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Address correspondence to Fred Sigworth, Dept. of Cellular and Molecular Physiology, Yale University, 333 Cedar Street, New Haven, CT 06520-8026. Fax: (203) 785-4951. email: fred.sigworth{at}yale.edu
For structural studies it would be useful to constrain the voltage sensor of a voltage-gated channel in its deactivated state. Here we consider one Shaker potassium channel mutant and speculate about others that might allow the channel to remain deactivated at zero membrane potential. Ionic and gating currents of F370C Shaker, expressed in Xenopus oocytes, were recorded in patches with internal application of the methanethiosulfonate reagent MTSET. It appears that the voltage dependence of voltage sensor movement is strongly shifted by reaction with internal MTSET, such that the voltage sensors appear to remain deactivated even at positive potentials. A disadvantage of this construct is that the rate of modification of voltage sensors by MTSET is quite low, 
0.17 mM–1·s–1 at –80 mV, and is expected to be much lower at depolarized potentials.
Key Words: patch clamp • gating current • MTSET • cysteine • S4
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