Role of the C-terminus of the high-conductance calcium-activated potassium channel in channel structure and function

The role of ion channels in cell physiology is regulated by processes occurring after protein biosynthesis, which are critical for both channel function and targeting of channels to appropriate cell compartments. Here we apply biochemical and electrophysiological methods to investigate the role of the high-conductance, calcium-activated potassium (Maxi-K) channel C-terminal domain in channel tetramerization, association with the β1 subunit, trafficking of the channel complex to the cell surface, and channel function. No evidence for channel tetramerization, cell surface expression, or function was observed with Maxi-K_1-323, a construct truncated three residues after the S₆ transmembrane domain. However, Maxi-K_1-343 and Maxi-K_1-441 are able to form tetramers and to associate with the β1 subunit. Maxi-K_1-343,-β1 and Maxi-K_1-441- β1 complexes are efficiently targeted to the cell surface and cannot be pharmacologically distinguished from full-length channels in binding experiments but do not form functional channels. Maxi-K_1-651 forms tetramers and associates with β1; however, the complex is not present at the cell surface, but is retained intracellularly. Maxi-K_1-651 surface expression and channel function can be fully rescued after coexpression with its C-terminal complement, Maxi-K_652-1113. However coexpression of Maxi-K_1-343 and Maxi-K_1-441 with their respective C-terminal complements did not rescue channel function. Together, these data demonstrate that the domain(s) in the Maxi-K channel necessary for formation of tetramers, coassembly with the β1 subunit, and cell surface expression resides within the S₀-S₆ linker domain of the protein, and that structural constraints within the gating ring in the C-terminal region can regulate trafficking and function of constructs truncated in this region.