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 (By similarity). Part of the essential Sec protein translocation apparatus which comprises SecA, SecYEG and auxiliary proteins SecDF-YajC and YidC. In the N-terminal section; belongs to the SecD/SecF family. SecD subfamily. In the C-terminal section; belongs to the SecD/SecF family. SecF subfamily.