Faculty of Science and Technology, Centre for Organelle Research, University of Stavanger, 4036, Stavanger, Norway.
Departamento de Genética Molecular de Plantas, Centro Nacional de Biotecnología, CSIC, Madrid, Spain.
Planta. 2017 Oct;246(4):701-710. doi: 10.1007/s00425-017-2726-4. Epub 2017 Jun 27.
PP2A catalytic subunit C2 is of special importance for light/dark regulation of nitrate reductase activity. The level of unmethylated PP2A catalytic subunits decreases in darkness. Protein phosphatase 2A (PP2A) dephosphorylates and activates nitrate reductase (NR) in photosynthetically active tissue when plants are transferred from darkness to light. In the present work, investigation of Arabidopsis thaliana PP2A mutant lines revealed that one of the five PP2A catalytic subunit genes, e.g., C2, was of special importance for NR activation. Impairment of NR activation was, especially pronounced in the c2c4 double mutant. Though weaker, NR activation was also impaired in the c2 single mutant, and c1c2 and c2c5 double mutants. On the other hand, NR activation in the c4c5 double mutant was as efficient as in WT. The c4 single mutant had low PP2A activity, whereas the c2 single mutant possessed WT levels of extractable PP2A activity. PP2A activity was low in both c2c4 and c4c5. Differences in extracted PP2A activity among mutants did not strictly correlate with differences in NR activation, but underpinned that C2 has a special function in NR activation in vivo. The terminal leucine in PP2A catalytic subunits is generally methylated to a high degree, but regulation and impact of methylation/demethylation is barely studied. In WT and PP2A mutants, the level of unmethylated PP2A catalytic subunits decreased during 45 min of darkness, but did not change much when light was switched on. In leucine carboxyl methyl transferase1 (LCMT1) knockout plants, which possess mainly unmethylated PP2A, NR was still activated, although not fully as efficient as in WT.
PP2A 催化亚基 C2 对硝酸盐还原酶活性的光/暗调节特别重要。在黑暗中，未甲基化的 PP2A 催化亚基水平降低。当植物从黑暗转移到光照时，蛋白磷酸酶 2A（PP2A）去磷酸化并激活光合作用组织中的硝酸盐还原酶（NR）。在本工作中，对拟南芥 PP2A 突变体系的研究表明，五个 PP2A 催化亚基基因之一，例如 C2，对 NR 激活特别重要。NR 激活的损伤在 c2c4 双突变体中尤为明显。虽然较弱，但 c2 单突变体和 c1c2 和 c2c5 双突变体也会损害 NR 激活。另一方面，c4c5 双突变体中的 NR 激活与 WT 一样有效。c4 单突变体的 PP2A 活性较低，而 c2 单突变体具有 WT 水平的可提取 PP2A 活性。c2c4 和 c4c5 中的 PP2A 活性均较低。突变体之间提取的 PP2A 活性差异与 NR 激活差异没有严格相关，但表明 C2 在体内 NR 激活中具有特殊功能。PP2A 催化亚基中的末端亮氨酸通常高度甲基化，但甲基化/去甲基化的调节和影响几乎没有研究。在 WT 和 PP2A 突变体中，在 45 分钟的黑暗期间，未甲基化的 PP2A 催化亚基水平降低，但当光照打开时，变化不大。在亮氨酸羧基甲基转移酶 1（LCMT1）敲除植物中，其主要具有未甲基化的 PP2A，NR 仍然被激活，尽管不如 WT 那样有效。
