Conducts protons, which can be regulated by a proton gradient or by binding of an unfolded protein. Proton conduction requires the large periplasmic domain of the SecD. 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. Part of the essential Sec protein translocation apparatus which comprises SecA, SecYEG and auxiliary protein SecDF. Other proteins may also be involved (By similarity). Monomer. 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.