Possible Mechanism of Calcium-Dependent Block of L-Type Calcium Channel by Glacontryphan-M

Abstract

Calcium channel-blocking peptide toxins have a large therapeutic potential. Electrostatic interactions play a major role in peptide-channel recognition. For example, homology models are proposed with cationic conotoxins blocking the cation-attractive outer vestibule of Cav2.2 (Lewis et al., 2012; Wang et al., 2016) and Nav1.4 (Korkosh et al, 2014) channels. Paradoxically, the Cav1.2 channel is blocked by an anionic peptide, 11-residue glacontryphan-M containing two γ-carboxyglutamates, and the block is calcium-dependent (Hansson et al., 2004; Grant et al., 2004). Here we employed Monte-Carlo energy minimizations to explore how glacontryphan-M can block Cav1.2. The cryo-EM structure of Cav1.1 and the X-ray structure of contryphan-Vn were used to model Cav1.2 and glacontryphan-M. Computations predicted that the selectivity-filter region can accommodate glacontryphan-M. Two calcium ions bound to γ-carboxyglutamates bridged the toxin to selectivity-filter carboxylates. We further computed energy profiles of glacontryphan-M pulled through the channel to the selectivity-filter region. The only pathway where the toxin did not encounter sterical or electrostatic repulsion from the channel was that through the fenestration between extracellular loops in domains III and IV. Energetically preferable pathway for a calcium ion from the extracellular space to the selectivity filter glutamates in the toxin-free channel involved several calcium binding sites where the ion is coordinated by at least two carboxylates. In the toxin-bound channel, calcium ions immobilized between γ-carboxyglutamates and the outer carboxylates repelled the incoming calcium ion, but did not move towards the selectivity-filer glutamates. Thus, the pore of cationic channels can be blocked by permeant ions bound to either small-molecule cation-attractive blockers (e.g. Tikhonov & Zhorov, 2017) or to anionic peptide toxins. Supported by RFBR (17-04-00549) and NSERC (RGPIN-2014-04894).