The E3 ubiquitin ligase domain found in the bacterial protein AvrPtoB inhibits immunity-associated programmed cell death (PCD) when translocated into plant cells, probably by recruiting E2 enzymes and transferring ubiquitin molecules to cellular proteins involved in regulation of PCD and targeting them for degradation. The structure of this domain reveals a globular fold centred on a four-stranded beta-sheet that packs against two helices on one face and has three very extended loops connecting the elements of secondary structure, with remarkable homology to the RING-finger and U-box families of proteins involved in ubiquitin ligase complexes in eukaryotes [1].

1: A bacterial inhibitor of host programmed cell death defenses is an E3 ubiquitin ligase. Janjusevic R, Abramovitch RB, Martin GB, Stebbins CE; Science. 2006;311:222-226. PMID:16373536