A type III effector ADP-ribosylates
RNA-binding proteins and quells plant
Zheng Qing Fu1
*, Ming Guo1
*, Byeong-ryool Jeong1
, Fang Tian1,2
, Thomas E. Elthon2,3
, Ronald L. Cerny4
& James R. Alfano1
The bacterial plant pathogen Pseudomonas syringae injects effector proteins into host cells through a type III protein
secretion system to cause disease. The enzymatic activities of most of P. syringae effectors and their targets remain obscure.
Here we show that the type III effector HopU1 is a mono-ADP-ribosyltransferase (ADP-RT). HopU1 suppresses plant innate
immunity in a manner dependent on its ADP-RT active site. The HopU1 substrates in Arabidopsis thaliana extracts were
RNA-binding proteins that possess RNA-recognition motifs (RRMs). A. thaliana knockout lines defective in the glycine-rich
RNA-binding protein GRP7 (also known as AtGRP7), a HopU1 substrate, were more susceptible than wild-type plants to P.
syringae. The ADP-ribosylation of GRP7 by HopU1 required two arginines within the RRM, indicating that this modification