Large conductance voltage- and calcium-dependent K+ channel, a distinct member of voltage-dependent ion channels with seven N-terminal transmembrane segments (SO-S6), an extracellular N terminus, and an intracellular (S9-S10) C terminus

Large conductance voltage-and Ca2+-dependent K+ (MaxiK) channels show sequence similarities to voltage-gated ion channels. They have a homologous S1-S6 region, but are unique at the N and C termini. At the C terminus, MaxiK channels have four additional hydrophobic regions (S7-S10) of unknown topology. At the N terminus, we have recently proposed a new model where MaxiK channels hare an additional transmembrane region (S0) that confers beta subunit regulation. Using transient expression of epitope tagged MaxiK channels, in vitro translation, functional, and ''in vivo'' reconstitution assays, we now show that MaxiK channels have seven transmembrane segments (S0-SG) at the N terminus and a S1-S6 region that folds in a similar may as in voltage-gated ion channels, Further, our results indicate that hydrophobic segments S9-S10 in the C terminus are cytoplasmic and unequivocally demonstrate that SO forms an additional transmembrane segment leading to an exoplasmic N terminus.