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.