Horrigan Frank T, Heinemann Stefan H, Hoshi Toshinori
Department of Physiology, School of Medecine, University of Pennsylvania, Philadelphia, PA 19104, USA.
J Gen Physiol. 2005 Jul;126(1):7-21. doi: 10.1085/jgp.200509262. Epub 2005 Jun 13.
Large conductance calcium-dependent (Slo1 BK) channels are allosterically activated by membrane depolarization and divalent cations, and possess a rich modulatory repertoire. Recently, intracellular heme has been identified as a potent regulator of Slo1 BK channels (Tang, X.D., R. Xu, M.F. Reynolds, M.L. Garcia, S.H. Heinemann, and T. Hoshi. 2003. Nature. 425:531-535). Here we investigated the mechanism of the regulatory action of heme on heterologously expressed Slo1 BK channels by separating the influences of voltage and divalent cations. In the absence of divalent cations, heme generally decreased ionic currents by shifting the channel's G-V curve toward more depolarized voltages and by rendering the curve less steep. In contrast, gating currents remained largely unaffected by heme. Simulations suggest that a decrease in the strength of allosteric coupling between the voltage sensor and the activation gate and a concomitant stabilization of the open state account for the essential features of the heme action in the absence of divalent ions. At saturating levels of divalent cations, heme remained similarly effective with its influence on the G-V simulated by weakening the coupling of both Ca(2+) binding and voltage sensor activation to channel opening. The results thus show that heme dampens the influence of allosteric activators on the activation gate of the Slo1 BK channel. To account for these effects, we consider the possibility that heme binding alters the structure of the RCK gating ring and thereby disrupts both Ca(2+)- and voltage-dependent gating as well as intrinsic stability of the open state.
大电导钙依赖性（Slo1 BK）通道通过膜去极化和二价阳离子进行变构激活，并具有丰富的调节机制。最近，细胞内血红素已被确定为Slo1 BK通道的一种有效调节剂（Tang, X.D., R. Xu, M.F. Reynolds, M.L. Garcia, S.H. Heinemann, and T. Hoshi. 2003. Nature. 425:531 - 535）。在此，我们通过分离电压和二价阳离子的影响，研究了血红素对异源表达的Slo1 BK通道调节作用的机制。在没有二价阳离子的情况下，血红素通常通过将通道的G-V曲线向更去极化的电压方向移动并使曲线变平缓来降低离子电流。相比之下，门控电流在很大程度上不受血红素影响。模拟结果表明，电压传感器与激活门之间变构偶联强度的降低以及开放状态的相应稳定解释了在没有二价离子时血红素作用的基本特征。在二价阳离子达到饱和水平时，血红素通过减弱Ca(2+)结合和电压传感器激活与通道开放的偶联来模拟其对G-V的影响，仍然具有类似的效果。因此，结果表明血红素减弱了变构激活剂对Slo1 BK通道激活门的影响。为了解释这些效应，我们考虑了血红素结合改变RCK门控环结构从而破坏Ca(2+)和电压依赖性门控以及开放状态固有稳定性的可能性。
