Department of Biomedical Sciences, Florida State University College of Medicine, Tallahassee, Florida, 32306, USA.
Sci Rep. 2019 Aug 27;9(1):12418. doi: 10.1038/s41598-019-48889-5.
The 26S proteasome conducts the majority of regulated protein catabolism in eukaryotes. At the heart of the proteasome is the barrel-shaped 20S core particle (CP), which contains two β-rings sandwiched between two α-rings. Whereas canonical CPs contain α-rings with seven subunits arranged α1-α7, a non-canonical CP in which a second copy of the α4 subunit replaces the α3 subunit occurs in both yeast and humans. The mechanisms that control canonical versus non-canonical CP biogenesis remain poorly understood. Here, we have repurposed a split-protein reporter to identify genes that can enhance canonical proteasome assembly in mutant yeast producing non-canonical α4-α4 CPs. We identified the proteasome subunit α1 as an enhancer of α3 incorporation, and find that elevating α1 protein levels preferentially drives canonical CP assembly under conditions that normally favor α4-α4 CP formation. Further, we demonstrate that α1 is stoichiometrically limiting for α-ring assembly, and that enhancing α1 levels is sufficient to increase proteasome abundance and enhance stress tolerance in yeast. Together, our data indicate that the abundance of α1 exerts multiple impacts on proteasome assembly and composition, and we propose that the limited α1 levels observed in yeast may prime cells for alternative proteasome assembly following environmental stimuli.
26S 蛋白酶体在真核生物中进行大多数受调控的蛋白质降解。蛋白酶体的核心是桶形的 20S 核心颗粒（CP），其中包含两个β环夹在两个α环之间。虽然规范的 CP 含有由七个亚基排列的α环，α1-α7，但在酵母和人类中，存在一种非规范的 CP，其中第二个α4 亚基副本取代了α3 亚基。控制规范 CP 与非规范 CP 生物发生的机制仍知之甚少。在这里，我们重新利用了一种分裂蛋白报告基因来鉴定可以增强产生非规范α4-α4 CP 的突变酵母中规范蛋白酶体组装的基因。我们鉴定出蛋白酶体亚基α1 是增强α3 掺入的增强子，并且发现在通常有利于α4-α4 CP 形成的条件下，升高α1 蛋白水平优先驱动规范 CP 组装。此外，我们证明α1 对α 环组装的量是有限的，并且提高α1 水平足以增加酵母中蛋白酶体的丰度并增强其对压力的耐受性。总之，我们的数据表明α1 的丰度对蛋白酶体组装和组成有多种影响，我们提出在酵母中观察到的有限α1 水平可能使细胞在环境刺激后为替代蛋白酶体组装做好准备。
