SecYEG and SecA Are the Stoichiometric Components of Preprotein Translocase

The transport of large preproteins across the Escherichia coli plasma membrane is catalyzed by preprotein translocase, comprised of the peripherally bound SecA subunit and an integrally bound heterotrimeric domain consisting of the SecY, SecE, and SecG subunits. We have now placed the secY, secE, and secG genes under the control of an arabinose-inducible promoter on a multicopy plasmid. Upon induction, all three of the proteins are strongly overexpressed and recovered in the plasma membrane fraction. These membranes show a strong enhancement of 1) translocation ATPase activity, 2) preprotein translocation, 3) capacity for SecA binding, and 4) formation of the membrane-inserted form of SecA. These data establish that SecY, SecE, and SecG constitute the integral membrane domain of preprotein translocase.