A neuronal β subunit (KCNMB4) makes the large conductance, voltage- and Ca2+-activated K+ channel resistant to chavybdotoxin and iberiotoxin

Large conductance voltage and Ca2+-activated K+ (MaxiK) channels couple intracellular Ca2+ with cellular excitability. They are composed of a pore-forming alpha subunit and modulatory beta subunits. The pore blockers charybdotoxin (CTx) and iberiotoxin (IbTx), at nanomolar concentrations, have been invaluable in unraveling MaxiK channel physiological role in vertebrates. However in mammalian brain, CTx-insensitive MaxiK channels have been described [Reinhart, P. H., Chung, S. & Levitan, I. B. (1989) Neuron 2, 1031-1041], but their molecular basis is unknown. Here we report a human MaxiK channel beta-subunit (beta 4), highly expressed in brain, which renders the MaxiK channel alpha-subunit resistant to nanomolar concentrations of CTx and IbTx. The resistance of MaxiK channel to toxin black, a phenotype conferred by the beta 4 extracellular loop, results from a dramatic (approximate to 1,000 fold) slowdown of the toxin association. However once bound, the toxin block is apparently irreversible. Thus. unusually high toxin concentrations and long exposure times are necessary to determine the role of "CTx/IbTx-insensitive" MaxiK channels formed by alpha + beta 4 subunits.