The Journal of General Physiology
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Published online January 29, 2007
doi:10.1085/jgp.200609655
The Journal of General Physiology, Vol. 129, No. 2, 109-120
The Rockefeller University Press, 0022-1295 $30.00
© 2007 Li et al.
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ARTICLE

 Gating and Inward Rectifying Properties of the MthK K+ Channel with and without the Gating Ring



Yang Li1, Ian Berke2, Liping Chen1, and Youxing Jiang1

1 Department of Physiology, University of Texas Southwestern Medical Center, Dallas, TX 75390
2 Department of Molecular Medicine, Cornell University, Ithaca, NY 14853

Correspondence to Youxing Jiang: youxing.jiang{at}utsouthwestern.edu

In MthK, a Ca2+-gated K+ channel from Methanobacterium thermoautotrophicum, eight cytoplasmic RCK domains form an octameric gating ring that controls the intracellular gate of the ion conduction pore. The binding of Ca2+ ions to the RCK domains alters the conformation of the gating ring, thereby opening the gate. In the present study, we examined the Ca2+- and pH-regulated gating and the rectifying conduction properties of MthK at the single-channel level. The open probability (Po) of MthK exhibits a sigmoidal relationship with intracellular [Ca2+], and a Hill coefficient >1 is required to describe the dependence of Po on [Ca2+], suggesting cooperative Ca2+ activation of the channel. Additionally, intracellular Ca2+ also blocks the MthK pore in a voltage-dependent manner, rendering an apparently inwardly rectifying I-V relation. Intracellular pH has a dual effect on MthK gating. Below pH 7.5, the channel becomes insensitive to Ca2+. This occurs because the gating ring is structurally unstable at this pH and tends to disassemble (Ye, S., Y. Li, L. Chen, and Y. Jiang. 2006. Cell. 126:1161–1173). In contrast, above pH 7.5, a further increase in pH shifts the Po-[Ca2+] relation towards a lower Ca2+ concentration, augments Po at saturating [Ca2+], and activates the channel even in the absence of Ca2+. Channel activity is marked by bursts of rapid openings and closings separated by relatively longer interburst closings. The duration of interburst closing and the burst length are highly Ca2+ and pH dependent, whereas the kinetics of intraburst events is Ca2+ and pH independent. The rapid intraburst openings and closings are also observed with the isolated MthK pore lacking the attached intracellular gating ring. The fast kinetic events, independent of both Ca2+ and pH, therefore appear to be determined by processes occurring within the ion conduction pore, whereas the slow events reflect the gating process controlled by Ca2+ and pH through the gating ring.

Abbreviations used in this paper: CTX, charybdotoxin; DM, n-decyl-ß-D-maltoside; NMG, N-methyl-D-glucamine.


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