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. The large periplasmic domain is thought to have a base and head domain joined by a hinge; movement of the hinge may be coupled to both proton transport and protein export, with the head domain capturing substrate, and a conformational change preventing backward movement and driving forward movement. Expression of V.alginolyticus SecD and SecF in E.coli confers Na(+)-dependent protein export, strongly suggesting SecDF functions via cation-coupled protein translocation. Part of the prokaryotic protein translocation apparatus which is comprised of SecA, SecB, SecD, SecE, SecF, SecG and SecY. The SecDF-YidC-YajC translocase forms a supercomplex with SecYEG, called the holo-translocon (HTL) (PubMed:27435098). The stoichiometry of the super complex may be SecYEG:YidC:SecDF 4:3:1, YajC is in the reconstituted complex (with SecDF) but as no antibody is available it could not be quantified (PubMed:27435098). Disruption of both secD and SecF confers cold-sensitive growth (strain secD1(Cs)). Belongs to the SecD/SecF family. SecF subfamily. Extended N-terminus.