A small viral potassium ion channel with an inherent inward rectification

Abstract

Some algal viruses have coding sequences for K+ channels with structural and functional characteristics of pore modules of complex K+ channels. Here we exploit the immense structural diversity of natural channel orthologs to discover new basic principles of structure/function correlates in K+ channels. The comparative analysis of three similar K+ channels with monomer sizes [≤] 86 amino acids (AA) shows that one channel (Kmpv1) generates an ohmic conductance in HEK293 cells while the other two channels (KmpvSP1, KmpvPL1) exhibit typical features of canonical Kir channels. Like Kir channels, the rectification of the viral channels is a function of the K+ driving force. Reconstitution of KmpvSP1 and KmpvPL1 in planar lipid bilayers showed rapid channel fluctuations only at voltages negative of the K+ reversal voltage. This rectification was maintained in KCl buffer with 1 mM EDTA, which excludes blocking cations as the source of rectification. This means that rectification of the viral channels must be, unlike Kir channels, an inherent property of the channel proteins. The structural basis for rectification was investigated by a chimera between rectifying and non-rectifying channels as well as point mutations, which made the rectifying channels similar to the ohmic conducting channel. The results of these experiments exclude the domain, which connects the two transmembrane helixes and which includes the pore helix and the selectivity filter, as playing a major role in rectification; inward rectification must be conferred by the transmembrane domains. The finding that a swapping of the AA, which is typical for the two inward rectifiers, with the respective AA from Kmpv1 did not compromise rectification suggests that tertiary or quaternary structural interactions are responsible for this type of gating.