Characterization and multiple molecular forms of TorD from Shewanella massilia, the putative chaperone of the molybdoenzyme TorA

Several bacteria use trimethylamine N‐oxyde (TMAO) as an exogenous electron acceptor for anaerobic respiration. This metabolic pathway involves expression of the tor operon that codes for a periplasmic molybdopterin‐containing reductase of the DMSO/TMAO family, a pentahemic c‐type cytochrome, and the TorD cytoplasmic chaperone, possibly required for acquisition of the molybdenum cofactor and translocation of the reductase by the twin‐arginine translocation system. In this report, we show that the TorD chaperone from Shewanella massilia forms multiple and stable oligomeric species. The monomeric, dimeric, and trimeric forms were purified to homogeneity and characterized by analytical ultracentrifugation. Small‐angle X‐ray scattering (SAXS) and preliminary diffraction data indicated that the TorD dimer is made of identical protein modules of similar size to the monomeric species. Interconversion of the native oligomeric forms occurred at acidic pH value. In this condition, ANS fluorescence indicates a non‐native conformation of the polypeptide chain in which, according to the circular dichroism spectra, the α‐helical content is similar to that of the native species. Surface plasmon resonance showed that both the monomeric and dimeric species bind the mature TorA enzyme, but that the dimer binds its target protein more efficiently. The possible biologic significance of these oligomers is discussed in relation to the chaperone activity of TorD, and to the ability of another member of the TorD family to bind the Twin Arginine leader sequences of the precursor of DMSO/TMAO reductases.