Fig. 4. The α6 helix of the Kv3.1 T1 domain forms a second cuff below the S4/S5 linker, stabilized by inter- and intrasubunit interactions.

a Cartoon representation of the Kv1.2-2.1 tetramer (light grey) with one protomer highlighted in dark grey. S4/S5 linker is represented in magenta, S6 is coloured in pale yellow and α6 of the T1 domain is shown in aquamarine blue. b Cartoon representation of the Kv3.1a tetramer, illustrating the unique orientation of α6 (teal blue) of the T1 domain. Colour-coding of other elements is the same as in (a). c Inset from b with an enlarged view of the α6 helix and its interactions with residues in the S4/S5 linker (magenta) and the lower gate formed by S6T (pale yellow). d Cytoplasmic view of the tetrameric “α6 gating cuff” (shown as teal cartoon), located below the gating machinery formed by S4/S5 (magenta) and S6T (yellow). Only helix α6 of T1 is shown for clarity. e Inset from d, detailing inter-subunit interactions between α6 helices from adjacent subunits and interactions between α6 and S6T. Zn²⁺ ions are shown as grey spheres.