Regulation of ion channels by GTP-binding Proteins

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GTP-binding protein (G proteins) consisting of alpha, beta, gamma subunits, carry signals from agonist-bound receptors to effectors such as enzymes or ion channels. Since the functions of G proteins as the signal transducers are profoundly affected by cholera or pertussis toxin which transfers the ADP-ribosyl moiety of NAD^+ to the alpha subunits of G proteins, the toxin-catalyzed ADP-ribosylation has widely been used as a tool to study the roles of G proteins in signal transduction processes. In the present studies, we investigated mechanisms by which the activities of an inwardly rectifying K^+ channel and adenylyl cyclase are regulated by the subunits of G proteins.1) Activation of inwardly rectifying K^+ channels by the betagamma subunits of G proteins. An inwardly rectifying K^+ channel in guinea pig cardiac atrial cells is activated by the stimulation of muscarinic cholinergic or A^1-purinergic receptors in a manner sensitive to pertussis toxin. In inside-out patches of the cell … More membranes, the betagamma subunits of G proteins activated the K^+ channels. Properties of the betagamma-induced activation of the K^+ channel appeared to be the same as those observed in GTP-dependent receptor-mediated activation of the K^+ channel. Moreover, the receptor-mediated activation of the K^+ channel was selectively inhibited by GDP-bound from of G_t (transducin). These results indicate that activation of inwardly rectifying K^+ channel coupling to membrane-bound receptors is mediated through the action] of betagammasubunits resolved from alphabetagammatrimer.2) Activation of adenylyl cyclase by protein kinase C.Cyclic AMP formation within cells is altered upon the activation of protein kinase C.However, the site (s) to be modified by the kinase in the cyclic AMP formation pathway remains unclear. Thus, effects of protein kinase C on the activity of adenylyl cyclase were investigated with purified proteins. Protein kinase C could phosphorylate adenylyl cyclase type V directly, leading to a 20-fold increase in its catalytic activity. This activation was much larger than that stimulated by forskolin (5 fold). When forskolin and the protein phosphorylation were combined, the type 5 cyclase activity was increased more than 100 fold over the basal. Thus, it appears that protein kinase C directly and potently activates adenylyl cyclase. Less