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J. Biol. Chem., Vol. 282, Issue 33, 24302-24309, August 17, 2007

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Molecular Architecture and Divalent Cation Activation of TvoK, a Prokaryotic Potassium Channel^*

From the Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229

RCK (regulator of conductance of potassium) domains form a family of ligand-binding domains found in many prokaryotic K⁺ channels and transport proteins. Although many RCK domains contain an apparent nucleotide binding motif, some are known instead to bind Ca²⁺, which can then facilitate channel opening. Here we report on the molecular architecture and ligand activation properties of an RCK-containing potassium channel cloned from the prokaryote Thermoplasma volcanium. This channel, called TvoK, is of an apparent molecular mass and subunit composition that is consistent with the hetero-octameric configuration hypothesized for the related MthK (Methanobacterium thermoautotrophicum potassium) channel, in which four channel-tethered RCK domains coassemble with four soluble (untethered) RCK domains. The expression of soluble TvoK RCK subunits arises from an unconventional UUG start codon within the TvoK gene; silent mutagenesis of this alternative start codon abolishes expression of the soluble form of the TvoK RCK domain. Using single channel recording of purified, reconstituted TvoK, we found that the channel is activated by Ca²⁺ as well as Mg²⁺, Mn²⁺, and Ni²⁺. This non-selective divalent activation is in contrast with the activation properties of MthK, which is selectively activated by Ca²⁺. Transplantation of the TvoK RCK domain into MthK generates a channel that can be activated by Mg²⁺, illustrating that the Mg²⁺ binding site is likely contained within the RCK domain. We present a working hypothesis for TvoK gating in which the binding of either Ca²⁺ or Mg²⁺ can contribute 5 kcal/mol toward stabilization of the open conformation of the channel.

Received for publication, May 2, 2007 , and in revised form, June 20, 2007.

^* This work was supported in part by National Institutes of Health Grant GM68523 (to B. S. R.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Physiology, MC7756, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Dr., San Antonio, TX 78229-3900. Tel.: 210-567-4342; Fax: 210-567-4410; E-mail: rothberg{at}uthscsa.edu.

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