TatD is a central component of a Tat translocon-initiated quality control system for exported FeS proteins in Escherichia coli

Bacterial Tat systems export folded proteins, including FeS proteins such as NrfC and NapG, which acquire their cofactors before translocation. NrfC and NapG are proofread by the Tat pathway, and misfolded examples are degraded after interaction with the translocon. Here, we identify TatD as a crucial component of this quality control system in Escherichia coli. NrfC/NapG variants lacking FeS centres are rapidly degraded in wild-type cells but stable in a DeltatatD strain. The precursor of another substrate, FhuD, is also transiently detected in wild-type cells but stable in the DeltatatD strain. Surprisingly, these substrates are stable in DeltatatD cells that overexpress TatD, and export of the non-mutated precursors is inhibited. We propose that TatD is part of a quality control system that is intimately linked to the Tat export pathway, and that the overexpression of TatD leads to an imbalance between the two systems such that both Tat-initiated turnover and export are prevented.