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

Abstract

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 (M agnaporthe 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.

Recognition of the Magnaporthe oryzae Effector AVR-Pia by the Decoy Domain of the Rice NLR Immune Receptor RGA5

“…M. oryzae isolates carrying the AVRPia-H3 allele are virulent on rice varieties carrying the Pia resistance locus and AVR-Pia-H3 does not interact in yeast two-hybrid (Y2H) assays with the C-terminal part of the rice NLR immune receptor RGA5 containing the RATX1 domain (RGA5 C-ter ). The NMR structure of AVR-Pia showed that both the F24 and T46 residues are surface exposed and suggested that the corresponding substitutions affect only AVR-Pia surface properties without major structural rearrangements (de Guillen et al, 2015).…”

“…Although AVR-Pik, AVR-Pia, and AVR1-CO39 do not share sequence similarities, they share a highly similar three-dimensional structure characterized by a six b-sandwich fold also present in two other effectors: AvrPiz-t from M. oryzae and ToxB from the wheat pathogenic fungus Pyrenophora tritici repentis (Zhang et al, 2013;de Guillen et al, 2015;Maqbool et al, 2015;Nyarko et al, 2014). The corresponding, structurally related Magnaporthe Avr and ToxB effectors were termed MAX effectors.…”

“…The corresponding, structurally related Magnaporthe Avr and ToxB effectors were termed MAX effectors. MAX effectors are present in other sometimes only distantly related phytopathogenic fungi and the MAX effector family underwent strong expansion in M. oryzae where it accounts for roughly 10% of the effectors (de Guillen et al, 2015).…”

Population diversity and genomics of the fungal pathogen of canola Leptosphaeria maculans

“…The majority of effectors in fungal pathogen genomes are single genes and a rare few genes form small paralog families comprising up to three genes (de Guillen et al, 2015). These effectors do not share sequence homology with other effectors and also do not share any conserved motifs (de Guillen et al, 2015).…”

“…These effectors do not share sequence homology with other effectors and also do not share any conserved motifs (de Guillen et al, 2015). Ohm et al (2012) (Zhang et al, 2013) and Pyrenophora tritici-repentis (Sarma et al, 2005) that all share β-sandwich structures.…”