Protein degradation maintains cellular integrity by regulating virtually all biological processes, whereas impaired proteolysis perturbs protein quality control, and often leads to human disease. Two major proteolytic systems are responsible for protein breakdown in all cells: autophagy, which facilitates the loss of organelles, protein aggregates, and cell surface proteins; and the ubiquitin-proteasome system (UPS), which promotes degradation of mainly soluble proteins. Recent findings indicate that more complex protein structures, such as filamentous assemblies, which are not accessible to the catalytic core of the proteasome in vitro, can be efficiently degraded by this proteolytic machinery in systemic catabolic states in vivo. Mechanisms that loosen the filamentous structure seem to be activated first, hence increasing the accessibility of protein constituents to the UPS. In this review, we will discuss the mechanisms underlying the disassembly and loss of the intricate insoluble filamentous myofibrils, which are responsible for muscle contraction, and whose degradation by the UPS causes weakness and disability in aging and disease. Several lines of evidence indicate that myofibril breakdown occurs in a strictly ordered and controlled manner, and the function of AAA-ATPases is crucial for their disassembly and loss.

中文翻译：

---

UPS逐步分解丝状肌原纤维

蛋白质降解通过调节几乎所有生物过程来维持细胞完整性，而蛋白水解受损会干扰蛋白质质量控​​制，并经常导致人类疾病。两个主要的蛋白水解系统负责所有细胞中的蛋白质分解：自噬，它促进细胞器，蛋白质聚集体和细胞表面蛋白质的损失；泛素-蛋白酶体系统（UPS），可促进主要可溶性蛋白质的降解。最近的发现表明，在体外系统分解代谢状态下，这种蛋白水解机制可以有效地降解更复杂的蛋白质结构，如丝状装配体，在体外是蛋白酶体的催化核心无法接近的。放松丝状结构的机制似乎首先被激活，因此，增加了蛋白质成分对UPS的可及性。在这篇综述中，我们将讨论复杂的不溶性丝状肌原纤维的分解和丢失的机制，这些机制负责肌肉收缩，UPS的降解会导致衰老和疾病中的虚弱和残疾。几条证据表明，肌原纤维的分解以严格有序和受控的方式发生，而AAA-ATPase的功能对其分解和丢失至关重要。