Structure Analysis Uncovers a Highly Diverse but Structurally Conserved Effector Family in Phytopathogenic Fungi

Phytopathogenic ascomycete fungi possess huge effector repertoires that are dominated by hundreds of sequence-unrelated small secreted proteins. the molecular function of these effectors and the evolutionary mechanisms that generate this tremendous number of singleton genes are largely unknown. to get a deeper understanding of fungal effectors,we determined by nmr spectroscopy the 3-dimensional structures of the magnaporthe oryzae effectors avr1-co39 and avr-pia. despite a lack of sequence similarity,both proteins have very similar 6 β-sandwich structures that are stabilized in both cases by a disulfide bridge between 2 conserved cysteins located in similar positions of the proteins. structural similarity searches revealed that avrpiz-t,another effector from m. oryzae,and toxb,an effector of the wheat tan spot pathogen pyrenophora tritici-repentis have the same structures suggesting the existence of a family of sequence-unrelated but structurally conserved fungal effectors that we named max-effectors (magnaporthe avrs and toxb like). structure-informed pattern searches strengthened this hypothesis by identifying max-effector candidates in a broad range of ascomycete phytopathogens. strong expansion of the max-effector family was detected in m. oryzae and m. grisea where they seem to be particularly important since they account for 5–10% of the effector repertoire and 50% of the cloned avirulence effectors. expression analysis indicated that the majority of m. oryzae max-effectors are expressed specifically during early infection suggesting important functions during biotrophic host colonization. we hypothesize that the scenario observed for max-effectors can serve as a paradigm for ascomycete effector diversity and that the enormous number of sequence-unrelated ascomycete effectors may in fact belong to a restricted set of structurally conserved effector families. © 2015 de guillen et al.