Expression of V.alginolyticus SecDF in an E.coli secDF mutant restores protein export in a Na(+)-dependent manner, strongly suggesting SecDF functions via cation-coupled protein translocation. 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). Forms a complex with SecD. Part of the essential Sec protein translocation apparatus which comprises SecA, SecYEG and auxiliary proteins SecDF-YajC and YidC (By similarity). Belongs to the SecD/SecF family. SecF subfamily.