Structural modeling of calcium binding in the selectivity filter of the L-type calcium channel.
ABSTRACT Calcium channels play crucial physiological roles. In the absence of high-resolution structures of the channels, the mechanism of ion permeation is unknown. Here we used a method proposed in an accompanying paper (Cheng and Zhorov in Eur Biophys J, 2009) to predict possible chelation patterns of calcium ions in a structural model of the L-type calcium channel. We compared three models in which two or three calcium ions interact with the four selectivity filter glutamates and a conserved aspartate adjacent to the glutamate in repeat II. Monte Carlo energy minimizations yielded many complexes with calcium ions bound to at least two selectivity filter carboxylates. In these complexes calcium-carboxylate attractions are counterbalanced by calcium-calcium and carboxylate-carboxylate repulsions. Superposition of the complexes suggests a high degree of mobility of calcium ions and carboxylate groups of the glutamates. We used the predicted complexes to propose a permeation mechanism that involves single-file movement of calcium ions. The key feature of this mechanism is the presence of bridging glutamates that coordinate two calcium ions and enable their transitions between different chelating patterns involving four to six oxygen atoms from the channel protein. The conserved aspartate is proposed to coordinate a calcium ion incoming to the selectivity filter from the extracellular side. Glutamates in repeats III and IV, which are most distant from the repeat II aspartate, are proposed to coordinate the calcium ion that leaves the selectivity filter to the inner pore. Published experimental data and earlier proposed permeation models are discussed in view of our model.
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ABSTRACT: Cav3 T-type channels are low-voltage-gated channels with rapid kinetics that are classified among the calcium-selective Cav1 and Cav2 type channels. Here, we outline the fundamental and unique regulators of T-type channels. An ubiquitous and proximally located "gating brake" works in concert with the voltage-sensor domain and S6 alpha-helical segment from domain II to set the canonical low-threshold and transient gating features of T-type channels. Gene splicing of optional exon 25c (and/or exon 26) in the short III-IV linker provides a developmental switch between modes of activity, such as activating in response to membrane depolarization, to channels requiring hyperpolarization input before being available to activate. Downstream of the gating brake in the I-II linker is a key region for regulating channel expression where alternative splicing patterns correlate with functional diversity of spike patterns, pacemaking rate (especially in the heart), stage of development, and animal size. A small but persistent window conductance depolarizes cells and boosts excitability at rest. T-type channels possess an ion selectivity that can resemble not only the calcium ion exclusive Cav1 and Cav2 channels but also the sodium ion selectivity of Nav1 sodium channels too. Alternative splicing in the extracellular turret of domain II generates highly sodium-permeable channels, which contribute to low-threshold sodium spikes. Cav3 channels are more ubiquitous among multicellular animals and more widespread in tissues than the more brain centric Nav1 sodium channels in invertebrates. Highly sodium-permeant Cav3 channels can functionally replace Nav1 channels in species where they are lacking, such as in Caenorhabditis elegans.Pflügers Archiv - European Journal of Physiology 02/2014; · 4.87 Impact Factor
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ABSTRACT: Invertebrate L-type calcium channel,LCav1, isolated from the pond snail Lymnaea stagnalis is nearly indistinguishable from mammalian Cav1.2 (α1C) calcium channel in biophysical characteristics observed in vitro. These L-type channels are likely constrained within a narrow range of biophysical parameters to perform similar functions in the snail and mammalian cardiovascular systems. What distinguishes snail and mammalian L-type channels is a difference in dihydropyridine sensitivity: 100 nM isradipine exhibits a significant block of mammalian Cav1.2 currents without effect on snail LCav1 currents. The native snail channel serves as a valuable surrogate for validating key residue differences identified from previous experimental and molecular modeling work. As predicted, three residue changes in LCav1 (N_3o18, F_3i10 and I_4i12) replaced with DHP -sensing residues in respective positions of Cav1.2,(Q_3o18, Y_3i10 and M_4i12) raises the potency of isradipine block of LCav1 channels to that of mammalian Cav1.2. Interestingly, the single N_3o18_Q mutation in LCav1 channels lowers DHP sensitivity even further and the triple mutation bearing enhanced isradipine sensitivity still retains a reduced potency of agonist, (S)-Bay K8644Channels 03/2011; 5(2):173-187. · 2.16 Impact Factor
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ABSTRACT: We have performed dynamic Monte Carlo simulations for a model calcium channel and present our results in terms of binding affinity (expressed as the occupancy of the selectivity filter by various ions) and dynamical selectivity (expressed as the flux carried by various ions). We show that the binding affinity of Ca2+ versus Na+ is always larger than the dynamical selectivity because Ca2+ ions are tightly bound to the binding site of the selectivity filter of the channel and, at the same time, their mobility and drift velocity is smaller in this region. We present density and drift velocity profiles of Ca2+ and Na+ in the channel for various mole fractions of Ca2+.Keywords (keywords): calcium channel; selectivity; dynamic Monte CarloJournal of Physical Chemistry Letters - J PHYS CHEM LETT. 06/2010; 1(14).