The energetics of molecular motors

肌动蛋白和肌球蛋白是组成分子马达的两种蛋白质，它们驱动动物的许多功能，包括细胞分裂、ATP生产、听力调节，最重要的是横纹肌(骨骼肌和心脏)的收缩。在过去的15年里，我们研究了肌肉收缩的分子机制，并意识到教科书版本的肌肉收缩无法解释许多实验观察到的现象。具体地说，当一块活跃的肌肉被拉伸时，它的力量、能量和僵硬不能用由肌动蛋白和肌球蛋白组成的分子马达来解释。就在十多年前，我们发现被认为只起到被动支持作用的结构蛋白有助于主动力量调节和收缩。我们通过分子、亚细胞和细胞实验证实，分子弹簧Titin是参与主动力调节的结构元件。我们认为，并有初步证据表明，Titin在肌肉激活时结合了钙，从而增加了肌肉的固有硬度和力，而Titin与肌动蛋白结合，从而缩短了其自由弹簧长度，从而增加了其在拉伸时的刚度和力。如果我们的理论是正确的，那么在主动肌肉拉伸期间和之后，单位力量的能量成本(ATP，三磷酸腺苷)应该比被动肌肉拉伸后相应力量的成本小得多。然而，单个肌原纤维和单个肌肉细胞的能量成本从未被测量过，因此我们的预测目前无法在实验中得到验证。这项应用的目的是获得倒置荧光显微镜和单细胞机械测试设备，将其集成到现有的单个肌原纤维系统中，该系统允许同时测量机械和能量特性，同时通过钙和ATP激活肌动蛋白-肌球蛋白马达。该系统将允许连续测量单个肌原纤维以及心肌和骨骼肌细胞中的力、硬度和三磷酸腺苷的使用。据我们所知，这样的系统在任何地方都不存在，而且肯定没有文献中发表的关于单个肌原纤维收缩的能量学描述。

Actin and myosin are two proteins that make up molecular motors that drive many functions in animals including cell division, ATP production, regulation of hearing, and most importantly contractions in striated (skeletal and heart) muscles. We have studied the molecular mechanisms of muscle contractions for the past 15 years and realized that the textbook version of muscle contraction cannot explain many of the experimentally observed phenomena. Specifically, when an active muscle is stretched, its force, energetics and stiffness are not explained by the molecular motor comprised of actin and myosin. Just over a decade ago, we found that structural proteins, thought to only play passive supportive roles, contribute to active force regulation and contraction. We identified through molecular, sub-cellular and cellular experiments that the structural element contributing to active force regulation is the molecular spring titin. We suggest, and have preliminary evidence, that titin binds calcium upon muscle activation, thereby increasing its inherent stiffness and force, and that titin binds to actin, thus shortening its free spring length, thereby increasing its stiffness and force upon stretch. If our theories are correct, then the energy cost (ATP, adenosine triphosphate) per unit of force should be much smaller during and after stretch of active muscle compared to the cost of the corresponding force following passive muscle stretching. However, the energetic costs of single myofibrils and single muscle cells have never been measured, thus our predictions cannot be tested experimentally at this time. The purpose of this application is to acquire an inverted fluorescent microscope and a single cell mechanical testing apparatus to be integrated into an existing single myofibril system that allows for simultaneous mechanical and energetic property measurements while the actin-myosin motors are activated through calcium and ATP. This system would allow for continuous measurement of the force, stiffness and ATP use in single myofibrils and cardiac and skeletal muscle cells. To our best knowledge, such a system does not exist anywhere and there are definitely no accounts of the energetics of single myofibril contraction published in the literature.