A Model Of The Inner Pore Of Ca Channels In The Open State

Known open state structures of K channels do not provide a useful template for open Ca channels, because the latter do not contain the hinge residues Gly and Pro. We, therefore, began with the closed KcsA structure (Doyle et al, Science 280:69, 1998) and sequentially outwardly displaced amino acids near the bundle crossing (Thr107-Ala111) and optimized the structures (restrained minimization with distance constraints). We then populated open channel structures with amino acid residues of T and P/Q channels and re-optimized. The Ca channel structures with openings of ∼11 Å were similar and allowed both symmetrical TEA and verapamil to enter and block from inside. However, simulations were in disagreement with the MTSET accessibility data for the P/Q channel (Zhen et al, JGP 126:193, 2005). Noting that amino acid residues near C-terminal ends of S6 helices of Ca channels have side chains facing the inner pore that are quite different from those in K channels, we speculated that these segments might contain intra-molecular deformations that lead to the reorientation of their side chains. We modeled these deformations by π-bulges, which produced wide turns, containing an additional amino acid residue. The very conserved Asn residues of Ca channels initiated the formation of π-bulges in the direction of the C-ends in all 4 S6 α-helices. Introduction of π-bulges achieved agreement between amino acid residues predicted to face the pore and MTSET accessibility data. Formation of π-bulges would be expected to stabilize the open state of the Ca channel, and MTSET modification of single cysteines at the C-ends of S6's could produce physical occlusion of the inner pore, i.e. full block of Ca current as observed experimentally. Supported by RO1HL065680.