Required for efficient translocation of secretory pre-proteins under conditions of hypersecretion but is not required for the release of mature proteins from the membrane. 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 protein SecDF. Other proteins may also be involved (By similarity). Found uniformly distributed in both the mother cell and forespore following sporulation. Shows maximal expression at the beginning of post-exponential growth phase and increased expression by glucose in the post-exponential growth phase when cells are cultured on rich medium. Expressed constitutively during growth in minimal medium. Cells lacking this gene show a cold-sensitive phenotype and a filamentous morphology. 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.