Molecular and functional targets of methionine oxidation in large-conductance calcium-activated potassium channels.

摘要

The activity of the large-conductance calcium-activated BK ion channel, also referred to as Slo1 or MaxiK, plays an important role in limiting neurotransmitter release, determining the length and frequency of the action potential, and relaxing smooth muscle. The opening of this channel in response to depolarization and/or intracellular calcium allows for selective conductance of potassium ions to the extracellular milieu, thereby driving the cellular membrane potential towards the resting state. Various cellular constituents, including auxiliary beta subunits and calcium, as well as reduction/oxidation processes influence channel performance.;Oxidation of the native BK channel leads to various alterations in its activity likely dependent upon the composition of the channel complex (i.e. alternative splice variants, presence of auxiliary subunits) and the nature of the oxidant. More controlled studies utilizing heterologously-expressed BK channels generally indicate that cysteine- and methionine-directed oxidation decrease and increase the channel open probability, respectively. Although a specific cysteine residue within the channel has been identified as an oxidative target, knowledge of such methionine residue/s is missing. Furthermore, prior oxidative investigations failed to co-express auxiliary beta subunits with the pore-forming BK channel.;Here, heterologously-expressed human BK channel (hSlo1) currents were electrophysiologically measured in the inside-out patch-clamp configuration before and after application of the methionine-preferring oxidant, chloramine-T. Inclusion of the beta1 subunit within the hSlo1 complex not only enhanced the characteristic functional effects exhibited by methionine-directed oxidative regulation of hSlo1 alone, but also accelerated the rate at which the channel opened. While this latter finding depended upon oxidation of a methionine residue within beta1 (M177), the increase in channel open probability due to oxidation relied on modification of at least one of three specific methionine residues (M536, M712 and M739) residing within the intracellular carboxyl terminus of the hSlo1 channel. Overall, methionine-directed oxidation of the native BK channel complex may in fact be involved in homeostatic redox regulatory processes and also vitally influence the membrane potential during times of oxidative stress, which clearly plays a role in numerous disease states and physiological aging.