The mammalian cholesterol synthesis enzyme squalene monooxygenase is proteasomally truncated to a constitutively active form.
The mammalian cholesterol synthesis enzyme squalene monooxygenase is proteasomally truncated to a constitutively active form.
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DOI：
10.1016/j.jbc.2021.100731
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发表时间：
2021-01
期刊：
影响因子：
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通讯作者：
Brown AJ
中科院分区：
文献类型：
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作者：
Coates HW;Capell-Hattam IM;Brown AJ
角鲨烯单加氧酶（SM，也称为角鲨烯环氧酶）是胆固醇合成的限速酶，其将角鲨烯转化为单氧化角鲨烯，并且在许多癌症类型中是致癌的。SM通过胆固醇诱导的蛋白酶体降解进行反馈调节，这取决于其脂质敏感的N-末端调节结构域。我们以前确定了一个内源性的截断形式的SM具有类似的丰度全长SM，但无论是这种截断形式的功能或受到相同的监管机制，全长SM是未知的。在这里，我们表明，截短SM不同于全长SM在两个主要方面：它是胆固醇抵抗，并采用了周边，而不是完整的关联与内质网膜。然而，截短的SM保留了完整的SM活性，因此具有组成型活性。SM的截短发生在其内质网相关的降解过程中，需要蛋白酶体，蛋白酶体部分降解SM N-末端并破坏调节结构域内的胆固醇敏感元件。此外，截短依赖于与胆固醇诱导的降解所需的信号不同的泛素信号。使用诱变，我们表明，部分蛋白酶体降解SM依赖于一个内在的无序区域附近的截断位点和稳定的相邻的催化结构域，逃脱降解。这些发现揭示了胆固醇合成的翻译后调控的额外复杂性层，并建立SM作为第一个发现进行蛋白酶体截短的真核酶。
Squalene monooxygenase (SM, also known as squalene epoxidase) is a rate-limiting enzyme of cholesterol synthesis that converts squalene to monooxidosqualene and is oncogenic in numerous cancer types. SM is subject to feedback regulation via cholesterol-induced proteasomal degradation, which depends on its lipid-sensing N-terminal regulatory domain. We previously identified an endogenous truncated form of SM with a similar abundance to full-length SM, but whether this truncated form is functional or subject to the same regulatory mechanisms as full-length SM is not known. Here, we show that truncated SM differs from full-length SM in two major ways: it is cholesterol resistant and adopts a peripheral rather than integral association with the endoplasmic reticulum membrane. However, truncated SM retains full SM activity and is therefore constitutively active. Truncation of SM occurs during its endoplasmic reticulum–associated degradation and requires the proteasome, which partially degrades the SM N-terminus and disrupts cholesterol-sensing elements within the regulatory domain. Furthermore, truncation relies on a ubiquitin signal that is distinct from that required for cholesterol-induced degradation. Using mutagenesis, we demonstrate that partial proteasomal degradation of SM depends on both an intrinsically disordered region near the truncation site and the stability of the adjacent catalytic domain, which escapes degradation. These findings uncover an additional layer of complexity in the post-translational regulation of cholesterol synthesis and establish SM as the first eukaryotic enzyme found to undergo proteasomal truncation.