The ER-Golgi Lipid Exchanger PsORP1 Is Essential for Phytophthora sojae Development and Is Blocked by Oxathiapiprolin via Its OSBP-Related Domain (ORD).

Oxysterol-binding protein (OSBP)-related proteins (ORPs) are indispensable eukaryotic lipid transporters and promising targets for pharmaceutical and agrochemical exploitation. Among ORP family members across kingdoms, oomycete ORP1 constitutes a phylogenetically distinct subtype that evolved independently in oomycetes, which is markedly divergent from ORP homologues in animals, plants and fungi. Oxathiapiprolin, the first commercial fungicide targeting OSBP family proteins, specifically acts on oomycete ORP1 and exhibits robust inhibitory efficacy against diverse Phytophthora pathogens and downy mildews. Nevertheless, the molecular function of oomycete ORP1 and the inhibitory mechanism of oxathiapiprolin remain poorly defined. Here, we demonstrate that PsORP1 of Phytophthora sojae specifically localises to ER-Golgi membrane contact sites (MCSs) and is indispensable for vegetative growth and asexual/sexual development of P. sojae. In vitro liposome reconstitution assays confirmed that the conserved C-terminal OSBP-related domain (ORD) of PsORP1 mediates canonical phosphatidylserine (PS)/phosphatidylinositol 4-phosphate (PI4P) counter-transport. Biochemical binding assays further verified that oxathiapiprolin directly interacts with the ORD of PsORP1 and competitively blocks PS/PI4P lipid exchange. Collectively, this study demonstrates that PsORP1 localises to ER-Golgi MCSs and mediates ORD-dependent PS/PI4P counter-transport, which is essential for P. sojae development, and elucidates that oxathiapiprolin directly targets the ORD to competitively block this lipid exchange, providing a structural framework for rational design of next-generation OSBP-targeting inhibitors against oomycetes.