Part of the Sec protein translocase complex. Interacts with the SecYEG preprotein conducting channel. SecDF uses the proton motive force (PMF) to complete protein translocation after the ATP-dependent function of SecA. Forms a complex with SecF. Part of the essential Sec protein translocation apparatus which comprises SecA, SecYEG and auxiliary proteins SecDF-YajC and YidC. Belongs to the SecD/SecF family. SecD subfamily.