Lytic polysaccharide monooxygenases (LPMOs) are oxidative biocatalysts important for biofuel development. LPMOs use molecular oxygen and an external electron donor to cleave chemically recalcitrant polysaccharide. Because of LPMOs important role in carbohydrate turnover, interest in their mechanism of action and identification of their redox partners has recently surged. Current knowledge of the electron donor partners in LPMO-catalyzed reactions solely relies on observations of LPMO

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... y proteins and on conjecture about the molecules that may act as LPMO-associated reductants. Here, using direct protein-protein interaction studies of a fungal secretome, we sought to identify LPMOs endogenous redox partner. To this end, we searched the genome and secretome of the fungus Botrytis cinerea for the presence of LPMOs belonging to auxiliary activity family 9 (AA9) superfamily and identified BcLPMO 9 C as the predominant secreted LPMO. Using pull-down, label-transfer, and other biochemical assays, we then scrutinized the fungal secretome for a probable redox partner of BcLPMO 9 C and identified cellobiose dehydrogenase_53425 (CDH_53425) as its electron donor partner. Their interaction was validated with experimental strategies such as competition ELISA, surface plasmon resonance and label-transfer studies. Further, using X-ray diffraction and SEM analyses, we established that BcLPMO 9 C and CDH_53425 are redox partners in degrading complex biomass. Overall, our study provides for the first time a rational approach for secretome analysis to identify an endogenous redox partner of LPMO. This advancement in our knowledge on the molecular mechanisms of the LPMO-CDH interaction will provide insights into improving cellulolytic enzyme cocktails used in the biofuels industry.