DELTA-MU-H+ AND ATP FUNCTION AT DIFFERENT STEPS OF THE CATALYTIC CYCLE OF PREPROTEIN TRANSLOCASE

Preprotein translocation in E. coli requires ATP, the membrane electrochemical potential DELTA-mu-H+, and translocase, an enzyme with an ATPase domain (SecA) and the membrane-embedded SecY/E. 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 approximately 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 DELTA-mu-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 DELTA-mu-H+ (fifth step), proOmpA rapidly completes translocation. DELTA-mu-H+ - driven translocation is blocked by SecA plus nonhydrolyzable ATP analogs, indicating that DELTA-mu-H+ drives translocation when ATP and proOmpA are not bound to SecA.