Voltage-gated potassium channels are proteins composed of four subunits consisting of six membrane-spanning segments S1-S6, with S4 as the voltage sensor. The region between S5 and S6 forms the potassium-selective ion-conducting central ␣-pore. Recent studies showed that mutations in the voltage sensor of the Shaker channel could disclose another ion permeation pathway through the voltage-sensing domain (S1-S4) of the channel, the -pore. In our studies we used the voltage-gated hKv1.3 channel, and the insertion of a cysteine at position V388C (Shaker position 438) generated a current through the ␣-pore in high potassium outside and an inward current at hyperpolarizing potentials carried by different cations like Na ؉ , Li ؉ , Cs ؉ , and NH 4 ؉ . The observed inward current looked similar to the -current described for the R1C/S Shaker mutant channel and was not affected by some pore blockers like charybdotoxin and tetraethylammonium but was inhibited by a phenylalkylamine blocker (verapamil) that acts from the intracellular side. Therefore, we hypothesize that the hKv1.3_V388C mutation in the P-region generated a channel with two ion-conducting pathways. One, the ␣-pore allowing K ؉ flux in the presence of K ؉ , and the second pathway, the -pore, functionally similar but physically distinct from the -pathway. The entry of this new pathway (-pore) is presumably located at the backside of Y395 (Shaker position 445), proceeds parallel to the ␣-pore in the S6 -S6 interface gap, ending between S5 and S6 at the intracellular side of one ␣-subunit, and is blocked by verapamil.The human voltage-gated potassium channel Kv1.3 is a member of the Shaker-related potassium channel family (1) and can be distinguished from the other members because of its characteristic C-type inactivation (2, 3). With the crystal structure data on the hKv1.2 channel (4), another member of the Shaker-related family, we have a good picture about the architecture of the classical voltage-gated potassium channels. The channels consist of four subunits composed of six membranespanning segments, termed S1-S6. The region between the segments S5 and S6, with the pore helix (P) in-between (S5-P-S6), forms, together with three other subunits, the central ion conduction pathway, the ␣-pore, which is potassium-selective. The S4 segment is known as the voltage sensor, and the helices S1-S4 form the voltage-sensing domain, which surrounds the pore domain and controls the gates (1, 5). Recent studies showed that mutations in the voltage sensor of the Shaker channel or of the voltage-gated sodium channel Na v 1.2 can disclose another ion permeation pathway, the -pore, a pathway through the voltage-sensing domain (S1-S4) of the channel (5-9). The substitution of the first S4 arginine (Arg-1) in the Shaker channel with smaller amino acids like serine or cysteine created a pathway for a leak current selective for monovalent cations, the -pore. This conductance pathway in the Shaker channel was located distinct from the central K ϩ -conducting ␣-pore and ran throu...
