The glycyl-radical enzyme 2-ketobutyrate formate-lyase, TdcE, interacts specifically with the formate-translocating FNT-channel protein FocA

Formate is a major product of mixed-acid fermentation in Escherichia coli. Because formate can act as an uncoupler at high concentration it must be excreted from the cell. The FNT (formate-nitrite transporter) membrane channel FocA ensures formate is translocated across the cytoplasmic membrane. Two glycyl-radical enzymes (GREs), pyruvate formate-lyase (PflB) and 2-ketobutyrate formate-lyase (TdcE), generate formate as a product of catalysis during anaerobic growth of Escherichia coli. We demonstrate in this study that TdcE, like PflB, interacts specifically with FocA. His-tagged variants of two other predicted GREs encoded in the genome of E. coli were over-produced and purified and were shown not to interact with FocA, indicating that interaction with FocA is not a general property of GREs per se. Together, these data show that only the GREs TdcE and PflB interact with the FNT channel protein and suggest that, like PflB, TdcE can control formate translocation by FocA.