Endogenous disulfide cross-linking between α R17C and a native Cys in pWT mβ3a. Either pWTα or R17C α was coexpressed with pWT mβ3a in HEK cells, and the channel complexes were extracted and denatured in SDS. The blots were developed with anti-BK α antibody, which reacts with both α (130 kD) and the cross-linked mβ3a dimer (∼160 kD).