Chapter 5 Nuclear Trafficking of Regulator of G Protein Signaling Proteins and Their Roles in the Nucleus

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It was generally believed that the main function of regulator of G protein signaling (RGS) proteins was to negatively regulate GPCR (G protein‐coupled receptor)‐G protein signal transduction near the cell surface. Recent studies reveal that instead of localizing at the plasma membrane, where GPCRs and G proteins reside, many RGS proteins accumulate in the nucleus or shuttle between the cytoplasm and the nucleus. Some RGS proteins are even targeted to unique subnuclear regions. Nuclear trafficking of RGS proteins is controlled by nuclear localization signals and nuclear export signals. The RGS domain, which is essential for interaction between Gα subunits and RGS proteins, has also been shown to play an important role in nuclear trafficking of RGS proteins. This review focuses on the structural basis of RGS proteins and mechanisms underlying their nuclear trafficking, as well as their potential actions in the nucleus.

Introduction

G protein‐coupled receptors (GPCRs) constitute one of the most important classes of cell surface signaling proteins, and recognize numerous environmental signals including hormones, neurotransmitters, growth factors, drugs, and sensory stimuli. GPCRs are intrinsic membrane proteins with seven transmembrane domains, which transmit extracellular signals into intracellular biological responses via their activation of heterotrimeric G proteins. Inactive G protein heterotrimers consist of Gα, Gβ, and Gγ subunits. Upon activation by ligand binding, GPCRs stimulate exchange of GTP for GDP on the Gα subunits via their guanine nucleotide exchange factor (GEF) activity. Binding of GTP promotes activation of the Gα subunits and dissociation of the Gα subunit and a Gβγ heterodimer, both of which are active signaling entities that activate downstream signaling by actions on adaptor proteins, effector enzymes, or ion channels.

Activation and termination of all signaling systems are tightly regulated to ensure intracellular responses of appropriate strength and duration. Regulator of G protein signaling (RGS) proteins have been identified as a class of major negative regulators of G protein signaling.1, 2, 3, 4 Members of this large protein family have been implicated as key regulators of a variety of G protein‐mediated cellular functions, including photoresponse,5, 6 chemokine response,7 embryonic development,8 motor ability,9 opiate response,10, 11 and blood pressure regulation via smooth muscle contraction.12

The first RGS protein, Sst2p, was identified in yeast.13 Later, Koelle and Horvitz identified a C. elegans RGS protein, Egl‐10, which shared a 120‐amino acid (aa) semiconserved RGS domain with Sst2p.14 Identification of the RGS domain enabled Koelle and Horvitz to demonstrate the existence of a family of mammalian proteins with this domain.14 The RGS domain mediates binding of RGS proteins to G proteins and contains a GTPase‐activating protein (GAP) activity that can enhance the intrinsic GTPase activity of Gα subunits. It is believed that the GAP activity of RGS proteins is the main determinant for the ability of RGS proteins to regulate G protein signaling. Association between RGS proteins and Gα subunits leads to an acceleration of the hydrolysis of their bound GTP to GDP, thereby converting the active Gα‐GTP to the inactive GDP‐bound form and resulting in the reassociation of Gαβγ subunits and termination of G protein signaling.1, 2, 3, 4 Other possible mechanisms by which RGS proteins attenuate G protein signaling involve direct interaction between RGS proteins and GPCRs15 or effectors.16, 17

Since the classic actions of RGS proteins depend largely upon their interaction with components of G protein signaling complexes, it was initially assumed that RGS proteins were localized mainly on the inner cytoplasmic membrane surface or in the periplasmic membrane region, where these cell surface signaling molecules reside. However, recent studies have revealed differential subcellular localizations of various RGS family members. Surprisingly, only a small fraction of RGS proteins are associated with the plasma membrane under basal conditions. Moreover, a number of RGS proteins exhibit a predominant or exclusive nuclear distribution in unstimulated cells or undergo a signal‐induced translocation from the cytoplasm into the nucleus, which suggests that not only plasma membrane‐associated RGS proteins but also nuclear RGS proteins play roles in signal transduction. This chapter summarizes recent studies of the subcellular localization and function of RGS proteins, with a focus on the mechanisms controlling their trafficking in and out of the nucleus and their possible nuclear functions apart from the modulation of G protein signaling.

Section snippets

Subcellular Localization of RGS Proteins

Compartmentalization of signaling molecules is one mechanism by which intracellular signal transduction is regulated. Components of a signaling complex are usually in close proximity to ensure specificity and efficiency of signal transduction. Therefore, studies of subcellular localization of different signaling molecules are important as they can shed light on the intracellular actions and physiological functions of these proteins. Understanding their subcellular localizations is particularly

Mechanisms Controlling Protein Nucleocytoplasmic Transport

The nucleus is an isolated compartment surrounded by two layers of membrane—the nuclear envelope. Exchange of materials between the cytoplasm and the nucleus occurs through the nuclear pore complex (NPC). The NPC is a large macromolecular complex forming an aqueous channel across the nuclear envelope of approximately 9 nm in diameter. Ions and small molecules generally can pass freely through this channel. Proteins smaller than 30 kDa are able to passively diffuse at a relatively faster rate

Potential Roles of RGS Proteins in the Nucleus

Although it is still widely accepted that nuclear RGS proteins are an inactive pool for proper regulation of G protein signaling, accumulating evidence showing signal‐induced nuclear import of RGS proteins suggests that they are actively involved in actions induced by these signals. In the following section, we will summarize investigations to date into the roles of nuclear RGS proteins in addition to or, distinct from their direct actions on G protein signaling.

Conclusions

We have discussed in this chapter the studies investigating the subcellular locations of RGS proteins and RGS protein functions in nonplasma membrane locations. Being members of such a large protein family and possessing various structural motifs, it is hard to imagine that RGS proteins only function as negative regulators for G protein signaling. These investigations provide important evidence supporting multiple functions of RGS proteins other than their actions on G protein signaling.

Acknowledgments

We thank Dr. John Koland for critical reading of the manuscript. The authors were supported by grants from the NIH (GM075033) and AHA (0750057Z). We also thank current and past laboratory members for their research contributions.

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