Effectors are microbial-derived secreted proteins with an essential function in modulating host immunity during infections. Most effectors were until recently assumed to be species-specific that originated late in a pathogen’s evolutionary history to facilitate disease on specific hosts. However, research in our lab and elsewhere has revealed the presence of abundant homologous “core” effector proteins across fungi, thus challenging the “species-specific” paradigm in plant-pathogen interactions. Remarkably, despite their low sequence identity many core effectors are shown to be pleiotropically recognized by a single plant immune receptor, thus establishing that the recognition specificity of some resistance proteins is not restricted to a single effector from a specific pathogen (Stergiopoulos et al. 2010).
In our research we have sought to provide answers to critical questions regarding the biology of core effector proteins in fungi, including (i) whether their biological function and contribution to virulence are conserved among different fungal species, (ii) what is the molecular basis for their pleiotropic recognition by single cognate plant immune receptors, and (iii) how new (core) effectors emerge and acquire their functions. To answer these questions we have structurally, biochemically, and functionally analyzed three members of the fungal Avr4 effector family, whose founding member (i.e. CfAvr4 from the tomato pathogen Cladosporium fulvum) was shown to promote parasitism through binding chitin in fungal cell-walls and protecting it from chitinases. To date, we have successfully solved the crystal structure of two Avr4 family members either alone (Kohler et al 2016) or in complex with chitin (Hurlburt et al. under review) and by subsequent structure-guided site-directed mutagenesis we have provided evidence for the structural separation between the virulence and avirulence properties in this effector family. Further on, we have found that gene duplication has been a major driver of evolution within this effector family that generated paralogs with a novel function that, to our knowledge, has not been described before in fungal effector proteins (Chen et al. in preparation). Thus, next to the structural analysis of the Avr4 effector family, we will present this putatively novel effector function.
Kohler A, Chen LH, Hurlburt N, Salvucci A, Schwessinger B, Fisher A, & Stergiopoulos I (2016). Structural analysis of an Avr4 effector ortholog offers insight into chitin-binding and recognition by the Cf-4 receptor. The Plant Cell, 28: 1945-1965.
Stergiopoulos I, van den Burg HA, Ökmen B, Beenen H, van Liere S, Kema GHJ & De Wit PJGM (2010). Tomato Cf resistance proteins mediate recognition of cognate homologous effectors from fungi pathogenic on dicots and monocots. PNAS 107 (16): 7610–7615.
Hurlburt N, Chen LH, Stergiopoulos I, & Fisher A. Structural analysis of an Avr4 effector ortholog offers insight into chitin-binding and recognition by the Cf-4 receptor. Submitted.