Intracellular cross-linking of BK α S0 to S1–S5 in the absence and the presence of β1. Expression, surface biotinylation, protein extraction, SDS-PAGE, Western blotting, and detection with an anti-HA antibody were as described previously (Liu et al., 2008a,b, 2010; Wu et al., 2009, 2013). The conditions of induction of disulfide bond formation with QPD are described in Materials and methods. (A) HRV-3C cleavage pattern. Only fragments containing the N-terminal HA epitope were detected. (B–D) Anti-HA immunoblots showing QPD-induced cross-linking between indicated Cys in S0 and S1–S5 in the absence (B and C) and presence (D) of pWT β1. (B) A highly cross-linked extracellular pair of Cys served as a positive control for the capture, cleavage, and detection of BK α. The calculated extents of cross-linking in the examples are given below the blots. (E) Mean extents (±SEM) of QPD-induced cross-linking. The wide bars are the means in the absence of β1 and are color-coded for the three residues in the intracellular flank of S0 substituted with Cys. The superimposed narrow yellow bars are the extents of cross-linking in the presence of β1. Where the extent of cross-linking is 2% or less, it is represented as 2% for visibility.