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The actinobacteriumCorynebacterium matruchotii has been implicated in nucleation of oral microbial consortia leading to biofilm formation. Due to the lack of genetic tools, little is known about basic cellular processes, including protein secretion and folding, in this organism. We report here a survey of theC. matruchotii genome, which encodes a large number of exported proteins containing paired cysteine residues, and identified an oxidoreductase that is highly homologous to theCorynebacterium diphtheriae thiol-disulfide oxidoreductase MdbA (MdbACd ). Crystallization studies uncovered that the 1.2-Å resolution structure ofC. matruchotii MdbA (MdbACm ) possesses two conserved features found in actinobacterial MdbA enzymes, a thioredoxin-like fold and an extended α-helical domain. By reconstituting the disulfide bond-forming machinein vitro , we demonstrated that MdbACm catalyzes disulfide bond formation within the actinobacterial pilin FimA. A new gene deletion method supported thatmdbA is essential inC. matruchotii . Remarkably, heterologous expression of MdbACm in theC. diphtheriae ΔmdbA mutant rescued its known defects in cell growth and morphology, toxin production, and pilus assembly, and this thiol-disulfide oxidoreductase activity required the catalytic motif CXXC. Altogether, the results suggest that MdbACm is a major thiol-disulfide oxidoreductase, which likely mediates posttranslocational protein folding inC. matruchotii by a mechanism that is conserved inActinobacteria .IMPORTANCE The actinobacteriumCorynebacterium matruchotii has been implicated in the development of oral biofilms or dental plaque; however, little is known about the basic cellular processes in this organism. We report here a high-resolution structure of aC. matruchotii oxidoreductase that is highly homologous to theCorynebacterium diphtheriae thiol-disulfide oxidoreductase MdbA. By biochemical analysis, we demonstrated thatC. matruchotii MdbA catalyzes disulfide bond formationin vitro . Furthermore, a new gene deletion method revealed that deletion ofmdbA is lethal inC. matruchotii . Remarkably,C. matruchotii MdbA can replaceC. diphtheriae MdbA to maintain normal cell growth and morphology, toxin production, and pilus assembly. Overall, our studies support the hypothesis thatC. matruchotii utilizes MdbA as a major oxidoreductase to catalyze oxidative protein folding.

Structural Basis of a Thiol-Disulfide Oxidoreductase in the Hedgehog-Forming Actinobacterium Corynebacterium matruchotii