A Point Mutation Causing Episodic Ataxia Reveals Functional Link between Voltage Sensor and Selectivity Filter in Shaker Kv Channels

Abstract

Episodic ataxia (EA) constitutes itself as sporadic episodes of vertigo, migraine and ataxia and is occasionally associated with epilepsy or hemiplegic migraine. EA type 1 is caused by mutations in KCNA1. We investigated the biophysical implications of one of these mutations, F244C, in the drosophila analogue the Shaker Kv channel (numbering according to Shaker). In spite of being located in the first transmembrane helix (S1), the mutation had drastic effects on the gating currents. The gating charge-voltage relation (QV) was shifted to more depolarized potentials. Although other EA1 mutations had been linked to electromechanical coupling, this was not the case for F244, as also the conductance-voltage relations of different amino acid replacements were proportionally shifted to more depolarized potentials. Instead, we found a stabilization of the second arginine in S4 in the intermediate state of the voltage sensor dependent on the hydrophobicity of the replacement residue.One striking feature of the F244C mutant was that it led to drastic development of leak in the W434F background. This leak current passed though the main ion conducting pore, and we were able to link it to reduced C-type inactivation. We established that the C-type inactivation of the neighboring subunit was effected. F244 therefore directly links the voltage sensor movement of one subunit with the selectivity filter of its neighboring one.This work is supported by grants of the Canadian Institutes of Health Research and the Canada Research Chairs.