Molecular regulation of the HERG potassium channel
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The dynamic regulation of membrane excitability determines the human heart rhythm and governs cardiac adaptation to changes in physiologic demands. Human Ether-a-go-go Related Gene (HERG) encodes the pore-forming K+ selective channel subunit that carries the rapidly activating delayed rectifier current (Ikr). Ikr is unique in its ability to respond to and modify the rate of membrane repolarization at the end of each action potential, making it essential to the maintenance of the cardiac rhythm. HERG K+ channel has been linked to both hereditary and acquired forms of the Long QT syndrome (LQT), a potentially fatal cardiac disorder with a characteristic polymorphic ventricular tachyarrhythmia. Given the critical role of this channel in controlling the cardiac rhythm, we studied mechanisms of molecular and cellular regulation of HERG and proposed a role for these mechanisms in both normal activity and the LQT Syndrome.;To examine the role of cellular proteins in the regulation of HERG K + channel function, we sought to identify additional proteins that interact with and modify channel activity. In a yeast-two-hybrid screen of a human heart library, we identified an interaction between HERG and 14-3-3 proteins providing a novel mechanism linking beta-adrenergic signaling and HERG K+ channel function. Association of HERG and 14-3-3 required protein kinase A (PKA) dependent phosphorylation of the channel on both the N- and C-termini. 14-3-3 binding stabilized the lifetime of the PKA-phosphorylated state of the channel by shielding HERG from phosphatases. 14-3-3 overexpression enhanced HERG current by accelerating activation and shifting the voltage dependence of channel activation to more hyperpolarized potentials. The functional effects of 14-3-3 on HERG current required dimerization of 14-3-3 and potential cross-bridging of the cytoplasmic termini of the channel. We confirmed an association of HERG and 14-3-3 in porcine myocardium, supporting a role for 14-3-3 in regulating endogenous HERG/Ikr. Overall, binding of 14-3-3 to HERG prolonged the effects of cAMP stimulation upon channel activity. These results describe a novel molecular mechanism by which adrenergic signaling alters HERG channel activity and provide initial evidence that a macromolecular complex couples intracellular signals with membrane excitability by dynamically regulating HERG.
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