title: Nucleotide exchange from the high-affinity ATP-binding site in SecA is the rate-limiting step in the ATPase cycle of the soluble enzyme and occurs through a specialized conformational state.
abstract: We have characterized the kinetic and thermodynamic consequences of adenine nucleotide interaction with the low-affinity and high-affinity nucleotide-binding sites in free SecA. ATP binds to the hydrolytically active high-affinity site approximately 3-fold more slowly than ADP when SecA is in its conformational ground state, suggesting that ATP binding probably occurs when the enzyme is in another conformational state during the productive ATPase/transport cycle. The steady-state ATP hydrolysis rate is equivalent to the rate of ADP release from the high-affinity site under a number of conditions, indicating that this process is the rate-limiting step in the ATPase cycle of the free enzyme. Because efficient protein translocation requires at least a 100-fold acceleration in the ATPase rate, the rate-limiting process of ADP release from the high-affinity site is likely to play a controlling role in the conformational reaction cycle of SecA. This release process involves a large enthalpy of activation, suggesting that it involves a protein conformational change, and two observations indicate that this conformational change is different from the well-characterized endothermic conformational transition believed to gate the binding of SecA to SecYEG. First, nucleotide binding to the low-affinity site strongly inhibits the endothermic transition but does not reduce the rate of ADP release. Second, removal of Mg(2+) from an allosteric binding site on SecA does not perturb the endothermic transition but produces a 10-fold acceleration in the rate of ADP release. These divergent effects suggest that a specialized conformational transition mediates the rate-limiting ADP-release process in SecA. Finally, ADP, 2'-O-(N-methylanthraniloyl)-adenosine-5'-diphosphate (MANT-ADP), and adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S) bind with similar affinities to the high-affinity site and also to the low-affinity site as inferred from their consistent effects in inhibiting the endothermic transition. In contrast, adenosine 5'-(beta,gamma-imino)triphosphate (AMPPNP) shows 100-fold weaker affinity than ADP for the high-affinity site and no detectable interaction with the low-affinity site at concentrations up to 1 mM, suggesting that this nonhydrolyzable analogue may not be a faithful mimic of ATP in its interactions with SecA.
authors.affiliation: Department of Biological Sciences, 702A Fairchild Center, MC2434, Columbia University, New York, New York 10027, USA.
authors.forename: John J
authors.initials: JJ
authors.lastname: Fak
authors.forename: Anna
authors.initials: A
authors.lastname: Itkin
authors.forename: Daita D
authors.initials: DD
authors.lastname: Ciobanu
authors.forename: Edward C
authors.initials: EC
authors.lastname: Lin
authors.forename: Xiang-Jin
authors.initials: XJ
authors.lastname: Song
authors.forename: Yi-Te
authors.initials: YT
authors.lastname: Chou
authors.forename: Lila M
authors.initials: LM
authors.lastname: Gierasch
authors.forename: John F
authors.initials: JF
authors.lastname: Hunt
mesh_terms: D000244:Adenosine Diphosphate / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000494:pharmacology; D000266:Adenylyl Imidodiphosphate / Q000378:metabolism / Q000494:pharmacology; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D011487:Protein Conformation; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D012697:Serine Endopeptidases / Q000378:metabolism; D012995:Solubility; D013696:Temperature; D062367:ortho-Aminobenzoates / Q000378:metabolism / Q000494:pharmacology
chemical_list: D001426:Bacterial Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; D062367:ortho-Aminobenzoates; D000266:Adenylyl Imidodiphosphate; D000244:Adenosine Diphosphate; C081523:3'-O-(N-methylanthraniloyl)adenosine 5'-diphosphate; C045473:3'-O-(N-methylanthraniloyl) ATP; D000255:Adenosine Triphosphate; D012697:Serine Endopeptidases; C026515:type I signal peptidase; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins; D008274:Magnesium