The SecDFyajC domain of preprotein translocase controls preprotein movement by regulating SecA membrane cycling

Escherichia coli preprotein translocase comprises a membrane-embedded hexameric complex of SecY, SecE, SecG, SecD, SecF and YajC (SecYEGDFyajC) and the peripheral ATPase SecA. The energy of ATP binding and hydrolysis promotes cycles of membrane insertion and deinsertion of SecA and catalyzes the movement of the preprotein across the membrane. The proton motive force (PMF), though not essential, greatly accelerates late stages of translocation. We now report that the SecDFyajC domain of translocase slows the movement of preprotein in transit against both reverse and forward translocation and exerts this control through stabilization of the inserted form of SecA. This mechanism allows the accumulation of specific translocation intermediates which can then complete translocation under the driving force of the PMF. These findings establish a functional relationship between SecA membrane insertion and preprotein translocation and show that SecDFyajC controls SecA membrane cycling to regulate the movement of the translocating preprotein.