講題：Contribution of the internal entrance structure to the conductance of KcsA channel

時間：1/16(二)下午 14：00

地點：第三綜合大樓837室

摘要：Potassium channels regulate membrane excitability and are critically involved in many human diseases. Yet, how various parts of the channel pore contribute towards the determination of K⁺ conductance remains elusive. Because all the crystallographic structures of K⁺ channel solved so far share an almost identical selective filter, the wide range of K⁺ channel conductance must be imparted by other parts of the channel pore. In BK channels, a ring of negative charges at the internal entrance raises local potassium concentration and facilitates outward K⁺ currents. The same mechanism was also demonstrated in the Streptomyces KcsA channel, but not thoroughly investigated. Also, substituting the charged side-chains at the internal entrance of BK channels with increasingly large hydrophobic side-chains decreases channel conductance. The decrease in channel conductance as a result of shrinking internal entrance could be explained by a simple two-resistor model. In this study we sought to investigate the effect of charges and increasing side-chain size or volume at the internal entrance of the KcsA channel on channel conductance by measuring single-channel current with the lipid-bilayer technique. We identified the position A111 as the innermost pore-facing residue at the KcsA internal entrance, because a positively charged substitution increased single-channel conductance, similar to the effect of A108D mutation. Based on the A108/A111 positions, we will introduce various hydrophobic side-chains to further quantify the effects of changing dimension of the internal entrance on channel conductance in the KcsA channel, whose better-defined geometry provides additional model constraints and will enable more meaningful fine tuning of model parameters. The data will be used to construct a theoretic model to correlate channel conductance with the dimension of the KcsA internal entrance. The results from our study will help to explain the wide range of channel conductance in different K⁺ channels and elucidate the transport mechanism of ions through the channel pore.

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