Raab Sabine, Drechsel Gabriele, Zarepour Maryam, Hartung Wolfram, Koshiba Tomokazu, Bittner Florian, Hoth Stefan
Molekulare Pflanzenphysiologie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstrasse 5, D-91058 Erlangen, Germany.
Plant J. 2009 Jul;59(1):39-51. doi: 10.1111/j.1365-313X.2009.03846.x. Epub 2009 Feb 26.
During leaf senescence, resources are recycled by redistribution to younger leaves and reproductive organs. Candidate pathways for the regulation of onset and progression of leaf senescence include ubiquitin-dependent turnover of key proteins. Here, we identified a novel plant U-box E3 ubiquitin ligase that prevents premature senescence in Arabidopsis plants, and named it SENESCENCE-ASSOCIATED E3 UBIQUITIN LIGASE 1 (SAUL1). Using in vitro ubiquitination assays, we show that SAUL1 has E3 ubiquitin ligase activity. We isolated two alleles of saul1 mutants that show premature senescence under low light conditions. The visible yellowing of leaves is accompanied by reduced chlorophyll content, decreased photochemical efficiency of photosystem II and increased expression of senescence genes. In addition, saul1 mutants exhibit enhanced abscisic acid (ABA) biosynthesis. We show that application of ABA to Arabidopsis is sufficient to trigger leaf senescence, and that this response is abolished in the ABA-insensitive mutants abi1-1 and abi2-1, but enhanced in the ABA-hypersensitive mutant era1-3. We found that increased ABA levels coincide with enhanced activity of Arabidopsis aldehyde oxidase 3 (AAO3) and accumulation of AAO3 protein in saul1 mutants. Using label transfer experiments, we showed that interactions between SAUL1 and AAO3 occur. This suggests that SAUL1 participates in targeting AAO3 for ubiquitin-dependent degradation via the 26S proteasome to prevent premature senescence.
在叶片衰老过程中，资源通过重新分配到较年轻的叶片和生殖器官而被循环利用。调控叶片衰老起始和进程的候选途径包括关键蛋白的泛素依赖性周转。在此，我们鉴定出一种新型植物U-box E3泛素连接酶，它可防止拟南芥植物过早衰老，并将其命名为衰老相关E3泛素连接酶1（SAUL1）。通过体外泛素化分析，我们表明SAUL1具有E3泛素连接酶活性。我们分离出了saul1突变体的两个等位基因，它们在弱光条件下表现出过早衰老。叶片明显变黄伴随着叶绿素含量降低、光系统II光化学效率下降以及衰老基因表达增加。此外，saul1突变体表现出脱落酸（ABA）生物合成增强。我们表明，对拟南芥施用ABA足以触发叶片衰老，并且这种反应在ABA不敏感突变体abi1-1和abi2-1中被消除，但在ABA超敏感突变体era1-3中增强。我们发现，saul1突变体中ABA水平升高与拟南芥醛氧化酶3（AAO3）活性增强和AAO3蛋白积累一致。通过标记转移实验，我们表明SAUL1与AAO3之间存在相互作用。这表明SAUL1通过26S蛋白酶体参与将AAO3靶向泛素依赖性降解，以防止过早衰老。
