Physiological actions of regulators of G-protein signaling (RGS) proteins

Phosphatidylinositol 3,4,5-trisphosphate and Ca2+/calmodulin competitively bind to the regulators of G-protein-signalling (RGS) domain of RGS4 and reciprocally regulate its action

RGS (regulators of G-protein signalling) are a diverse group of proteins, which accelerate intrinsic GTP hydrolysis on heterotrimeric G-protein a subunits. They are involved in the control of a physiological behaviour known as 'relaxation' of G-protein-gated K+ channels in cardiac myocytes. The GTPase-accelerating activity of cardiac RGS proteins, such as RGS4, is inhibited by PtdIns(3,4,5)P3 (phosphatidylinositol 3,4,5-trisphosphate) and this inhibition is cancelled by Ca2+/calmodulin (CaM) formed during membrane depolarization. G-protein-gated K+ channel activity decreases on depolarization owing to the facilitation of GTPase-activating protein activity by RGS proteins and vice versa on hyperpolarization. The molecular mechanism responsible for this reciprocal control of RGS action by PtdIns(3,4,5)P3 and Ca2+/CaM, however, has not been fully elucidated. Using lipid-protein co-sedimentation assay and surface plasmon resonance measurements, we show in the present study that the control of the GTPase-accelerating activity of the RGS4 protein is achieved through the competitive binding of PtdIns(3,4,5)P3 and Ca2+/CaM within its RGS domain. Competitive binding occurs exclusively within the RGS domain and involves a cluster of positively charged residues located on the surface opposite to the Ga interaction site. In the RGS proteins conserving these residues, the reciprocal regulation by PtdIns(3,4,5)P3 and Ca2+/CaM may be important for their physiological regulation of G-protein signalling.

Desensitization to gonadotropic hormones: a model system for the regulation of a G-protein-coupled receptor with 7-transmembrane spanning regions

Gonadotropic hormone, luteinizing hormone, and follicle-stimulating hormone exert their effect via activation of G-coupled receptors, which activate the hormone sensitive adenylyl cyclase, protein kinase A, and cyclic AMP responsive elements. This activation leads to specific de novo synthesis of steroidogenic factors and steroidogenic enzymes. In normal cells and following activation of this signaling pathway, desensitization period will be followed. This down-regulation, which was studied in detail for the last three decays, was found to take place at various steps of these signal transduction pathways as well as at different kinetics. A common and diverse feature of the mechanism of desensitization in other G-coupled-7-transmembrane receptor system is also discussed.

Expression of regulators of g protein signaling mRNA is differentially regulated in hot and cold thyroid nodules

Because of their regulatory properties on cellular proliferation and differentiation, regulators of G protein signaling (RGS) have been suggested as potential tumor suppressors. The aim of this study was to describe the normal pattern of RGS transcripts in the thyroid gland systematically and to elucidate their potential role in common thyroid pathologies. Real-time polymerase chain reaction (PCR) was applied to quantify mRNA expression of RGS transcripts in 10 hot thyroid nodules (HTN), 10 cold thyroid nodules (CTN), and corresponding surrounding tissues (ST). We have found that 9 of 13 tested RGS transcripts were expressed in the human thyroid gland. Expression of several RGS transcripts was altered in thyroid nodules compared to corresponding normal tissue. In HTN and CTN, mRNA transcripts of RGS 2, 9, and 12 were significantly downregulated. In contrast, mRNA expression of RGS 3, 6, 10 was differentially regulated in HTN and CTN compared to corresponding normal tissue. RGS 3 transcripts were significantly upregulated in CTN. RGS 6 transcripts were significantly downregulated in CTN. RGS 10 mRNA was significantly reduced in HTN. We therefore propose that downregulation of several RGS transcripts in thyroid nodules might contribute to tumor growth within the thyroid gland.

Assays of RGS protein modulation by phosphatidylinositides and calmodulin

Regulator of G-protein signaling (RGS) proteins are a family of proteins that accelerate intrinsic GTP hydrolysis on alpha subunits of trimeric G proteins. They play crucial roles in the physiological regulation of G-protein-mediated cell signaling. If RGS proteins were active unrestrictedly, they would completely suppress G-protein-mediated signaling. Therefore, it is important to understand how the actions of RGS proteins are regulated under different physiological conditions. We have discovered a physiological mode of regulation of a RGS protein in cardiac myocytes. The voltage-dependent formation of Ca2+/calmodulin (CaM) facilitated the GTPase activity of RGS proteins by removing intrinsic inhibition mediated by the phospholipid phosphatidylinositol-3,4,5-trisphosphate. This modulation of RGS protein action underlies the characteristic "relaxation" behavior of G-protein-gated K+ channels in native cardiac myocytes. This article describes briefly the discovery of this novel mode of RGS protein modulation in native cardiac myocytes and then gives details of the biochemical and electrophysiological assays used for the functional investigation of this modulation. These assays would be useful for dissecting the physiological modes of action of RGS proteins in controlling G-protein-mediated signaling machinery.