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J. Biol. Chem., Vol. 282, Issue 6, 3478-3486, February 9, 2007

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An Amino Acid Outside the Pore Region Influences Apamin Sensitivity in Small Conductance Ca²⁺-activated K⁺ Channels^*

From the Laboratory of Molecular Pharmacology, Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, United Kingdom

Small conductance calcium-activated potassium channels (SK, K_Ca) are a family of voltage-independent K⁺ channels with a distinct physiology and pharmacology. The bee venom toxin apamin inhibits exclusively the three cloned SK channel subtypes (SK1, SK2, and SK3) with different affinity, highest for SK2, lowest for SK1, and intermediate for SK3 channels. The high selectivity of apamin made it a valuable tool to study the molecular makeup and function of native SK channels. Three amino acids located in the outer vestibule of the pore are of particular importance for the different apamin sensitivities of SK channels. Chimeric SK1 channels, enabling the homomeric expression of the rat SK1 (rSK1) subunit and containing the core domain (S1–S6) of rSK1, are apamin-insensitive. By contrast, channels formed by the human orthologue human SK1 (hSK1) are sensitive to apamin. This finding hinted at the involvement of regions beyond the pore as determinants of apamin sensitivity, because hSK1 and rSK1 have an identical amino acid sequence in the pore region. Here we investigated which parts of the channels outside the pore region are important for apamin sensitivity by constructing chimeras between apamin-insensitive and -sensitive SK channel subunits and by introducing point mutations. We demonstrate that a single amino acid situated in the extracellular loop between the transmembrane segments S3 and S4 has a major impact on apamin sensitivity. Our findings enabled us to convert the hSK1 channel into a channel that was as sensitive for apamin as SK2, the SK channel with the highest sensitivity.

Received for publication, July 31, 2006 , and in revised form, November 20, 2006.

^* This work was supported by a Wellcome Prize studentship (to T. F.) and a Wellcome Trust Senior Research fellowship (to M. S.). 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 Present address: Laboratorium voor Fysiologie, Katholieke Universiteit Leuven, Herestraat 49, B-3000, Leuven, Belgium.

² To whom correspondence should be addressed. Tel.: 44-20-7679-7244; Fax: 44-20-7679-7245; E-mail: M.Stocker{at}ucl.ac.uk.

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