Mining of a Plant Pathogen Genome to Identify Candidate WY-Domain Containing Pathogenicity Proteins

The most destructive plant pathogens use secreted proteins known as effectors to hijack host metabolism and suppress the immune system, often by direct protein-protein interaction with their host targets. Effector-target interactions are not well understood on a biochemical level; however, crystal structures of several oomycete effectors have recently revealed a structurally conserved WY-domain that may play a role in these protein-protein interactions. The genome of Bremia lactucae, the causative agent of lettuce downy mildew, has been sequenced by the Michelmore lab and comparative and functional genomic analysis of the draft genome is underway. We have mined the B. lactucae genome for genes encoding WY-domain candidate effectors. We identified a family of expressed proteins with variable numbers of WY-domains per protein. Several candidates have no known homologs in the oomycetes and those with homologs were only distantly related, suggesting lineage specific divergence. Many of these WY-proteins are predicted to be secreted and a subset of them also contains the RXLR motif that is characteristic of secreted oomycete effectors. These candidate effectors will be functionally tested in planta for suppression of cell death and effector targets will be identified using high-throughput yeast two-hybrid assays and co-immunoprecipitation. Effector and host target identification will provide the basis for future research into the molecular mechanisms of pathogenesis.