De novo assembly of myofibrils in vertebrate cross‐striated muscles progresses in three distinct steps, first from a minisarcomeric alignment of several nonmuscle and muscle proteins in premyofibrils, followed by insertions of additional proteins and increased organization in nascent myofibrils, ending with mature contractile myofibrils. In a search for controls of the process of myofibril assembly, we discovered that the transition from nascent to mature myofibrils could be halted by inhibitors of three distinct functions of the ubiquitin proteasome system (UPS). First, inhibition of pathway to E3 Cullin ligases that ubiquitinate proteins led to an arrest of myofibrillogenesis at the nascent myofibril stage. Second, inhibition of p97 protein extractions of ubiquitinated proteins led to a similar arrest of myofibrillogenesis at the nascent myofibril stage. Third, inhibitors of proteolytic action by proteasomes also blocked nascent myofibrils from transitioning to mature myofibrils. In contrast, inhibitors of autophagy or lysosomes did not affect myofibrillogenesis. To probe for differences in the effects of UPS inhibitors during myofibrillogenesis, we analyzed by fluorescence recovery after photobleaching the exchange rates of two selected sarcomeric proteins (muscle myosin II heavy chains and light chains). In the presence of p97 and proteasomal inhibitors, the dynamics of each of these two myosin proteins decreased in the nascent myofibril stage, but were unaffected in the mature myofibril stage. The increased stability of myofibrils occurring in the transition from nascent to mature myofibril assembly indicates the importance of dynamics and selective destruction in the muscle myosin II proteins for the remodeling of nascent to mature myofibrils.

中文翻译：

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肌原纤维组装和泛素蛋白酶体系统的作用

脊椎动物横纹肌中肌原纤维的从头组装分三个不同的步骤进行，首先是前肌原纤维中几种非肌肉和肌肉蛋白的小肌节排列，然后是插入额外的蛋白质并增加新生肌原纤维的组织，最后是成熟的收缩性肌原纤维。在寻找肌原纤维组装过程的控制过程中，我们发现泛素蛋白酶体系统 (UPS) 的三种不同功能的抑制剂可以阻止从新生肌原纤维到成熟肌原纤维的过渡。首先，抑制泛素化蛋白质的 E3 Cullin 连接酶途径导致肌原纤维发生在新生肌原纤维阶段停滞。第二，抑制泛素化蛋白的 p97 蛋白提取导致在新生肌原纤维阶段类似的肌原纤维生成停滞。第三，蛋白酶体的蛋白水解作用抑制剂也阻止新生肌原纤维转变为成熟肌原纤维。相反，自噬或溶酶体抑制剂不影响肌原纤维生成。为了探讨 UPS 抑制剂在肌原纤维形成过程中的影响差异，我们通过光漂白两种选定的肌节蛋白（肌肉肌球蛋白 II 重链和轻链）的交换率后的荧光恢复进行了分析。在 p97 和蛋白酶体抑制剂存在的情况下，这两种肌球蛋白的动力学在新生肌原纤维阶段下降，但在成熟肌原纤维阶段不受影响。