Periodic perturbations in Shaker K+ channel gating kinetics by deletions in the S3-S4 linker

Upon depolarization positive charges contained in the transmembrane segment S4 of voltage-dependent channels are displaced from the cytoplasmic to the external milieu, This charge movement leads to channel opening. In Shaker K+ channels four positively charged arginines in the S4 domain are transfer red from the internal to the external side of the channel during activation . The distance traveled by the S4 segment during activation is unknown, but large movements should be constrained by the S3-S4 linker. Constructing de letion mutants, we show that the activation time constant and the midpoint of the voltage activation curve of the Shaker K+ channel macroscopic curren ts becomes a periodic function of the S3-S4 linker length for linkers short er than 7 aa residues. The periodicity is that typical of a-helices. Moreov er, a linker containing only 3 aa is enough to recover the wild-type phenot ype. The deletion method revealed the importance of the S3-S4 linker in det ermining the channel gating kinetics and indicated that the a-helical natur e of S4 extends toward its N terminus. These results support the notion tha t a small displacement of the S4 segment suffices to displace the four gati ng charges involved in channel opening.