Schweighofer K J, Pohorille A
Department of Pharmaceutical Chemistry, University of California San Francisco, San Francisco, California 94143, USA.
Biophys J. 2000 Jan;78(1):150-63. doi: 10.1016/S0006-3495(00)76581-7.
The transmembrane fragment of the influenza virus M(2) protein forms a homotetrameric channel that transports protons. In this paper, we use molecular dynamics simulations to help elucidate the mechanism of channel gating by four histidines that occlude the channel lumen in the closed state. We test two competing hypotheses. In the "shuttle" mechanism, the delta nitrogen atom on the extracellular side of one histidine is protonated by the incoming proton, and, subsequently, the proton on the epsilon nitrogen atom is released on the opposite side. In the "water-wire" mechanism, the gate opens because of electrostatic repulsion between four simultaneously biprotonated histidines. This allows for proton transport along the water wire that penetrates the gate. For each system, composed of the channel embedded in a hydrated phospholipid bilayer, a 1.3-ns trajectory was obtained. It is found that the states involved in the shuttle mechanism, which contain either single-protonated histidines or a mixture of single-protonated histidines plus one biprotonated residue, are stable during the simulations. Furthermore, the orientations and dynamics of water molecules near the gate are conducive to proton transfer. In contrast, the fully biprotonated state is not stable. Additional simulations show that if only two histidines are biprotonated, the channel deforms but the gate remains closed. These results support the shuttle mechanism but not the gate-opening mechanism of proton gating in M(2).
流感病毒M(2)蛋白的跨膜片段形成一个能运输质子的同四聚体通道。在本文中，我们使用分子动力学模拟来帮助阐明由四个组氨酸介导的通道门控机制，这四个组氨酸在通道处于关闭状态时会堵塞通道内腔。我们测试了两种相互竞争的假说。在“穿梭”机制中，一个组氨酸胞外侧的δ氮原子被进入的质子质子化，随后，ε氮原子上的质子在另一侧释放。在“水线”机制中，通道门由于四个同时被双质子化的组氨酸之间的静电排斥而打开。这使得质子能够沿着穿透通道门的水线进行运输。对于每个嵌入水合磷脂双分子层中的通道组成的系统，都获得了一条1.3纳秒的轨迹。研究发现，“穿梭”机制中涉及的状态，即包含单质子化组氨酸或单质子化组氨酸与一个双质子化残基的混合物的状态，在模拟过程中是稳定的。此外，通道门附近水分子的取向和动力学有利于质子转移。相比之下，完全双质子化的状态不稳定。额外的模拟表明，如果只有两个组氨酸被双质子化，通道会变形但通道门仍保持关闭。这些结果支持了“穿梭”机制，而不支持M(2)中质子门控的通道门打开机制。
