Kambara Taketoshi, Ikebe Mitsuo
Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
J Biol Chem. 2006 Feb 24;281(8):4949-57. doi: 10.1074/jbc.M509141200. Epub 2005 Dec 7.
Recent studies have revealed that myosin IX is a single-headed processive myosin, yet it is unclear how myosin IX can achieve the processive movement. Here we studied the mechanism of ATP hydrolysis cycle of actomyosin IXb. We found that myosin IXb has a rate-limiting ATP hydrolysis step unlike other known myosins, thus populating the prehydrolysis intermediate (M.ATP). M.ATP has a high affinity for actin, and, unlike other myosins, the dissociation of M.ATP from actin was extremely slow, thus preventing myosin from dissociating away from actin. The ADP dissociation step was 10-fold faster than the overall ATP hydrolysis cycle rate and thus not rate-limiting. We propose the following model for single-headed processive myosin. Upon the formation of the M.ATP intermediate, the tight binding of actomyosin IX at the interface is weakened. However, the head is kept in close proximity to actin due to the tethering role of loop 2/large unique insertion of myosin IX. There is enough freedom for the myosin head to find the next location of the binding site along with the actin filament before complete dissociation from the filament. After ATP hydrolysis, Pi is quickly released to form a strong actin binding form, and a power stroke takes place.
最近的研究表明，肌球蛋白IX是一种单头持续性肌球蛋白，但尚不清楚肌球蛋白IX如何实现持续性运动。在此，我们研究了肌动球蛋白IXb的ATP水解循环机制。我们发现，与其他已知的肌球蛋白不同，肌球蛋白IXb存在限速ATP水解步骤，从而形成预水解中间体（M.ATP）。M.ATP对肌动蛋白具有高亲和力，与其他肌球蛋白不同，M.ATP从肌动蛋白上的解离极其缓慢，从而阻止肌球蛋白从肌动蛋白上解离。ADP解离步骤比整个ATP水解循环速率快10倍，因此不是限速步骤。我们提出了以下单头持续性肌球蛋白模型。在形成M.ATP中间体后，肌动球蛋白IX在界面处的紧密结合减弱。然而，由于肌球蛋白IX的环2/大的独特插入片段的拴系作用，头部与肌动蛋白保持紧密接近。在从肌动蛋白丝完全解离之前，肌球蛋白头部有足够的自由度沿着肌动蛋白丝找到结合位点的下一个位置。ATP水解后，磷酸迅速释放，形成强肌动蛋白结合形式，并发生动力冲程。
