title: A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ.
abstract: G protein-gated K+ channels (GIRK; Kir3), activated by Gβγ subunits derived from Gi/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (Ievoked) and neurotransmitter-independent basal (Ibasal) GIRK activities are physiologically important, but mechanisms of Ibasal and its relation to Ievoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that Ibasal and Ievoked are interrelated: the extent of activation by neurotransmitter (activation index, Ra) is inversely related to Ibasal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates Ibasal and Ievoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse Ibasal-Ra correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gαi/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gαi/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα.
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Daniel
authors.initials: D
authors.lastname: Yakubovich
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Shai
authors.initials: S
authors.lastname: Berlin
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Uri
authors.initials: U
authors.lastname: Kahanovitch
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Moran
authors.initials: M
authors.lastname: Rubinstein
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Isabella
authors.initials: I
authors.lastname: Farhy-Tselnicker
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Boaz
authors.initials: B
authors.lastname: Styr
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Tal
authors.initials: T
authors.lastname: Keren-Raifman
authors.affiliation: Department of Integrative Biology and Pharmacology, University of Texas Health Science Center, Houston, Texas, United States of America.
authors.forename: Carmen W
authors.initials: CW
authors.lastname: Dessauer
authors.affiliation: Department of Physiology and Pharmacology and Sagol School of Neuroscience, Tel Aviv University, Tel Aviv, Israel.
authors.forename: Nathan
authors.initials: N
authors.lastname: Dascal
mesh_terms: D000818:Animals; D019295:Computational Biology; D005260:Female; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels / Q000737:chemistry / Q000378:metabolism*; D044385:GTP-Binding Protein alpha Subunits / Q000737:chemistry / Q000378:metabolism*; D044387:GTP-Binding Protein beta Subunits / Q000737:chemistry / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008954:Models, Biological; D009865:Oocytes / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D044385:GTP-Binding Protein alpha Subunits; D044387:GTP-Binding Protein beta Subunits