Interactions between the Powdery Mildew Effector BEC1054 and Barley Proteins Identify Candidate Host Targets

There are over 500 candidate secreted effector proteins (CSEPs) or Blumeria effector candidates (BECs) specific to the barley powdery mildew pathogen Blumeria graminis f.sp. hordei. The CSEP/ BEC proteins are expressed and predicted to be secreted by biotrophic feeding structures called haustoria. Eight BECs are required for the formation of functional haustoria. These include the RNAse-like effector BEC1054 (synonym CSEP0064). In order to identify host proteins targeted by BEC1054, recombinant BEC1054 was expressed in E. coli, solubilised and used in pull-down assays from total barley protein extracts. Many putative interactors were identified by LC-MS/MS, despite subtraction of unspecific binders in negative controls. Therefore, a directed yeast-2-hybrid assay, developed to measure the effectiveness of the interactions in yeast, was used to validate putative interactors. We conclude that BEC1054 may target several host proteins including a glutathione-S-transferase, a malate dehydrogenase and a pathogen-related-5 protein isoform, indicating a possible role for BEC1054 in compromising well known key players of resistance and response to pathogens. In addition, pull-down data indicated the possible interaction of BEC1054 with ribosomal proteins, including an elongation factor 1 gamma. This data supports the idea that BEC1054 binds to barley ribosomal proteins. The biological significance of this remains to be defined.