Ubiquitination regulates many aspects of host immunity and thus is a common target for infectious agents. Recent studies have revealed that members of the SidE effector family of the bacterial pathogen Legionella pneumophila attack several small GTPases associated with the endoplasmic reticulum by a novel ubiquitination mechanism that does not require the E1 and E2 enzymes of the host ubiquitination machinery. In this case, ubiquitin is first activated by ADP-ribosylation at Arg₄₂ by a mono-ADP-ribosyltransferase activity; the intermediate is then cleaved by a phosphodiesterase activity also residing within SdeA, concomitant with the attachment of ubiquitin to serine residues of substrate proteins via a phosphoribosyl linker. Here we demonstrate that the effect of SidEs is antagonized by SidJ, an effector encoded by a gene situated in the locus coding for three members of the SidE family (SdeC, SdeB and SdeA). SidJ reverses ubiquitination of SidEs-modified substrates by cleaving the phosphodiester bond that links phosphoribosylated ubiquitin to protein substrates. SidJ also displays classical deubiquitinase activity but does not require catalytic cysteine residues. Further, these deubiquitinase activities of SidJ are essential for its role in L. pneumophila infection. Finally, the activity of SidJ is required for efficiently reducing the abundance of ubiquitinated Rab33b in infected cells within a few hours after bacterial uptake. Our results establish SidJ as a ubiquitin-deconjugating enzyme that functions to impose temporal regulation on the activity of SidE effectors. SidJ may be important in future studies of signaling cascades mediated by this unique ubiquitination, one that also potentially regulates cellular processes in eukaryotic cells.

中文翻译：

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一种独特的去泛素酶，可与磷酸核糖基连接的蛋白泛素结合。

泛素化调节宿主免疫力的许多方面，因此是传染原的常见靶标。最近的研究表明，细菌病原体肺炎军团菌SidE效应子家族的成员通过不需要宿主泛素化机制的E1和E2酶的新型泛素化机制攻击与内质网相关的几个小GTP酶。在这种情况下，泛素首先在Arg ₄₂处通过ADP-核糖基化被激活通过单-ADP-核糖基转移酶活性；然后该中间体被同样存在于SdeA中的磷酸二酯酶活性切割，并伴随泛素通过磷酸核糖基连接子与底物蛋白的丝氨酸残基连接。在这里，我们证明SidE的作用被SidJ拮抗，SidJ是一种由位于编码SidE家族三个成员（SdeC，SdeB和SdeA）的基因座中的基因编码的效应子。SidJ通过裂解磷酸二酯键使磷酸化的泛素连接到蛋白质底物上，从而逆转SidEs修饰的底物的泛素化。SidJ还显示出经典的去泛素酶活性，但不需要催化的半胱氨酸残基。此外，SidJ的这些去泛素酶活性对其在肺炎支原体感染中的作用至关重要。最后，SidJ的活性是细菌吸收后数小时内有效减少感染细胞中泛素化的Rab33b丰度所必需的。我们的研究结果将SidJ建立为一种泛素解偶联酶，其功能是对SidE效应子的活性施加时间调控。SidJ在这种独特的泛素化介导的信号级联的未来研究中可能很重要，后者也可能调节真核细胞的细胞过程。