Ubiquitin-dependent switch during assembly of the proteasomal ATPases mediated by Not4 ubiquitin ligase
Ubiquitin-dependent switch during assembly of the proteasomal ATPases mediated by Not4 ubiquitin ligase
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DOI：
10.1073/pnas.1805353115
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发表时间：
2018-12-26
影响因子：
11.1
通讯作者：
Park, Soyeon
中科院分区：
文献类型：
--
作者：
Fu, Xinyi;Sokolova, Vladyslava;Park, Soyeon
在蛋白酶体全酶中，六聚体 ATP 酶 (Rpt1Rpt6) 通过将泛素化蛋白质展开并易位到蛋白水解核心颗粒中来降解它们。在早期蛋白酶体组装过程中，各个 Rpt 蛋白通过与其同源伴侣：Nas2、Hsm3、Nas6 和 Rpn14 结合，组装成六聚体“Rpt 环”。在这里，我们证明 Rpt 环组装采用特定的泛素化介导的控制。 E3 连接酶 Not4 在 Rpt 环组装过程中选择性泛素化 Rpt5。为了访问 Rpt5，Not4 与 Nas2 竞争直到倒数第二步，然后在 Rpt 环完成的最后一步与 Hsm3 竞争。利用已知的 Rpt 伴侣共晶结构，我们发现 Rpt5 中 Not4 介导的泛素化位点被 Nas2 和 Hsm3 阻碍。因此，Not4 可以根据 Rpt5 的可及性来区分在没有这些伴侣的情况下成熟的 Rpt 环。 Rpt5 泛素化不会破坏环的稳定性，但会阻碍传入亚基（Rpn1 泛素受体和 Ubp6 去泛素酶）的掺入，从而阻止蛋白酶体组装和从蛋白酶体底物再生泛素的进程。我们的研究结果揭示了一个组装检查点，Not4 在六聚体 ATP 酶环组装过程中监测分子伴侣的作用，从而确保蛋白酶体全酶成熟的准确性。
In the proteasome holoenzyme, the hexameric ATPases (Rpt1Rpt6) enable degradation of ubiquitinated proteins by unfolding and translocating them into the proteolytic core particle. During early-stage proteasome assembly, individual Rpt proteins assemble into the hexameric "Rpt ring" through binding to their cognate chaperones: Nas2, Hsm3, Nas6, and Rpn14. Here, we show that Rpt ring assembly employs a specific ubiquitination-mediated control. An E3 ligase, Not4, selectively ubiquitinates Rpt5 during Rpt ring assembly. To access Rpt5, Not4 competes with Nas2 until the penultimate step and then with Hsm3 at the final step of Rpt ring completion. Using the known Rpt-chaperone cocrystal structures, we show that Not4-mediated ubiquitination sites in Rpt5 are obstructed by Nas2 and Hsm3. Thus, Not4 can distinguish a Rpt ring that matures without these chaperones, based on its accessibility to Rpt5. Rpt5 ubiquitination does not destabilize the ring but hinders incorporation of incoming subunits-Rpn1 ubiquitin receptor and Ubp6 deubiquitinase-thereby blocking progression of proteasome assembly and ubiquitin regeneration from proteasome substrates. Our findings reveal an assembly checkpoint where Not4 monitors chaperone actions during hexameric ATPase ring assembly, thereby ensuring the accuracy of proteasome holoenzyme maturation.