Both transmembrane domains of BK ?1 subunits are essential to confer the normal phenotype of ?1-containing BK channels.

ABSTRACT: Voltage/Ca²?(i)-gated, large conductance K+ (BK) channels result from tetrameric association of ? (slo1) subunits. In most tissues, BK protein complexes include regulatory ? subunits that contain two transmembrane domains (TM1, TM2), an extracellular loop, and two short intracellular termini. Four BK ? types have been identified, each presenting a rather selective tissue-specific expression profile. Thus, BK ? modifies current phenotype to suit physiology in a tissue-specific manner. The smooth muscle-abundant BK ?1 drastically increases the channel's apparent Ca²?(i) sensitivity. The resulting phenotype is critical for BK channel activity to increase in response to Ca2+ levels reached near the channel during depolarization-induced Ca2+ influx and myocyte contraction. The eventual BK channel activation generates outward K+ currents that drive the membrane potential in the negative direction and eventually counteract depolarization-induced Ca2+ influx. The BK ?1 regions responsible for the characteristic phenotype of ?1-containing BK channels remain to be identified. We used patch-clamp electrophysiology on channels resulting from the combination of smooth muscle slo1 (cbv1) subunits with smooth muscle-abundant ?1, neuron-abundant ?4, or chimeras constructed by swapping ?1 and ?4 regions, and determined the contribution of specific ?1 regions to the BK phenotype. At Ca2+ levels found near the channel during myocyte contraction (10 µM), channel complexes that included chimeras having both TMs from ?1 and the remaining regions ("background") from ?4 showed a phenotype (V(half), ?(act), ?(deact)) identical to that of complexes containing wt ?1. This phenotype could not be evoked by complexes that included chimeras combining either ?1 TM1 or ?1 TM2 with a ?4 background. Likewise, ? "halves" (each including ?1 TM1 or ?1 TM2) resulting from interrupting the continuity of the EC loop failed to render the normal phenotype, indicating that physical connection between ?1 TMs via the EC loop is also necessary for proper channel function.

SUBMITTER: Kuntamallappanavar G 

PROVIDER: S-EPMC4183656 | biostudies-literature | 2014

REPOSITORIES: biostudies-literature