The β subunit of the high-conductance calcium-activated potassium channel contributes to the high-affinity receptor for charybdotoxin

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Abstract
Transient expression of either alpha or alpha + beta subunits of the high-conductance Ca2+-activated K+ (maxi-K) channel has been achieved in COS-1 cells. Expression has been studied using charybdotoxin (ChTX), a peptidyl inhibitor that binds in the pore on the alpha subunit. Although some properties of monoiodotyrosine-ChTX (125I-ChTX) binding to membranes derived from each type of transfected cells appear to be identical, other parameters of the binding reaction are markedly different. Under low ionic strength conditions, the affinity constant for 125I-ChTX measured under equilibrium binding conditions is increased ca. 50-fold in the presence of the beta subunit. The rate constant for 125I-ChTX association is enhanced ca. 5-fold, whereas the dissociation rate constant is decreased more than 7-fold when the beta subunit is present. These data indicate that functional coassembly of maxi-K channel subunits can be obtained in a transient expression system, and that the beta subunit has profound effects on 125I-ChTX binding. We postulate that certain negatively charged residues in the large extracellular loop of beta attract the positively charged 125I-ChTX to its binding site on alpha through electrostatic interactions, and account for effects observed on ligand association kinetics. Moreover, another residue(s) in the loop of beta must contribute to stabilization of the toxin-bound state, either by a direct interaction with toxin, or through an allosteric effect on the alpha subunit. Certain regions in the extracellular loop of the beta subunit may be in close proximity to the pore of the channel, and could play an important role in maxi-K channel function.