Liu Jun, Taylor Dianne W, Krementsova Elena B, Trybus Kathleen M, Taylor Kenneth A
The Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306-4380, USA.
Nature. 2006 Jul 13;442(7099):208-11. doi: 10.1038/nature04719. Epub 2006 Apr 16.
Unconventional myosin V (myoV) is an actin-based molecular motor that has a key function in organelle and mRNA transport, as well as in membrane trafficking. MyoV was the first member of the myosin superfamily shown to be processive, meaning that a single motor protein can 'walk' hand-over-hand along an actin filament for many steps before detaching. Full-length myoV has a low actin-activated MgATPase activity at low [Ca2+], whereas expressed constructs lacking the cargo-binding domain have a high activity regardless of [Ca2+] (refs 5-7). Hydrodynamic data and electron micrographs indicate that the active state is extended, whereas the inactive state is compact. Here we show the first three-dimensional structure of the myoV inactive state. Each myoV molecule consists of two heads that contain an amino-terminal motor domain followed by a lever arm that binds six calmodulins. The heads are followed by a coiled-coil dimerization domain (S2) and a carboxy-terminal globular cargo-binding domain. In the inactive structure, bending of myoV at the head-S2 junction places the cargo-binding domain near the motor domain's ATP-binding pocket, indicating that ATPase inhibition might occur through decreased rates of nucleotide exchange. The actin-binding interfaces are unobstructed, and the lever arm is oriented in a position typical of strong actin-binding states. This structure indicates that motor recycling after cargo delivery might occur through transport on actively treadmilling actin filaments rather than by diffusion.
非常规肌球蛋白V（myoV）是一种基于肌动蛋白的分子马达，在细胞器和mRNA运输以及膜运输中具有关键作用。MyoV是肌球蛋白超家族中首个被证明具有持续性的成员，这意味着单个马达蛋白在脱离之前可以沿着肌动蛋白丝“手拉手”地行走许多步。全长myoV在低[Ca2+]时具有低肌动蛋白激活的MgATPase活性，而缺乏货物结合结构域的表达构建体无论[Ca2+]如何都具有高活性（参考文献5 - 7）。流体动力学数据和电子显微镜照片表明，活性状态是伸展的，而非活性状态是紧凑的。在这里，我们展示了myoV非活性状态的首个三维结构。每个myoV分子由两个头部组成，头部包含一个氨基末端马达结构域，后面跟着一个结合六个钙调蛋白的杠杆臂。头部后面是一个卷曲螺旋二聚化结构域（S2）和一个羧基末端球状货物结合结构域。在非活性结构中，myoV在头部 - S2连接处弯曲，使货物结合结构域靠近马达结构域的ATP结合口袋，这表明ATPase抑制可能是通过降低核苷酸交换速率而发生的。肌动蛋白结合界面畅通无阻，杠杆臂处于典型的强肌动蛋白结合状态的位置。这种结构表明，货物递送后马达蛋白的循环利用可能是通过在活跃踏车运动的肌动蛋白丝上运输而不是通过扩散来实现的。
