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AvrPtoB: A Bacterial E3 Ubiquitin Ligase

Type III secretion systems (T3SS) translocate bacterial virulence proteins into host cells to modulate diverse eukaryotic biochemical processes. Pseudomonas syringae pv. tomato DC3000 causes disease in tomato and Arabidopsis and utilizes a T3SS to evade the host programmed cell death (PCD) response, which sacrifices a limited portion of the plant to protect the rest from systemic infection. The Pseudomonas syringae type III effector AvrPtoB is delivered into plant cells, eliciting a host response that varies between resistant and susceptible tomato lines. In resistant plants, AvrPtoB is recognized by the Pto resistance (R) protein and elicits PCD that limits pathogen spread, thus resulting in host immunity. In susceptible plants, however, AvrPtoB suppresses PCD associated with plant immunity, allowing the pathogen to multiply in the host and cause disease. AvrPtoB targets a conserved PCD pathway because AvrPtoB inhibits PCD induced by diverse agonists in plants (such as mouse BAX) and also suppresses PCD in yeast. The primary amino acid sequence of AvrPtoB has provided no clues about its possible function.

The structure of a C-terminal domain of AvrPtoB that is essential for anti-PCD activity shows an unexpected homology to the U-box and RING-finger components of eukaryotic E3 ubiquitin ligases. We show that AvrPtoB possesses ubiquitin ligase activity. Mutation of conserved residues involved in the binding of E2 ubiquitin conjugating enzymes abolishes this activity in vitro as well as anti-PCD activity in vivo, dramatically decreasing virulence. Our data suggest that AvrPtoB functions as an E3 ligase in the infected cell, recruiting E2 enzymes and substrates, transferring ubiquitin or a ubiquitin-like molecule to cellular proteins involved in the regulation of PCD. Since AvrPtoB links E3 ubiquitin ligases and programmed cell death, and furthermore extends beyond plants to yeast cells as well as PCD activated in plants by the BAX protein, the bacterium has evolved this E3 ligase mimic to target a fundamental aspect of PCD that is highly conserved across several branches of eukaryota.

 

R. Janjusevic, R.B. Abramovitch, G.B. Martin, and C.E. Stebbins (2006). "A Bacterial Inhibitor of Host Programmed Cell Death Defenses is an E3 Ubiquitin Ligase." Science, 311, 222-6. PMID: 16373536.  [Abstract] [pdf]

A.B. Abramovitch, R. Janjusevic, C.E. Stebbins, and Martin GB. (2006) “Type III effector AvrPtoB requires intrinsic E3 ubiquitin ligase activity to suppress plant cell death and immunity. Proc Natl Acad Sci U S A. 103(8):2851-6. PMID: 16477026.  [Abstract] [pdf]

 

Dr. Radmila Janjusevic was the lead scientist on the AvrPtoB project. Our collaborators were the group of Gregory Martin at Cornell University.