Fig. 1.

Modularity of Kv channels. (A) Structural modularity of Kv channels. (Left) The crystal structure of the Kv1.2-Kv2.1 chimeric channel (Protein Data Bank ID code 2R9R) (28), a member of the Shaker family of eukaryotic Kv channels, showing the four well separated voltage-sensor modules, S1–S4 (red, magenta, green, and gray), surrounding a pore module (blue). (Right) A single subunit of the Kv1.2-Kv2.1 chimera with the sensor and pore depicted in red and blue. The yellow spheres represent K⁺ ions. All structures were rendered using YASARA (http://www.yasara.org). (B) Representations of tetrameric assemblies with different numbers of voltage sensors (from left to right: four, three, two, one, and zero). (C) Assembly modularity of KvLm: heterotetramers assembled by coexpressing FL and PM polypeptides in a cell-free expression system. The tetramers were separated in a 7.5% Tris-HCl SDS polyacrylamide gel. The polypeptides were labeled with L-[³⁵S]methionine and visualized by autoradiography. KvLm DNA ratios in the expression system were (FL∶PM): 1∶1 and 1∶0. Empty lanes were left between samples to avoid cross-contamination when extracting tetrameric protein assemblies from the gel. The cartoons illustrate the combinations of subunits in the tetramers. Note that FL₂PM₂ can adopt two possible configurations (i.e., FL subunits adjacent to each other or diagonally opposed); only one is depicted here for clarity.