Mouse BK α. (A) The N-terminal 333 residues of BK α are shown in bold letters. The predicted seven TM helices, S0–S6, and the pore helix are underlined and labeled. Predicted extracellular residues are shown in upper case and the rest in lower case. The extracellular residues that we mutated to Cys are shown in reverse contrast. These Cys substitutions were made in a pseudo-wild-type (pWT) construct, the sequence of which was altered as indicated in boxes above the wild-type sequence (see Materials and methods). We used ClustalW to align BK α residues 106–333, which contain S1–S6, with K_vAP residues 31–260 and K_v1.2 residues 97–387. (B) Scheme of the threading of BK α through the membrane. The locations of the changes we made in wild-type α to generate pseudo-wild-type α with a HRV-3C protease cleavage site are shown. The extracellular regions flanking S0–S6, in which Cys were substituted, are indicated by filled rectangles.