RGS4 regulates partial agonism of the M2 muscarinic receptor-activated K+ currents.

Partial agonists are used clinically to avoid overstimulation of receptor-mediated signalling, as they produce a submaximal response even at 100% receptor occupancy. The submaximal efficacy of partial agonists is due to conformational change of the agonist-receptor complex, which reduces effector activation. In addition to signalling activators, several regulators help control intracellular signal transductions. However, it remains unclear whether these signalling regulators contribute to partial agonism. Here we show that regulator of G-protein signalling (RGS) 4 is a determinant for partial agonism of the M2 muscarinic receptor (M2R). In rat atrial myocytes, pilocarpine evoked smaller G-protein-gated K(+) inwardly rectifying (KG) currents than those evoked by ACh. In a Xenopus oocyte expression system, pilocarpine acted as a partial agonist in the presence of RGS4 as it did in atrial myocytes, while it acted like a full agonist in the absence of RGS4. Functional couplings within the agonist-receptor complex/G-protein/RGS4 system controlled the efficacy of pilocarpine relative to ACh. The pilocarpine-M2R complex suppressed G-protein-mediated activation of KG currents via RGS4. Our results demonstrate that partial agonism of M2R is regulated by the RGS4-mediated inhibition of G-protein signalling. This finding helps us to understand the molecular components and mechanism underlying the partial agonism of M2R-mediated physiological responses

Conformational changes underlying pore dilation in the cytoplasmic domain of mammalian inward rectifier K^+ channels

Inanobe A, Nakagawa A, Kurachi Y (2013) Conformational Changes Underlying Pore Dilation in the Cytoplasmic Domain of Mammalian Inward Rectifier K^+ Channels. PLOS ONE 8(11): e79844. https://doi.org/10.1371/journal.pone.007984

Interactions of Cations with the Cytoplasmic Pores of Inward Rectifier K^+ Channels in the Closed State

This research was originally published in Journal of Biological Chemistry. Atsushi Inanobe, Atsushi Nakagawa, and Yoshihisa Kurachi. Interactions of Cations with the Cytoplasmic Pores of Inward Rectifier K^+ Channels in the Closed State. Journal of Biological Chemistry. 2011; 286, 41801-41811. © the American Society for Biochemistry and Molecular Biology

A structural determinant for the control of PIP_2 sensitivity in G protein-gated inward rectifier K^+ channels

Inward rectifier K^+ (Kir) channels are activated by phosphatidylinositol-( 4,5)-bisphosphate (PIP_2), but G protein-gated Kir (K_G) channels further require either G protein βγ subunits (Gβγ) or intracellular Na^+ for their activation. To reveal the mechanism(s) underlying this regulation, we compared the crystal structures of the cytoplasmic domain of K_G channel subunit Kir3.2 obtained in the presence and the absence of Na^+. The Na^+ -free Kir3.2, but not the Na^+ -plus Kir3.2, possessed an ionic bond connecting the N terminus and the CD loop of the C terminus. Functional analyses revealed that the ionic bond between His-69 on theNterminus and Asp-228 on the CD loop, which are known to be critically involved in Gβγ- and Na^+ -dependent activation, lowered PIP_2 sensitivity. The conservation of these residues within the K_G channel family indicates that the ionic bond is a character that maintains the channels in a closed state by controlling the PIP_2 sensitivity.This research was originally published in Journal of Biological Chemistry. Atsushi Inanobe, Atsushi Nakagawa, Takanori Matsuura and Yoshihisa Kurachi. A structural determinant for the control of PIP2 sensitivity in G protein-gated inward rectifier K^+ channels. Journal of Biological Chemistry. 2010; 285, 38517-38523. © the American Society for Biochemistry and Molecular Biology

Somatostatin induces hyperpolarization in pancreatic islet α cells by activating a G protein-gated K+ channel

AbstractSomatostatin inhibits glucagon-secretion from pancreatic α cells but its underlying mechanism is unknown. In mouse α cells, we found that somatostatin induced prominent hyperpolarization by activating a K+ channel, which was unaffected by tolbutamide but prevented by pre-treating the cells with pertussis toxin. The K+ channel was activated by intracellular GTP (with somatostatin), GTPγS or Gβγ subunits. It was thus identified as a G protein-gated K+ (KG) channel. RT-PCR and immunohistochemical analyses suggested the KG channel to be composed of Kir3.2c and Kir3.4. This study identified a novel ionic mechanism involved in somatostatin-inhibition of glucagon-secretion from pancreatic α cells

Inwardly rectifying potassium channels (KIR) in GtoPdb v.2021.3

The 2TM domain family of K channels are also known as the inward-rectifier K channel family. This family includes the strong inward-rectifier K channels (Kir2.x) that are constitutively active, the G-protein-activated inward-rectifier K channels (Kir3.x) and the ATP-sensitive K channels (Kir6.x, which combine with sulphonylurea receptors (SUR1-3)). The pore-forming α subunits form tetramers, and heteromeric channels may be formed within subfamilies (e.g. Kir3.2 with Kir3.3)

Inwardly rectifying potassium channels (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

The 2TM domain family of K channels are also known as the inward-rectifier K channel family. This family includes the strong inward-rectifier K channels (Kir2.x) that are constitutively active, the G-protein-activated inward-rectifier K channels (Kir3.x) and the ATP-sensitive K channels (Kir6.x, which combine with sulphonylurea receptors (SUR1-3)). The pore-forming α subunits form tetramers, and heteromeric channels may be formed within subfamilies (e.g. Kir3.2 with Kir3.3)

An Epithelial Ca2+-Sensor Protein is an Alternative to Calmodulin to Compose Functional KCNQ1 Channels

Background/Aims: KCNQ channels transport K+ ions and participate in various cellular functions. The channels directly assemble with auxiliary proteins such as a ubiquitous Ca2+-sensor protein, calmodulin (CaM), to configure the physiological properties in a tissue-specific manner. Although many CaM-like Ca2+-sensor proteins have been identified in eukaryotes, how KCNQ channels selectively interact with CaM and how the homologues modulate the functionality of the channels remain unclear. Methods: We developed protocols to evaluate the interaction between the green fluorescent protein-tagged C-terminus of KCNQ1 (KCNQ1cL) and Ca2+-sensors by detecting its fluorescence in size exclusion chromatography and electrophoresed gels. The effects of Ca2+-sensor proteins on KCNQ1 activity was measured by two electrode voltage clamp technique of Xenopus oocytes. Results: When co-expressed CaM and KCNQ1cL, they assemble in a 4:4 stoichiometry, forming a hetero-octamer. Among nine CaM homologues tested, Calml3 was found to form a hetero-octamer with KCNQ1cL and to associate with the full-length KCNQ1 in a competitive manner with CaM. When co-expressed in oocytes, Calml3 rendered KCNQ1 channels resistant to the voltage-dependent depletion of phosphatidylinositol 4,5-bisphosphate by voltage-sensitive phosphatase. Conclusion: Since Calml3 is closely related to CaM and is prominently expressed in epithelial cells, Calml3 may be a constituent of epithelial KCNQ1 channels and underscores the molecular diversity of endogenous KCNQ1 channels