Abstract
A protein in the pathogenic bacterium Legionella pneumophila has been found to attach the modifying molecule ubiquitin to human proteins, using a mechanism that, surprisingly, does not involve cellular E1 and E2 enzymes. See Letter p.120 Ubiquitination is an important regulatory mechanism for many cellular processes, and several bacterial pathogens have evolved to exploit and target this protein degradation pathway to facilitate their own proliferation. Ubiquitination is catalysed by a sequential multi-enzyme cascade that involves three enzymes: E1, E2 and E3. In this study, Zhao-Qing Luo and colleagues describe an unprecedented mechanism of ubiquitination that is independent of E1 and E2 enzymes, and which is mediated by members of the SidE effector family encoded by the bacterial pathogen Legionella pneumophila. They show that SidE effectors ubiquitinate multiple Rab small GTPases in a mechanism that relies on activation of ubiquitin by ADP-ribosylation, thereby circumventing the requirement for activation by the E1 and E2 enzymes. This work establishes that ubiquitination can be carried out by a single enzyme, but future work is required to resolve the functional implications of this unique mechanism of post-translational modification.
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