Department of Systems Biology, Yonsei University, Seoul, 03722, Korea.
Future Technology Research Center, Corporate R&D, LG Chem/LG Science Park, Seoul, 07796, Korea.
Planta. 2019 Feb;249(2):527-542. doi: 10.1007/s00425-018-3024-5. Epub 2018 Oct 6.
Maf1 repressor activity is critical for plant survival during environmental stresses, and is regulated by its phosphorylation/dephosphorylation through the activity of TOR and PP4/PP2A phosphatases. Maf1 is a global repressor of RNA polymerase III (Pol III), and is conserved in eukaryotes. Pol III synthesizes small RNAs, 5S rRNA, and tRNAs that are essential for protein translation and cell growth. Maf1 is a phosphoprotein and dephosphorylation of Maf1 promotes its repressor activity in yeast and mammals. Plant Maf1 was identified in citrus plants as a canker elicitor-binding protein, and citrus Maf1 represses cell growth associated with canker development. However, functions of plant Maf1 under diverse stress conditions and its regulation by the target of rapamycin (TOR) signaling components are poorly understood. In this study, the Arabidopsis maf1 mutants were more susceptible to diverse stresses and treatment with the TOR inhibitor Torin-1 than wild-type plants. The maf1 mutants expressed higher levels of Maf1 target RNAs, including 5S rRNA and pre-tRNAs in leaf cells, supporting Pol III repressor activity of Arabidopsis Maf1. Cellular stresses and Torin-1 treatment induced dephosphorylation of Maf1, suggesting Maf1 activation under diverse stress conditions. TOR silencing also stimulated Maf1 dephosphorylation, while silencing of catalytic subunit genes of PP4 and PP2A repressed it. Thus, TOR kinase and PP4/PP2A phosphatases appeared to oppositely modulate the Maf1 phosphorylation status. TOR silencing decreased the abundance of the target RNAs, while silencing of the PP4 and PP2A subunit genes increased it, supporting the positive correlation between Maf1 dephosphorylation and its repressor activity. Taken together, these results suggest that repressor activity of Maf1, regulated by the TOR signaling pathway, is critical for plant cell survival during environmental stresses.
Maf1 抑制物的活性对于植物在环境胁迫下的生存至关重要，其活性通过 TOR 和 PP4/PP2A 磷酸酶的磷酸化/去磷酸化来调节。Maf1 是 RNA 聚合酶 III（Pol III）的全局抑制剂，在真核生物中保守。Pol III 合成小 RNA、5S rRNA 和 tRNA，这些对于蛋白质翻译和细胞生长是必不可少的。Maf1 是一种磷酸化蛋白，在酵母和哺乳动物中，Maf1 的去磷酸化促进其抑制物活性。在柑橘植物中，植物 Maf1 被鉴定为溃疡诱导物结合蛋白，柑橘 Maf1 抑制与溃疡发展相关的细胞生长。然而，植物 Maf1 在各种胁迫条件下的功能及其受雷帕霉素（TOR）信号成分的调节知之甚少。在这项研究中，与野生型植物相比，拟南芥 maf1 突变体对各种胁迫和 TOR 抑制剂 Torin-1 的处理更敏感。maf1 突变体在叶片细胞中表达更高水平的 Maf1 靶标 RNA，包括 5S rRNA 和前 tRNA，支持拟南芥 Maf1 的 Pol III 抑制物活性。细胞应激和 Torin-1 处理诱导 Maf1 的去磷酸化，表明 Maf1 在各种胁迫条件下被激活。TOR 沉默也刺激了 Maf1 的去磷酸化，而 PP4 和 PP2A 催化亚基基因的沉默则抑制了它。因此，TOR 激酶和 PP4/PP2A 磷酸酶似乎对 Maf1 的磷酸化状态具有相反的调节作用。TOR 沉默降低了靶标 RNA 的丰度，而 PP4 和 PP2A 亚基基因的沉默增加了靶标 RNA 的丰度，这支持了 Maf1 去磷酸化与其抑制物活性之间的正相关关系。综上所述，这些结果表明，受 TOR 信号通路调节的 Maf1 抑制物活性对于植物细胞在环境胁迫下的生存至关重要。
