The crystal structure of SnTox3 from the necrotrophic fungusParastagonospora nodorumreveals a unique effector fold and insights into Kex2 protease processing of fungal effectors

Abstract

SummaryPlant pathogens cause disease through secreted effector proteins, which act to modulate host physiology and promote infection. Typically, the sequences of effectors provide little functional information and further targeted experimentation is required. Here, we utilised a structure/function approach to study SnTox3, an effector from the necrotrophic fungal pathogenParastagonospora nodorum, which causes cell death in wheat-lines carrying the sensitivity geneSnn3.We developed a workflow for the production of SnTox3 in a heterologous host that enabled crystal structure determination. We show this approach can be successfully applied to effectors from other pathogenic fungi. Complementing this, anin-silicostudy uncovered the prevalence of an expanded subclass of effectors from fungi.The β-barrel fold of SnTox3 is a novel fold among fungal effectors. We demonstrate that SnTox3 is a pre-pro-protein and that the protease Kex2 removes the pro-domain. Ourin-silicostudies suggest that Kex2-processed pro-domain (designated here as K2PP) effectors are common in fungi, and we demonstrate this experimentally for effectors fromFusarium oxysporumf sp.lycopersici.We propose that K2PP effectors are highly prevalent among fungal effectors. The identification and classification of K2PP effectors has broad implications for the approaches used to study their function in fungal virulence.