title: Modeling of the outer vestibule and selectivity filter of the L-type Ca2+ channel.
abstract: Using the KcsA bacterial K+ channel crystal structure [Doyle, D. A., et al. (1998) Science 280, 69-74] and the model of the outer vestibule of the Na+ channel [Lipkind, G. M., and Fozzard, H. A. (2000) Biochemistry 39, 8161-8170] as structural templates, we propose a structural model of the outer vestibule and selectivity filter of the pore of the Ca2+ channel (alpha1C or Ca(v)1.2). The Ca2+ channel P loops were modeled by alpha-helix-turn-beta-strand motifs, with the glutamate residues of the EEEE motif located in the turns. P loops were docked in the extracellular part of the inverted teepee structure formed by S5 and S6 alpha-helices with backbone coordinates from the M1 and M2 helices of the KcsA crystal structure. This construction results in a conical outer vestibule that tapers to the selectivity filter at the bottom. The modeled selectivity ring forms a wide open pore ( approximately 6 A) in the absence of Ca2+. When Ca2+ is present ( approximately 1 microM), all four glutamate side chains move to the center and form a cage around the dehydrated Ca2+ ion, blocking the pore. In the millimolar concentration range, Ca2+ also interacts with two low-affinity sites located externally and internally, which were modeled by the same carboxylate groups of the selectivity filter. Calculation of the resulting electrostatic potentials show that the single Ca2+ ion is located in an electrostatic trap. Only when three Ca2+ ions are bound simultaneously in the high- and low-affinity sites of the selectivity filter is Ca2+ able to overcome electrostatic attraction, permitting Ca2+ flux.
authors.affiliation: Cardiac Electrophysiology Laboratories, Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: G M
authors.initials: GM
authors.lastname: Lipkind
authors.forename: H A
authors.initials: HA
authors.lastname: Fozzard
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000737:chemistry* / Q000378:metabolism; D002413:Cations, Divalent / Q000378:metabolism; D003198:Computer Simulation; D015640:Ion Channel Gating; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010539:Permeability; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment
chemical_list: D001426:Bacterial Proteins; D020746:Calcium Channels, L-Type; D002413:Cations, Divalent; C419385:L-type calcium channel alpha(1C); D002118:Calcium