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SYMPOSIUM REPORT |
Department of Physiology and Biophysics, Cellular and Molecular Neurobiology Research Group, University of Calgary, Calgary T2N 4 N1, Canada
Voltage-dependent inactivation of calcium channels is a key mechanism for regulating intracellular calcium levels and neuronal excitability. In sodium and potassium channels, the molecular determinants that govern fast inactivation involve pore block by a cytoplasmic gating particle. As we discuss here, there is an increasing body of evidence that is consistent with a qualitatively similar inactivation mechanism in high-voltage-activated calcium channels. Work from a number of laboratories has implicated both cytoplasmic regions and the pore-lining S6 transmembrane helices in the inactivation process. Together with our recent findings, this leads us to propose a model in which the intracellular domain I–II linker region acts as a ‘hinged lid’ that physically occludes the pore by docking to the cytoplasmic ends of the S6 segments. We further propose that the ancillary calcium channel ß subunits differentially modulate inactivation kinetics by binding to and thereby regulating the mobility of the putative inactivation gate. Indeed, additional evidence suggests that the carboxy terminus, amino terminus and domain III–IV linker regions of the channel modulate inactivation rates through interactions with the I–II linker per se, or indirectly via the ancillary ß subunits. Taken together, the fast voltage-dependent inactivation of calcium channels appears reminiscent of that of sodium channels, but appears to show a more complex regulation through intramolecular interactions between the putative inactivation gate and other cytoplasmic regions.
(Received 13 May 2003;
accepted after revision 5 June 2003;
first published online 18 June 2003)
Corresponding author G. W. Zamponi: Department of Physiology and Biophysics, University of Calgary, 3330 Hospital Drive NW, Calgary T2N 4N1, Canada. Email: zamponi{at}ucalgary.ca
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