title: Structural basis for Ca2+ selectivity of a voltage-gated calcium channel.
abstract: Voltage-gated calcium (CaV) channels catalyse rapid, highly selective influx of Ca(2+) into cells despite a 70-fold higher extracellular concentration of Na(+). How CaV channels solve this fundamental biophysical problem remains unclear. Here we report physiological and crystallographic analyses of a calcium selectivity filter constructed in the homotetrameric bacterial NaV channel NaVAb. Our results reveal interactions of hydrated Ca(2+) with two high-affinity Ca(2+)-binding sites followed by a third lower-affinity site that would coordinate Ca(2+) as it moves inward. At the selectivity filter entry, Site 1 is formed by four carboxyl side chains, which have a critical role in determining Ca(2+) selectivity. Four carboxyls plus four backbone carbonyls form Site 2, which is targeted by the blocking cations Cd(2+) and Mn(2+), with single occupancy. The lower-affinity Site 3 is formed by four backbone carbonyls alone, which mediate exit into the central cavity. This pore architecture suggests a conduction pathway involving transitions between two main states with one or two hydrated Ca(2+) ions bound in the selectivity filter and supports a 'knock-off' mechanism of ion permeation through a stepwise-binding process. The multi-ion selectivity filter of our CaVAb model establishes a structural framework for understanding the mechanisms of ion selectivity and conductance by vertebrate CaV channels.
authors.affiliation: 1] Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA [2] Howard Hughes Medical Institute, University of Washington, Seattle, Washington 98195, USA [3].
authors.forename: Lin
authors.initials: L
authors.lastname: Tang
authors.affiliation: 1] Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA [2].
authors.forename: Tamer M
authors.initials: TM
authors.lastname: Gamal El-Din
authors.affiliation: 1] Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA [2] Department of Structural Biology, Genentech Inc., South San Francisco, California 94080, USA.
authors.forename: Jian
authors.initials: J
authors.lastname: Payandeh
authors.affiliation: Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Gilbert Q
authors.initials: GQ
authors.lastname: Martinez
authors.affiliation: Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Teresa M
authors.initials: TM
authors.lastname: Heard
authors.affiliation: Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Todd
authors.initials: T
authors.lastname: Scheuer
authors.affiliation: 1] Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA [2] Howard Hughes Medical Institute, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Ning
authors.initials: N
authors.lastname: Zheng
authors.affiliation: Department of Pharmacology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: William A
authors.initials: WA
authors.lastname: Catterall
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D055162:Biocatalysis; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism; D018360:Crystallography, X-Ray; D004553:Electric Conductivity; D015640:Ion Channel Gating; D008954:Models, Biological; D008958:Models, Molecular; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D015220:Calcium Channels; D002413:Cations, Divalent; D002118:Calcium