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: Ion selectivity of four-domain voltage-gated Ca2+ and Na+ channels, which is controlled by the selectivity filter (the narrowest region of an open pore), is crucial for electrical signaling. Over billions of years of evolution, mutation of the Glu from domain II/III in the EEEE/DEEA selectivity filters of Ca2+-selective channels to Lys made these channels Na+-selective. Why Lys is sufficient for Na+ selectivity and why the DKEA selectivity filter is less Na+-selective than the DEKA one are intriguing, fundamental questions. We address these questions by computing the free energy for replacing Ca2+ inside model selectivity filters with Na+. The nonmetal-ligating Lys in the DKEA/DEKA selectivity filter attenuates metal-protein interactions to such an extent that solvation effects become dominant, favoring Na+. It constricts and rigidifies the DEKA pore to bind Na+ optimally, highlighting the importance of lysine's non-obvious structural role, in addition to its electrostatic role, in the selectivity of Na+ over Ca2+.Journal of the American Chemical Society 02/2014; 136(9). DOI:10.1021/ja4121132 · 11.44 Impact Factor
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ABSTRACT: T-type (Cav3) channels are categorized as calcium channels, but invertebrate ones can be highly sodium-selective channels. We illustrate that the snail LCav3 T-type channel becomes highly sodium-permeable through exon splicing of an extracellular turret and descending helix in Domain II of the four domain Cav3 channel. Highly sodium permeable T-type channels are generated without altering the invariant ring of charged residues in the selectivity filter that governs calcium selectivity in calcium channels. The highly sodium permeant T-type channel expresses in the brain and is the only splice isoform expressed in the snail heart. This unique splicing of turret residues offers T-type channels a capacity to serve as a pacemaking sodium current in the primitive heart and brain in lieu of Nav1 type sodium channels, and substitute for voltage-gated sodium channels lacking in many invertebrates. T-type channels would also contribute substantially to sodium leak conductances at rest in invertebrates because of their large window currents.Journal of Biological Chemistry 03/2014; 289:11952-11969. DOI:10.1074/jbc.M114.551473 · 4.60 Impact Factor
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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; DOI:10.1007/s00424-014-1449-7 · 4.87 Impact Factor