alpha-helical structural elements within the voltage-sensing domains of a K+ channel

Voltage-gated K+ channels are tetramers with each subunit containing six (S 1-S6) putative membrane spanning segments. The fifth through sixth transmem brane segments (S5-S6) from each of four subunits assemble to form a centra l pore domain. A growing body of evidence suggests that the first four segm ents (S1-S4) comprise a domain-like voltage-sensing structure. While the to pology of this region is reasonably well defined, the secondary and tertiar y structures of these transmembrane segments are not. To explore the second ary structure of the voltage-sensing domains, we used alanine-scanning muta genesis through the region encompassing the first four transmembrane segmen ts in the drk1 voltage-gated K+ channel. We examined the mutation-induced p erturbation in gating free energy for periodicity characteristic of alpha-h elices. Our results are consistent with at least portions of S1, S2, S3, an d S4 adopting alpha-helical secondary structure. In addition, both the S1-S 2 and S3-S4 linkers exhibited substantial helical character: The distributi on of gating perturbations for S1 and S2 suggest that these two helices int er-act primarily with two environments. In contrast, the distribution of pe rturbations for S3 and S4 were more complex, suggesting that the latter two helices make more extensive protein contacts, possibly interfacing directl y with the shell of the pore domain.