Involvement of a mate chaperone (TorD) in the maturation pathway of molybdoenzyme TorA

As many prokaryotic molybdoenzymes, the trimethylamine oxide reductase ( TorA) of Escherichia coli requires the insertion of a bis(molybdopterin guanine dinucleotide) molybdenum cofactor in its catalytic site to be active and translocated to the periplasm. We show in vitro that the purified apo form of TorA was activated weakly when an appropriate bis( molybdopterin guanine dinucleotide) molybdenum source was provided, whereas addition of the TorD chaperone increased apo-TorA activation up to 4-fold, allowing maturation of most of the apoprotein. We demonstrate that TorD alone is sufficient for the efficient activation of apoTorA by performing a minimal in vitro assay containing only the components for the cofactor synthesis, apoTorA and TorD. Interestingly, incubation of apoTorA with TorD before cofactor addition led to a significant increase of apoTorA activation, suggesting that TorD acts on apoTorA before cofactor insertion. This result is consistent with the fact that TorD binds to apoTorA and probably modifies its conformation in the absence of cofactor. Therefore, we propose that TorD is involved in the first step of TorA maturation to make it competent to receive the cofactor.