Inoue Akira, Saito Junya, Ikebe Reiko, Ikebe Mitsuo
Department of Physiology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655-0127, USA.
Nat Cell Biol. 2002 Apr;4(4):302-6. doi: 10.1038/ncb774.
Myosin is an actin-based molecular motor that constitutes a diverse superfamily. In contrast to conventional myosin, which binds to actin for only a short time during cross-bridge cycling, recent studies have demonstrated that class V myosin moves along actin filaments for a long distance without dissociating. This would make it suitable for supporting cargo movement in cells. Because myosin V has a two-headed structure with an expanded neck domain, it has been postulated to 'walk' along the 36-nm helical repeat of the actin filament, with one head attached to the actin and leading the other head to the neighbouring helical pitch. Here, we report that myosin IXb, a single-headed myosin, moves processively on actin filaments. Furthermore, we found that myosin IXb is a minus-end-directed motor. In addition to class VI myosin, this is the first myosin superfamily member identified that moves in the reverse direction. The processive movement of the single-headed myosin IXb cannot be explained by a 'hand-over-hand' mechanism. This suggests that an alternative mechanism must be operating for the processive movement of single-headed myosin IXb.
肌球蛋白是一种基于肌动蛋白的分子马达，构成了一个多样化的超家族。与传统肌球蛋白在横桥循环过程中仅短暂结合肌动蛋白不同，最近的研究表明，V类肌球蛋白可沿肌动蛋白丝长距离移动而不解离。这使其适合于支持细胞内的货物运输。由于肌球蛋白V具有双头结构且颈部结构域扩展，因此推测它沿着肌动蛋白丝的36纳米螺旋重复序列“行走”，一个头部附着在肌动蛋白上并引导另一个头部至相邻的螺旋间距。在此，我们报告单头肌球蛋白IXb可在肌动蛋白丝上持续移动。此外，我们发现肌球蛋白IXb是一种向负端移动的马达。除了VI类肌球蛋白外，这是首个被鉴定出向相反方向移动的肌球蛋白超家族成员。单头肌球蛋白IXb的持续移动无法用“手换手”机制来解释。这表明单头肌球蛋白IXb的持续移动必定存在另一种作用机制。
