Editors' ChoicePosttranslational Modifications

An all-in-one bacterial ubiquitylation machine

Sci. Signal.  10 May 2016:
Vol. 9, Issue 427, pp. ec109
DOI: 10.1126/scisignal.aag0757

In eukaryotes, covalent attachment of the small regulatory protein ubiquitin controls the stability, activity, or localization of proteins. The ubiquitin-activating enzyme E1 and the ubiquitin-conjugating enzyme E2 are necessary for the ubiquitin ligase E3 to conjugate ubiquitin to target proteins (see Bhogaraju and Dikic). During ubiquitylation mediated by the E1-E2-E3 system, ubiquitin is first covalently attached to E1 during activation and then covalently attached to E2 before transfer from E2 to the substrate by E3. Although this ubiquitylation system does not exist in bacteria, some bacteria produce proteins that mimic or interfere with components of the host’s ubiquitylation machinery. Qiu et al. found that the members of the SidE family of proteins of Legionella pneumophila, which are required for this bacterium to replicate inside eukaryotic cells, ubiquitylated host cell Rab guanosine triphosphatases (GTPases) in a manner that did not depend on E1 or E2. The SidE family members SdeA, SdeB, SdeC, and SidE each contain a predicted mono-adenosine diphosphate (ADP)–ribosyltransferase (mART) motif, which can transfer the ADP-ribosyl group from nicotinamide adenine dinucleotide (NAD) to arginine residues on target proteins. The mART motif in SdeA was necessary for L. pneumophila toxicity in yeast, a protozoan host, and cultured human cells. Coexpression in mammalian cells of SdeA, SdeB, SdeC, or SidE with any one of several mammalian endoplasmic reticulum–associated Rab small GTPases resulted in an increase in the mass of the Rab proteins. Rab proteins are targeted by many L. pneumophila effectors and are recruited to the vacuoles in which L. pneumophila replicates. Rab33b and Rab1 were ubiquitylated in mammalian cells infected with L. pneumophila. In vitro, the only cofactor required for purified SdeA to ubiquitylate purified Rab33b in the presence of ubiquitin was NAD. Biochemical analysis indicated that SdeA ADP-ribosylated ubiquitin and then transferred ubiquitin directly to Rab33b without becoming covalently attached to ubiquitin. Thus, SdeA functions as a self-contained ubiquitylation machine that can both activate ubiquitin and transfer it to the substrate.

J. Qiu, M. J. Sheedlo, K. Yu, Y. Tan, E. S. Nakayasu, C. Das, X. Liu, Z.-Q. Luo, Ubiquitination independent of E1 and E2 enzymes by bacterial effectors. Nature 533, 120–124 (2016). [PubMed]

S. Bhogaraju, I. Dikic, Ubiquitination without E1 and E2 enzymes. Nature 533, 43–44 (2016). [PubMed]