Featured Article - May 2013
Short description: A novel bacterial effector protein functions as a ubiquitin E3 ligase.
The type III secretion system (T3SS) is used by most Gram-negative pathogenic bacteria to inject effector proteins directly into host cells, where they manipulate diverse cellular processes and suppress host immune responses. Despite key roles in infection, the biochemical mechanisms employed by the majority of bacterial effectors remain unknown.
Bonas, Savchenko and colleagues (MLU Halle Dept. of Genetics and PSI MCSG Toronto) have recently identified Xanthomonas outer protein L (XopL) as a T3SS effector from X. campestris, a model plant pathogen. Initially identified by a genome search, the protein contains leucine-rich repeats (LRRs), which are typically found only in eukaryotic proteins. Since several LRR-containing effectors from animal pathogens can function as ubiquitin ligases, the authors tested XopL and found that it functions as a robust and specific E3 ubiquitin ligase, mediating the transfer of ubiquitin from plant E2 conjugating enzymes.
Ubiquitination requires the C-terminal domain of XopL, a region that lacks any homology to known E3 ligases, prompting the authors to determine the crystal structures of fragments of XopL using single-wavelength anomalous dispersion data. The N-terminal domain, solved to 2.0 Å (PDB 4FCG), adopts a canonical LRR architecture containing a consensus sequence similar to plant LRR-containing proteins. Unexpectedly, the LRR domain alone is sufficient to suppress plant immunity responses.
The C-terminal domain, solved to 1.8 Å (PDB 4FC9), is composed of a four-helix bundle with no structural homology to E3 ligases or other proteins. This domain, termed XL-box, folds into two lobes surrounding a negatively charged cleft harboring conserved residues. Since the XL-box lacks cysteine residues, the E3 ligase activity of XopL apparently does not involve thioester intermediates as for eukaryotic (HECT-type) and other effector E3 ligases. The authors propose that XopL could transfer ubiquitin directly from cognate E2 to substrates, using a mechanism similar to that of RING/U-box E3 ligases. While mutational analysis identified residues involved in E2 complex formation, the unique XL-box domain structure necessitates the identification of interacting host factors and structural analysis of an E2-E3 complex to elucidate the transfer mechanism.
A.U. Singer et al. A pathogen type III effector with a novel E3 ubiquitin ligase architecture.
PLoS Pathog. 9, e1003121 (2013). doi:10.1371/journal.ppat.1003121