Structural basis for toxin resistance of β4-associated calcium-activated potassium (BK) channels

The functional diversity of large conductance Ca2+ - and voltage-dependent K+ (BK) channels arises mainly from co-assembly of the pore-forming mSlo alpha subunits with four tissue-enriched auxiliary beta subunits. The structural basis of the interaction between alpha subunits with beta subunits is not well understood. Using computational and experimental methods, we demonstrated that four mSlo turrets decentralized distally from the channel pore to provide a wide open conformation and that the mSlo and h beta 4 subunits together formed a helmet containing three basic residues (Lys-120, Arg-121, and Lys-125), which impeded the entry of charybdotoxin ( ChTX) by both the electrostatic interaction and limited space. In addition, the tyrosine insert mutant (in100Y) showed 56% inhibition, with a K-d = 17 nM, suggesting that the h beta 4 lacks an external ChTX-binding site (Tyr-100). We also found that mSlo had an internal binding site (Tyr-294) in the alpha subunits that could permanently block 15% of mSlo+h beta 4 currents in the presence of 100 nM ChTX. These findings provide a better understanding of the diverse interactions between alpha and beta subunits and will improve the design of channel inhibitors.