The Legionella Effector RavZ Inhibits Host Autophagy through Irreversible Atg8 Deconjugation

Augustine Choy, et al. 2012. Science 338, 1072

Speaker: Kuan Chih Wu (武冠志)                                        Time: 14:10~15:00, Mar 20, 2012

Commentator: Dr. I Hsiu Huang (黃一修 博士)         Place: Room 601

Abstract:

Legionella pneumophila, a Gram-negative, rod-shaped bacterium, is usually found in freshwater and amoeba. This bacterium can cause infectious diseases, Pontiac fever and Legionnaires’ disease, in children, elder people and immune suppressed patients by air born  or wound infection. L.pneumophila is known as an intracellular pathogen, which can hide and replicate in vacuoles of host phagocytes. For surviving in host cell, intracellular pathogens must have developed several strategies to escape from host phagocytosis and autophagy pathway. Previous studies have indicated that the type IV secretion system (T4SS) is required for L. pneumophila intracellular survival^1,2. In this study, the authors further searched for effectors involved in autophagy inhibition. They first monitored the level of LC3-II, an autophagy activation indicator, in HEK293 cells infected by L.pneumophila, and found that infection resulted in reduced autophagy activation. Next, by comparing the LC3-II levels in HEK293 cells infected with a variety of L. pneumophila mutant strains, they discovered that a L. pneumophila T4SS effector, RavZ, could inhibit autophagy activation in host cell after infection. They then found that the concentration of RavZ for effective inhibition of autophagy was much lower than those of Atg7 and Atg3, enzymes conjugating Atg8 to phosphotidylethanolamine (PE), required for autophagy activation. They suspected that RavZ may inhibit autophagy by acting as an enzyme, and they later found that this protein resembled Atg4, a cysteine proteinase, in cleaving Atg8 and releasing it from PE. Indeed, by using a cysteine proteinase inhibitor, this hypothesis was confirmed. The amino acid residues of Atg8 recognized and cleaved byRavZ were further identified by peptide mass fingerprinting of the digested Atg8. They also showed that, the effect of RavZ on Atg8 activity is irreversible.

References:

1.      G. Segal, et al. 1998. Host cell killing and bacterial conjugation require overlapping sets of genes within a 22-kb region of the Legionella pneumophila genome. Proc Natl Acad Sci U S A. 95:1669-74.

2.      J. P. Vogel, et al. 1998. Conjugative transfer by the virulence system of Legionella pneumophila. Science 279, 873-876