Abstract：
Titin is a molecular spring in parallel with myosin motors in each muscle half-sarcomere, responsible for passive force development at sarcomere length (SL) above the physiological range (>2.7 μm). The role of titin at physiological SL is unclear and is investigated here in single intact muscle cells of the frog (Rana esculenta), by combining half-sarcomere mechanics and synchrotron X-ray diffraction in the presence of 20 μM para-nitro-blebbistatin, which abolishes the activity of myosin motors and maintains them in the resting state even during activation of the cell by electrical stimulation. We show that, during cell activation at physiological SL, titin in the I-band switches from an SL-dependent extensible spring (OFF-state) to an SL-independent rectifier (ON-state) that allows free shortening while resisting stretch with an effective stiffness of ~3 pN nm-1 per half-thick filament. In this way, I-band titin efficiently transmits any load increase to the myosin filament in the A-band. Small-angle X-ray diffraction signals reveal that, with I-band titin ON, the periodic interactions of A-band titin with myosin motors alter their resting disposition in a load-dependent manner, biasing the azimuthal orientation of the motors toward actin. This work sets the stage for future investigations on scaffold and mechanosensing-based signaling functions of titin in health and disease.
摘要：
Titin是一种分子弹簧，与每个肌肉半肌节中的肌球蛋白马达平行，负责生理范围（>2.7μm）以上的肌节长度（SL）处的被动力发展。Titin在生理SL中的作用尚不清楚，本文在青蛙（Ranaesculenta）的单个完整肌肉细胞中进行了研究，在20μM对硝基blebbistatin的存在下，通过结合半肌节力学和同步加速器X射线衍射，即使在通过电刺激激活细胞的过程中，也可以消除肌球蛋白马达的活动并将其维持在静息状态。我们证明,在生理SL的细胞活化过程中，在I波段的titin从依赖SL的可伸缩弹簧（OFF状态）切换到独立于SL的整流器（ON状态），允许自由缩短，同时抵抗拉伸，有效刚度为〜3pNnm-1每半厚灯丝。这样,I-bandtitin有效地将任何负荷增加传递到A-band中的肌球蛋白丝。小角度X射线衍射信号显示,随着I-BandTitin开启,A带titin与肌球蛋白马达的周期性相互作用以负载依赖的方式改变其静息状态，将电机的方位角朝向肌动蛋白偏置。这项工作为未来研究titin在健康和疾病中的支架和基于机械传感的信号传导功能奠定了基础。
