THE P-REGION AND S6 OF KV3.1 CONTRIBUTE TO THE FORMATION OF THE ION CONDUCTION PATHWAY

The loop between transmembrane regions S5 and S6 (P-region) of voltage -gated K+ channels has been proposed to form the ion-conducting pore, and the internal part of this segment is reported to be responsible fo r ion permeation and internal tetraethylammonium (TEA) binding. The tw o T-cell K+ channels, Kv3.1 and Kv1.3, with widely divergent pore prop erties, differ by a single residue in this internal P-region, leucine 401 in Kv3.1 corresponding to valine 398 in Kv1.3. The L401V mutation in Kv3.1 was created with the anticipation that the mutant channel wou ld exhibit Kvl.3-like deep-pore properties. Surprisingly, this mutatio n did not alter single channel conductance and only moderately enhance d internal TEA sensitivity, indicating that residues outside the P-reg ion influence these properties. Our search for additional residues was guided by the model of Durell and Guy, which predicted that the C-ter minal end of S6 formed part of the K+ conduction pathway. In this segm ent, the two channels diverge at only one position, Kv3.1 containing M 430 in place of leucine in Kv1.3. The M430L mutant of Kv3.1 exhibited permeant ion- and voltage-dependent flickery outward single channel cu rrents, with no obvious changes in other pore properties. Modification of one or more ion-binding sites located in the electric field and po ssibly within the channel pore could give rise to this type of channel flicker.