Abstract：
Protein ubiquitination is one of the most important posttranslational modifications (PTMs) in eukaryotes and is involved in the regulation of almost all cellular signaling pathways. The intracellular bacterial pathogen Legionella pneumophila translocates at least 26 effectors to hijack host ubiquitination signaling via distinct mechanisms. Among these effectors, SidC/SdcA are novel E3 ubiquitin ligases with the adoption of a Cys-His-Asp catalytic triad. SidC/SdcA are critical for the recruitment of endoplasmic reticulum (ER)-derived vesicles to the Legionella-containing vacuole (LCV). However, the ubiquitination targets of SidC/SdcA are largely unknown, which restricts our understanding of the mechanisms used by these effectors to hijack the vesicle trafficking pathway. Here, we demonstrated that multiple Rab small GTPases and target soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins are bona fide ubiquitination substrates of SidC/SdcA. SidC/SdcA-mediated ubiquitination of syntaxin 3 and syntaxin 4 promotes their unconventional pairing with the vesicle-SNARE protein Sec22b, thereby contributing to the membrane fusion of ER-derived vesicles with the phagosome. In addition, our data reveal that ubiquitination of Rab7 by SidC/SdcA is critical for its association with the LCV membrane. Rab7 ubiquitination could impair its binding with the downstream effector Rab-interacting lysosomal protein (RILP), which partially explains why LCVs avoid fusion with lysosomes despite the acquisition of Rab7. Taken together, our study reveals the biological mechanisms employed by SidC/SdcA to promote the maturation of the LCVs.
摘要：
蛋白质泛素化是真核生物中最重要的翻译后修饰（PTM）之一，并参与几乎所有细胞信号通路的调节。细胞内细菌病原体嗜肺军团菌通过不同的机制易位至少26个效应子劫持宿主泛素化信号。在这些效应物中,SidC/SdcA是采用Cys-His-Asp催化三联体的新型E3泛素连接酶。SidC/SdcA对于将内质网（ER）衍生的囊泡募集到含军团菌的液泡（LCV）至关重要。然而,SidC/SdcA的泛素化靶标在很大程度上是未知的，这限制了我们对这些效应子劫持囊泡运输途径的机制的理解。这里,我们证明了多种Rab小GTP酶和目标可溶性N-乙基马来酰亚胺敏感因子附着蛋白受体（SNARE）蛋白是SidC/SdcA的真正泛素化底物。SidC/SdcA介导的突触素3和突触素4的泛素化促进了它们与囊泡-SNARE蛋白Sec22b的非常规配对，从而有助于ER衍生的囊泡与吞噬体的膜融合。此外,我们的数据表明，SidC/SdcA对Rab7的泛素化对于其与LCV膜的关联至关重要。Rab7泛素化可能损害其与下游效应Rab相互作用溶酶体蛋白（RILP）的结合，这部分解释了为什么尽管获得了Rab7，但LCV仍避免与溶酶体融合。一起来看,我们的研究揭示了SidC/SdcA促进LCV成熟的生物学机制。
