PID14 processing by Maize AF PLCPs

Ustilago maydis is a biotrophic fungus causing corn smut disease in maize. To downregulate immune responses and promote host colonization, U. maydis secretes a set of effector proteins into the plant apoplast. An effector essential for U. maydis virulence is Pit2, an inhibitor of papain-like cysteine proteases (PLCPs). Pit2 virulence function relies on a 14 amino acids motif (PID14). While sequence of the Pit2 effector is highly diverse amongst related pathogen species, the PID14 motif is highly conserved. Interestingly, synthetic PID14 peptides act more efficiently as PLCP inhibitors than the full-length Pit2 effector. In line with this finding, mass spectrometry showed processing of Pit2 by maize PLCPs, which releases an inhibitory core motif of the PID14. Mutational analysis demonstrated that two residues of the released inhibitor peptide are essential for Pit2 function and, consequently, for U. maydis virulence. Based on these findings, we propose a model, in which the Pit2 effector functions as a decoy: Pit2 represents a favorable substrate for apoplastic PLCPs, which are central hubs of the maize immune system. Processing of Pit2 releases the inhibitor peptide, which in turn efficiently blocks PLCPs to modulate host immunity.