Proteomic investigation of the secretome of cellvibrio japonicus during growth on chitin.

Studies of the secretomes of microbes grown on insoluble substrates are important for the discovery of novel proteins involved in biomass conversion. However, data in literature and this study indicate that secretome samples tend to be contaminated with cytoplasmic proteins. We have examined the secretome of the Gram-negative soil bacterium Cellvibrio japonicus using a simple plate-based culturing technique that yields samples with high fractions (60-75%) of proteins that are predicted to be secreted. By combining this approach with label-free quantification using the MaxLFQ algorithm, we have mapped and quantified proteins secreted by C. japonicus during growth on alpha- and beta-chitin. Hierarchical clustering of the detected protein quantities revealed groups of up-regulated proteins that include all five putative C. japonicus chitinases as well as a chitin-specific lytic polysaccharide monooxygenase (CjLPMO10A). A small set of secreted proteins were co-regulated with known chitin-specific enzymes, including several with unknown catalytic functions. These proteins provide interesting targets for further studies aimed at unraveling the enzymatic machineries used by C. japonicus for recalcitrant polysaccharide degradation. Studies of chitin degradation indicated that C. japonicus indeed produces an efficient chitinolytic enzyme cocktail. All MS data have been deposited in the ProteomeXchange with the dataset identifier PXD002843 (http://proteomecentral.proteomexchange.org/dataset/PXD002843).