Incremental reductions of positive charge within the S4 region of a voltage-gated K+ channel result in corresponding decreases in gating charge.

The S4 region of voltage-dependent ion channels is involved in the voltage-sensing mechanism of channel activation. Previous studies in fast inactivating channels have used non-steady-state measurements and thus have not allowed the quantitative assessment of activation parameters. Using site-directed mutagenesis and voltage-clamp recordings in a noninactivating channel (RCK1), we demonstrate that stepwise reductions of positive charge within the S4 region correlate with a progressive decrease in the channel's overall gating valence. In addition to testing for electrostatic behavior of individual charged residues, our study was designed to probe nonelectrostatic influences on charge movement. We provide evidence that individual charged residues behave differentially in response to the electric field, so that purely electrostatic influences cannot fully account for the gating movement of certain charges.