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¹ Department of Physiology and Membrane Biology, University of California, Davis, Davis, CA 95616
² Department of Biochemistry, Howard Hughes Medical Institute/Brandeis University, Waltham, MA 02454

Correspondence to Crina M. Nimigean: cnimigean{at}ucdavis.edu

MthK is a calcium-gated, inwardly rectifying, prokaryotic potassium channel. Although little functional information is available for MthK, its high-resolution structure is used as a model for eukaryotic Ca²⁺-dependent potassium channels. Here we characterize in detail the main gating characteristics of MthK at the single-channel level with special focus on the mechanism of Ca²⁺ activation. MthK has two distinct gating modes: slow gating affected mainly by Ca²⁺ and fast gating affected by voltage. Millimolar Ca²⁺ increases MthK open probability over 100-fold by mainly increasing the frequency of channel opening while leaving the opening durations unchanged. The Ca²⁺ dose–response curve displays an unusually high Hill coefficient (n = 8), suggesting strong coupling between Ca²⁺ binding and channel opening. Depolarization affects both the fast gate by dramatically reducing the fast flickers, and to a lesser extent, the slow gate, by increasing MthK open probability. We were able to capture the mechanistic features of MthK with a modified MWC model.

Abbreviations used in this paper: BK, large conductance Ca²⁺-activated K⁺; CTX, charybdotoxin; MWC, Monod-Wyman-Changeux; RCK, regulator of K⁺ conductance.

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