From the Department of Molecular, Cellular and Developmental Biology, University of Colorado Boulder, Boulder, Colorado 80309.
From the Department of Molecular, Cellular and Developmental Biology, University of Colorado Boulder, Boulder, Colorado 80309
J Biol Chem. 2019 Apr 19;294(16):6562-6577. doi: 10.1074/jbc.RA118.006298. Epub 2019 Feb 27.
The proteasome holoenzyme is a molecular machine that degrades most proteins in eukaryotes. In the holoenzyme, its heterohexameric ATPase injects protein substrates into the proteolytic core particle, where degradation occurs. The heterohexameric ATPase, referred to as 'Rpt ring', assembles through six ATPase subunits (Rpt1-Rpt6) individually binding to specific chaperones (Rpn14, Nas6, Nas2, and Hsm3). Here, our findings suggest that the onset of Rpt ring assembly can be regulated by two alternative mechanisms. Excess Rpt subunits relative to their chaperones are sequestered into multiple puncta specifically during early-stage Rpt ring assembly. Sequestration occurs during stressed conditions, for example heat, which transcriptionally induce Rpt subunits. When the free Rpt pool is limited experimentally, Rpt subunits are competent for proteasome assembly even without their cognate chaperones. These data suggest that sequestration may regulate amounts of individual Rpt subunits relative to their chaperones, allowing for proper onset of Rpt ring assembly. Indeed, Rpt subunits in the puncta can later resume their assembly into the proteasome. Intriguingly, when proteasome assembly resumes in stressed cells or is ongoing in unstressed cells, excess Rpt subunits are recognized by an alternative mechanism-degradation by the proteasome holoenzyme itself. Rpt subunits undergo proteasome assembly until the holoenzyme complex is generated at a sufficient level. The fully-formed holoenzyme can then degrade any remaining excess Rpt subunits, thereby regulating its own Rpt ring assembly. These two alternative mechanisms, degradation and sequestration of Rpt subunits, may help control the onset of chaperone-mediated Rpt ring assembly, thereby promoting proper proteasome holoenzyme formation.
蛋白酶体全酶是一种分子机器，可降解真核生物中的大多数蛋白质。在全酶中，其异六聚体 ATP 酶将蛋白质底物注入蛋白酶核心颗粒中，在那里发生降解。异六聚体 ATP 酶，称为“Rpt 环”，通过六个 ATP 酶亚基（Rpt1-Rpt6）分别与特定伴侣（Rpn14、Nas6、Nas2 和 Hsm3）结合而组装。在这里，我们的发现表明 Rpt 环组装的起始可以通过两种替代机制进行调节。相对于其伴侣，过量的 Rpt 亚基在早期 Rpt 环组装期间特异性地被隔离到多个斑点中。隔离发生在应激条件下，例如热，这会转录诱导 Rpt 亚基。当实验中游离 Rpt 池受到限制时，即使没有其同源伴侣，Rpt 亚基也有能力进行蛋白酶体组装。这些数据表明，隔离可能调节单个 Rpt 亚基相对于其伴侣的数量，从而适当起始 Rpt 环组装。事实上，斑点中的 Rpt 亚基以后可以重新开始组装到蛋白酶体中。有趣的是，当蛋白酶体组装在应激细胞中恢复或在未应激细胞中持续进行时，过量的 Rpt 亚基通过另一种机制被识别-由蛋白酶体全酶本身降解。Rpt 亚基经历蛋白酶体组装，直到生成足够水平的全酶复合物。然后，完全形成的全酶可以降解任何剩余的过量 Rpt 亚基，从而调节其自身的 Rpt 环组装。这两种替代机制，即 Rpt 亚基的降解和隔离，可能有助于控制伴侣介导的 Rpt 环组装的起始，从而促进适当的蛋白酶体全酶形成。
