Killer proteins 4 and 6 from the fungal wheat pathogen Zymoseptoria tritici are toxic to fungi and structurally related to effector families
Effectors from fungal plant pathogens are challenging to classify due to their low sequence identity. However, understanding their structure allows for their classification into families. The structures of two putative effectors from the wheat fungal pathogen Zymoseptoria tritici were determined using X-ray crystallography. Zt-Mycgr3-91409 is a candidate MAX effector, while Zt-NIP1 induces wheat leaf necrosis. Surprisingly, these two proteins were structurally related to KP6α and KP4 killer toxins, respectively, encoded by UmV dsRNA viruses infecting the corn smut Ustilago maydis. Zt-Mycgr3-91409 and Zt-NIP1 were renamed Zt-KP6-1 and Zt-KP4-1. Homologs of Zt-KP6-1 and Zt-KP4-1 were identified in Zymoseptoria species. Fungal proteins structurally related to Zt-KP6-1 and Zt-KP4-1 were identified using a novel pipeline based on Foldseek and HMM searches. These Zt-KP6-1 and Zt-KP4-1 like proteins were widely distributed across fungi. The biological activity of Zt-KP6-1 and Zt-KP4-1 was assessed on plants and fungi. Infiltration of Zt-KP6-1 and Zt-KP4-1 into wheat leaves did not induce visible symptoms. Zt-KP6-1 completely inhibited the growth of Botrytis cinerea, while it was less active on Z. tritici. Zt-KP4-1 was toxic to both B. cinerea and Z. tritici, demonstrating that these effectors are toxic to fungi. This study showed the importance of structure prediction in understanding the function of fungal effectors.