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Position and Role of the BK Channel Subunit S0 Helix Inferred from Disulfide Crosslinking

¹ Division of Cardiology, Department of Medicine; ² Division of Experimental Therapeutics, Department of Medicine; ³ Center for Molecular Recognition, Departments of Biochemistry, Physiology, and Neurology; ⁴ Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, NY 10032

Correspondence to A. Karlin: ak12{at}columbia.edu; or S.O. Marx: sm460{at}columbia.edu

The position and role of the unique N-terminal transmembrane (TM) helix, S0, in large-conductance, voltage- and calcium-activated potassium (BK) channels are undetermined. From the extents of intra-subunit, endogenous disulfide bond formation between cysteines substituted for the residues just outside the membrane domain, we infer that the extracellular flank of S0 is surrounded on three sides by the extracellular flanks of TM helices S1 and S2 and the four-residue extracellular loop between S3 and S4. Eight different double cysteine–substituted alphas, each with one cysteine in the S0 flank and one in the S3–S4 loop, were at least 90% disulfide cross-linked. Two of these alphas formed channels in which 90% cross-linking had no effect on the V₅₀ or on the activation and deactivation rate constants. This implies that the extracellular ends of S0, S3, and S4 are close in the resting state and move in concert during voltage sensor activation. The association of S0 with the gating charge bearing S3 and S4 could contribute to the considerably larger electrostatic energy required to activate the BK channel compared with typical voltage-gated potassium channels with six TM helices.

Abbreviations used in this paper: BK, large-conductance, voltage- and calcium-activated potassium; HRV, human rhinovirus; PDI, protein disulfide isomerase; pWT, pseudo-wild-type; QPD, quaternary piperazinium diamide; TM, transmembrane; WT, wild-type.

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