Structural and functional analyses of Phytophthora sojae RxLR effector PSR2 reveal functional domains for host RNA silencing suppression activity
D. CHOI (1), J. He (2), S. Duan (1), J. Ma (2), W. Ma (1) (1) University of California, Riverside, U.S.A.; (2) Fudan University, China

Small RNA silencing is a universal gene regulation mechanism in eukaryotes. There is an emerging body of evidence suggesting that small RNAs are integral regulators of plant immunity during the infection of filamentous eukaryotic pathogens including fungi and oomycetes. To overcome this defense, the destructive oomycete pathogens in the genus Phytophthora have evolved effector proteins to specifically suppress small RNA silencing. Previously, Phytophthora suppressor of RNA silencing 2 (PSR2) was identified from Phytophthora sojae as an essential virulence factor. Here, we report the motif arrangement and crystal structure of PSR2. PSR2 is an RxLR effector containing multiple L-W-Y motif modules that form tandem repeats. Each L-W-Y module folds into a five-helix bundle that is stabilized by a hydrophobic core. The overall structure looks like a string of helix-bundles linked by loops. Using mutagenesis analysis, we found that the C-terminal L-W-Y module is required for the RNA silencing suppression activity; this region is enriched for basic amino acids and required for the nuclear localization of PSR2 in plant cells. Furthermore, a fragment within the central region that forms a negatively charged surface is also required for PSR2 function, possibly through mediating its interaction with host proteins. Effectors containing similar tandem repeats are prevalent in Phytophthora species, supporting essential biological significance of this architecture.

Abstract Number: P9-246
Session Type: Poster