The antifungal protein AfpB induces regulated cell death in its parental fungus Penicillium digitatum

Filamentous fungi produce small cysteine-rich proteins with potent, specific antifungal activity, offering the potential to fight fungal infections that severely threaten human health and food safety and security. The genome of the citrus postharvest fungal pathogen Penicillium digitatum encodes one of these antifungal proteins, namely AfpB. Biotechnologically produced AfpB inhibited the growth of major pathogenic fungi at minimal concentrations, surprisingly including its parental fungus, and conferred protection to crop plants against fungal infections. This study reports an in-depth characterization of the AfpB mechanism of action, showing that it is a cell-penetrating protein that triggers a regulated cell death program in the target fungus. We prove the importance of AfpB interaction with the fungal cell wall to exert its killing activity, for which protein mannosylation is required. We also show that the potent activity of AfpB correlates with its rapid and efficient uptake by fungal cells through an energy-dependent process. Once internalized, AfpB induces a transcriptional reprogramming signaled by reactive oxygen species that ends in cell death. Our data show that AfpB activates a self-injury program, suggesting that this protein has a biological function in the parental fungus beyond defense against competitors, presumably more related to regulation of the fungal population. Our results demonstrate that this protein is a potent antifungal that acts through various targets to kill fungal cells through a regulated process, making AfpB a promising compound for the development of novel biofungicides with multiple fields of application in crop and postharvest protection, food preservation, and medical therapies.

This work was supported by grants BIO2015-68790-C2-1-R, BIO2015-68790-C2-2-R, RTI2018-101115B-C21, and RTI2018-101115B-C22, the Severo Ochoa Program for Centers of Excellence in R&D (SEV-2015-0533) from the Spanish Ministerio de Ciencia, Innovación y Universidades (cofinanced with FEDER funds), and the CERCA Program/Generalitat de Catalunya. S.G. was a recipient of a predoctoral scholarship (FPU13/04584) within the FPU program from the Ministerio de Educación, Cultura y Deporte (MECD, Spain).

Peer reviewed