Determinant for β -subunit Regulation in High-Conductance Voltage-Activated and Ca2+-sensitive K+Channels: An Additional Transmembrane Region at the N Terminus

The pore-forming α subunit of large conductance voltage- and Ca2+-sensitive K (MaxiK) channels is regulated by a β subunit that has two membrane-spanning regions separated by an extracellular loop. To investigate the structural determinants in the pore-forming α subunit necessary for β -subunit modulation, we made chimeras constructs between a human MaxiK channel and the Drosophila homologue, which we show is insensitive to β -subunit modulation, and analyzed the topology of the α subunit. A comparison of multiple sequence alignments with hydrophobicity plots revealed that MaxiK channel α subunits have a unique hydrophobic segment (S0) at the N terminus. This segment is in addition to the six putative transmembrane segments (S1-S6) usually found in voltage-dependent ion channels. The transmembrane nature of this unique S0 region was demonstrated by in vitro translation experiments. Moreover, normal functional expression of signal sequence fusions and in vitro N-linked glycosylation experiments indicate that S0 leads to an exoplasmic N terminus. Therefore, we purpose a new model where MaxiK channels have a seventh transmembrane segment at the N terminus (S0). Chimeric exchange of 41 N-terminal amino acids, including S0, from the human MaxiK channel to the Drosophila homologue transfers β -subunit regulation to the otherwise unresponsive Drosophila channel. Both the unique S0 region and the exoplasmic N terminus are necessary for this gain of function.