly practical to test every mutant for all the functions that corresponding WT arrestin fulfills, so these mutants might have limited therapeutic usability.

In contrast, monofunctional elements extracted from multi-functional arrestin proteins hold greater promise. So far two of these were constructed. Separated arrestin-2 C-terminus carrying both clathrin and AP2 binding sites, effectively outcompetes the arrestin-receptor complexes in coated pits, suppressing arrestin-dependent GPCR endocytosis in cells Krupnick et al. Thus, at least two arrestin functions can be manipulated independently of the others.

Of course, this is just the beginning, but these findings are encouraging. All of the examples above imply gene therapy, which is currently in its infancy. Theoretically, identification of arrestin sites responsible for the binding of any particular partner enable the search for small molecules that can bind to the site and selectively inhibit an individual interaction.

This avenue also needs to be explored, although it might yield fewer useful molecules than one would expect. Protein-protein interaction sites often involve relatively flat surfaces Gurevich and Gurevich, or disordered protein elements Gurevich et al. To summarize, selective regulation of GRKs and arrestins appears promising as a therapeutic approach. Many avenues of this regulation, involving both conventional small molecule therapeutics and gene therapy, must be explored.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abraham, A. Estrogen regulation of GRK2 inactivates kappa opioid receptor signaling mediating analgesia, but not aversion. Ahmed, M. Ubiquitin ligase parkin promotes Mdm2-arrestin interaction but inhibits arrestin ubiquitination.

Biochemistry 50, — Alvarez-Curto, E. Targeted elimination of G proteins and arrestins defines their specific contributions to both intensity and duration of G protein-coupled receptor signaling. Arshavsky, V. The effect of rhodopsin phosphorylation on the light-dependent activation of phosphodiesterase from bovine rod outer segments.

FEBS Lett. Attramadal, H. PubMed Abstract Google Scholar. Baameur, F. Role for the regulator of G-protein signaling homology domain of G protein-coupled receptor kinases 5 and 6 in beta 2-adrenergic receptor and rhodopsin phosphorylation.

Baylor, D. Responses of retinal rods to single photons. Google Scholar. Benovic, J. Beta-adrenergic receptor kinase: primary structure delineates a multigene family. Science , — Plant Signal Behav. Narendra Tuteja. Author information Article notes Copyright and License information Disclaimer.

Corresponding author. Correspondence to: Narendra Tuteja; Email: ni. Received Jul 13; Accepted Jul This article has been cited by other articles in PMC.

Key words: heterotrimeric G proteins, GPCRs, seven-transmembrane receptors, signal transduction, stress signaling. Introduction Cell signaling is one of the important processes required for the normal growth and development of cell. Open in a separate window. Figure 1.

Footnotes Previously published online: www. References 1. Redhead CR, Palme K. The genes of plant signal transduction. Crit Rev Plant Sci. Mahajan S, Tuteja N. Cold, salinity and drought stresses: An overview. Arch Biochem Biophys. Tuteja N. Mechanisms of high salinity tolerance in plants. Meth Enzymol. Trewavas AJ, Malho R. Signal perception and Transduction: The origin of the phenotype.

Plant Cell. Hucho F, Buchner K. Signal transduction and protein kinases: the long way from the plasma membrane into the nucleus. The plant heterotrimeric G-protein complex. Annu Rev Plant Biol. Tuteja N, Sopory SK. Plant signaling in stress: G-protein coupled receptors, heterotrimeric G-proteins and signal coupling via phospholipases.

Structure and function of G protein-coupled receptors. Annu Rev Biochem. Identification of the hormone-binding subunit. J Biol Chem. Bockaert J, Pin JP. Molecular tinkering of G-protein-coupled receptors: An evolutionary success. EMBO J. Hamm HE. How activated receptors coupled to G proteins. Hamm H. The many faces of G-protein signaling. Arrestins were first discovered as key players in the desensitization of G protein-coupled receptors GPCRs.

Author : Gary B. Author : Vsevolod L. This article focuses on the review of G-protein coupled receptor GPCR signaling pathways in insect physiology, including insects' reproduction, growth and development, stress responses, feeding, behaviors, and other physiological processes.

Hormones and polypeptides that are involved in insect GPCR regulatory pathways are reviewed.