DRP蛋白的SUMO化修饰参与线粒体分裂调节的研究

　　SUMO (small ubiquition-like modifier) 是一类小的类泛素修饰物，能在一系列酶的作用下共价结合到目标蛋白上。 这种可逆的SUMO化修饰过程 (SUMOylation) 是调节细胞内蛋白质功能的重要方式之一，参与真核细胞内各种代谢途径。已有的研究发现，植物的SUMO化修饰参与植物花期调控，激素信号转导，抗病防御及逆境应答等生理过程。 　　线粒体是重要的细胞器，它的形态是多样和动态的。在活细胞中，线粒体不断地运动，并频繁地进行分裂与融合，维持着形态的平衡。而这种形态的动态平衡对线粒体功能的行使有着重要的意义。线粒体的分裂需要dynamin相关蛋白DRPs的参与，该类蛋白在进化中是保守的，从植物中已经鉴定出DRP3A和DRP3B两个蛋白。DRP蛋白由胞质招募到线粒体外膜上来参与膜的剪切，但有关DRP向线粒体招募和组装的机制尚不清楚。 　　本文通过对稳定表达GFP-SUMO1的BY-2悬浮细胞观察发现，SUMO1主要定位于细胞核中，在细胞质中也有少量分布；过表达SUMO1能使线粒体的片段化程度增强。而在SAE-RNAi植株及siz1突变体中，线粒体片段化程度减弱，由此提示SUMO化修饰的下调降低了线粒体的分裂水平。经序列分析发现，线粒体分裂蛋白DRP3A和DRP3B有SUMO化修饰的识别序列，而通过表达SUMO-DRP融合蛋白来加强DRP蛋白的SUMO化修饰，则能使线粒体分裂增加，从而表明SUMO化是通过对DRP的调节来影响线粒体的分裂。 我们推测SUMO化修饰的作用是能特异地保护DRP不被降解，最终导致DRP库更为稳定与活跃。总之，本文首次在植物中鉴定出SUMO1在线粒体上的作用，并暗示它可能通过调节DRP蛋白来参与线粒体分裂的新功能。

SUMOylation represses SnRK1 signaling in Arabidopsis

The SnRK1 protein kinase balances cellular energy levels in accordance with extracellular conditions and is thereby key for plant stress tolerance. In addition, SnRK1 has been implicated in numerous growth and developmental processes from seed filling and maturation to flowering and senescence. Despite its importance, the mechanisms that regulate SnRK1 activity are poorly understood. Here, we demonstrate that the SnRK1 complex is SUMOylated on multiple subunits and identify SIZ1 as the E3 Small Ubiquitin-like Modifier (SUMO) ligase responsible for this modification. We further show that SnRK1 is ubiquitinated in a SIZ1-dependent manner, causing its degradation through the proteasome. In consequence, SnRK1 degradation is deficient in siz1-2 mutants, leading to its accumulation and hyperactivation of SnRK1 signaling. Finally, SnRK1 degradation is strictly dependent on its activity, as inactive SnRK1 variants are aberrantly stable but recover normal degradation when expressed as SUMO mimetics. Altogether, our data suggest that active SnRK1 triggers its own SUMOylation and degradation, establishing a negative feedback loop that attenuates SnRK1 signaling and prevents detrimental hyperactivation of stress responses.