TORC1 shapes the SUMO proteome to control RNA polymerase III activity

Loss of nutrient supply elicits alterations of the SUMO proteome and sumoylation is crucial to various cellular processes including transcription. However, the physiological significance of sumoylation of transcriptional regulators is unclear. To begin clarifying this, we mapped the SUMO proteome under nitrogen-limiting conditions in Saccharomyces cerevisiae. Interestingly, several RNA polymerase III (RNAPIII) components are major SUMO targets under normal growth conditions, including Rpc53, Rpc82, and Ret1, and nutrient starvation results in rapid desumoylation of these proteins. These findings are supported by ChIP-seq experiments that show that SUMO is highly enriched at tDNA genes. Furthermore, RNA-seq experiments revealed that preventing sumoylation results in significantly decreased tRNA transcription. TORC1 inhibition resulted in the same effect, and our data indicate that the SUMO and TORC1 pathways are both required for robust tDNA expression. Importantly, tRNA transcription was strongly reduced in cells expressing a non-sumoylatable Rpc82-4KR mutant, which correlated with a misassembled RNAPIII transcriptional complex. Our data suggest that in addition to TORC1 activity, sumoylation of RNAPIII is key to reaching full translational capacity under optimal growth conditions.营养供给缺失会引发SUMO修饰组（SUMO proteome）的改变，而SUMO化修饰对包括转录在内的多种细胞过程至关重要。然而，转录调控因子的SUMO化修饰的生理学意义尚不明确。为厘清这一问题，我们在酿酒酵母（Saccharomyces cerevisiae）的氮限制条件下绘制了SUMO修饰组图谱。有趣的是，在正常生长条件下，多个RNA聚合酶III（RNAPIII）复合物组分是主要的SUMO修饰靶点，包括Rpc53、Rpc82与Ret1；而营养饥饿会快速引发这些蛋白的去SUMO化修饰。上述结论得到了染色质免疫共沉淀测序（ChIP-seq）实验的佐证：该实验显示SUMO在tDNA基因区域呈现高度富集。此外，RNA测序（RNA-seq）实验揭示，阻断SUMO化修饰会显著降低tRNA的转录水平。TORC1复合物抑制也产生了相同效果，我们的数据表明，SUMO通路与TORC1通路均是维持高效tDNA表达所必需的。重要的是，在表达无法被SUMO化修饰的Rpc82-4KR突变体的细胞中，tRNA转录水平显著降低，这与组装异常的RNAPIII转录复合物相关。我们的研究结果表明，除TORC1活性外，RNAPIII的SUMO化修饰也是在最优生长条件下达到完整翻译能力的关键因素。