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 Arg(42) 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.
[1]Jilin Univ, Hosp 1, Ctr Infect & Immun, Changchun 130001, Jilin, Peoples R China.
[2]Purdue Univ, Purdue Inst Inflammat Immunol & Infect Dis, W Lafayette, IN 47907 USA.
[3]Purdue Univ, Dept Biol Sci, W Lafayette, IN 47907 USA.
[4]Peking Univ, Coll Chem & Mol Engn, Inst Analyt Chem, Beijing 100871, Peoples R China.
[5]Peking Univ, Coll Chem & Mol Engn, Synthet & Funct Biomol Ctr, Beijing 100871, Peoples R China.
[6]Hainan Med Coll, Dept Biochem & Mol Biol, Haikou 571101, Hainan, Peoples R China.
[7]Pacific Northwest Natl Lab, Div Biol Sci, Richland, WA 99352 USA.
[8]Pacific Northwest Natl Lab, Environm Mol Sci Lab, Richland, WA 99352 USA.
[9]Purdue Univ, Dept Chem, 560 Oval Dr, W Lafayette, IN 47907 USA.
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