A Disulfide Oxidoreductase (CHU_1165) Is Essential for Cellulose Degradation by Affecting Outer Membrane Proteins in Cytophaga hutchinsonii.

Cytophaga hutchinsonii cells can bind to the surface of insoluble cellulose and degrade it by utilizing a novel cell contact-dependent mechanism, in which the outer membrane proteins may play important roles. In this study, the deletion of a gene locus, chu_1165, which encodes a hypothetical protein with 32% identity with TlpB, a disulfide oxidoreductase in Flavobacterium psychrophilum, caused a complete cellulolytic defect in C. hutchinsonii Further study showed that cells of the Δ1165 strain could not bind to cellulose, and the levels of many outer membrane proteins that can bind to cellulose were significantly decreased. The N-terminal region of CHU_1165 is anchored to the cytoplasmic membrane with five predicted transmembrane helices, and the C-terminal region is predicted to stretch to the periplasm and has a similar thioredoxin (Trx) fold containing a Cys-X-X-Cys motif that is conserved in disulfide oxidoreductases. Recombinant CHU_1165His containing the Cys-X-X-Cys motif was able to reduce the disulfide bonds of insulin in vitro Site-directed mutation showed that the cysteines in the Cys-X-X-Cys motif and at residues 106 and 108 were indispensable for the function of CHU_1165. Western blotting showed that CHU_1165 was in an oxidized state in vivo, suggesting that it may act as an oxidase to catalyze disulfide bond formation. However, many of the decreased outer membrane proteins that were essential for cellulose degradation contained no or one cysteine, and mutation of the cysteine in these proteins did not affect cellulose degradation, indicating that CHU_1165 may have an indirect or pleiotropic effect on the function of these outer membrane proteins.IMPORTANCE Cytophaga hutchinsonii can rapidly digest cellulose in a contact-dependent manner, in which the outer membrane proteins may play important roles. In this study, a hypothetical protein, CHU_1165, characterized as a disulfide oxidoreductase, is essential for cellulose degradation by affecting the cellulose binding ability of many outer membrane proteins in C. hutchinsonii Disulfide oxidoreductases are involved in disulfide bond formation. However, our studies show that many of the decreased outer membrane proteins that were essential for cellulose degradation contained no or one cysteine, and mutation of cysteine did not affect their function, indicating that CHU_1165 did not facilitate the formation of a disulfide bond in these proteins. It may have an indirect or pleiotropic effect on the function of these outer membrane proteins. Our study provides an orientation for exploring the proteins that assist in the appropriate conformation of many outer membrane proteins essential for cellulose degradation, which is important for exploring the novel mechanism of cellulose degradation in C. hutchinsonii.