Yang Huanghe, Zhang Guohui, Shi Jingyi, Lee Urvi S, Delaloye Kelli, Cui Jianmin
Department of Biomedical Engineering and Cardiac Bioelectricity and Arrhythmia Center, Washington University, St. Louis, Missouri 63130, USA.
Biophys J. 2008 Jun;94(12):4678-87. doi: 10.1529/biophysj.107.121590. Epub 2008 Mar 13.
Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels, composed of pore-forming alpha-subunits and auxiliary beta-subunits, play important roles in diverse physiological processes. The differences in BK channel phenotypes are primarily due to the tissue-specific expression of beta-subunits (beta1-beta4) that modulate channel function differently. Yet, the molecular basis of the subunit-specific regulation is not clear. In our study, we demonstrate that perturbation of the voltage sensor in BK channels by mutations selectively disrupts the ability of the beta1-subunit--but not that of the beta2-subunit--to enhance apparent Ca(2+) sensitivity. These mutations change the number of equivalent gating charges, the voltage dependence of voltage sensor movements, the open-close equilibrium of the channel, and the allosteric coupling between voltage sensor movements and channel opening to various degrees, indicating that they alter the conformation and movements of the voltage sensor and the activation gate. Similarly, the ability of the beta1-subunit to enhance apparent Ca(2+) sensitivity is diminished to various degrees, correlating quantitatively with the shift of voltage dependence of voltage sensor movements. In contrast, none of these mutations significantly reduces the ability of the beta2-subunit to enhance Ca(2+) sensitivity. These results suggest that the beta1-subunit enhances Ca(2+) sensitivity by altering the conformation and movements of the voltage sensor, whereas the similar function of the beta2-subunit is governed by a distinct mechanism.
大电导钙（2+）和电压激活钾（BK）通道由形成孔道的α亚基和辅助β亚基组成，在多种生理过程中发挥重要作用。BK通道表型的差异主要归因于β亚基（β1-β4）的组织特异性表达，这些β亚基对通道功能的调节方式不同。然而，亚基特异性调节的分子基础尚不清楚。在我们的研究中，我们证明，BK通道中的电压感受器因突变而受到扰动，会选择性地破坏β1亚基（而非β2亚基）增强表观钙（2+）敏感性的能力。这些突变在不同程度上改变了等效门控电荷的数量、电压感受器运动的电压依赖性、通道的开闭平衡以及电压感受器运动与通道开放之间的变构偶联，表明它们改变了电压感受器和激活门的构象及运动。同样，β1亚基增强表观钙（2+）敏感性的能力在不同程度上减弱，与电压感受器运动的电压依赖性的变化呈定量相关。相比之下，这些突变均未显著降低β2亚基增强钙（2+）敏感性的能力。这些结果表明，β1亚基通过改变电压感受器的构象和运动来增强钙（2+）敏感性，而β2亚基的类似功能则由不同的机制所调控。
