ΔµH+ and ATP Function at Different Steps of the Catalytic Cycle of Preprotein Translocase

Preprotein translocation in E. coli requires ATP, the membrane electrochemical potential Δμ H +, and translocase, an enzyme with an ATPase domain (SecA) and the membrane-embedded SecYE. Studies of translocase and proOmpA reveal a five-step catalytic cycle: First, proOmpA binds to the SecA domain. Second, SecA binds ATP. Third, ATP-binding energy permits translocation of ∼20 residues of proOmpA. Fourth, ATP hydrolysis releases proOmpA. ProOmpA may then rebind to SecA and reenter this cycle, allowing progress through a series of transmembrane intermediates. In the absence of Δμ H + or association with SecA, proOmpA passes backward through the membrane, but moves forward when either ATP and SecA or a membrane electrochemical potential is supplied. However, in the presence of Δμ H + (fifth step), proOmpA rapidly completes translocation. Δμ H + — driven translocation is blocked by SecA plus nonhydrolyzable ATP analogs, indicating that Δμ H + drives translocation when ATP and proOmpA are not bound to SecA.