title: A two-site mechanism for ATP hydrolysis by the asymmetric Rep dimer P2S as revealed by site-specific inhibition with ADP-A1F4.
abstract: The Escherichia coli Rep helicase is a dimeric motor protein that catalyzes the transient unwinding of duplex DNA to form single-stranded (ss) DNA using energy derived from the binding and hydrolysis of ATP. In an effort to understand this mechanism of energy transduction, we have used pre-steady-state methods to study the kinetics of ATP binding and hydrolysis by an important intermediate in the DNA unwinding reaction--the asymmetric Rep dimer state, P2S, where ss DNA [dT(pT)15] is bound to only one subunit of the Rep dimer. To differentiate between the two potential ATPase active sites inherent in the dimer, we constructed dimers with one subunit covalently cross-linked to ss DNA and where one or the other of the ATPase sites was selectively complexed to the tightly bound transition state analog ADP-A1F4. We found that when ADP-A1F4 is bound to the Rep subunit in trans from the subunit bound to ss DNA, steady-state ATPase activity of 18 s(-1) per dimer (equivalent to wild-type P2S) was recovered. However, when the ADP-A1F4 and ss DNA are both bound to the same subunit (cis), then a titratable burst of ATP hydrolysis is observed corresponding to a single turnover of ATP. Rapid chemical quenched-flow techniques were used to resolve the following minimal mechanism for ATP hydrolysis by the unligated Rep subunit of the cis dimer: E + ATP <==> E-ATP <==> E'-ATP <==> E'-ADP-Pi <==> E-ADP-Pi <==> E-ADP + Pi <==> E + ADP + Pi, with K1 = (2.0 +/- 0.85) x 10(5) M(-1), k2 = 22 +/- 3.5 s(-1), k(-2) < 0.12 s(-1), K3 = 4.0 +/- 0.4 (k3 > 200 s(-1)), k4 = 1.2 +/- 0.14 s(-1), k(-4) << 1.2 s(-1), K5 = 1.0 +/- 0.2 mM, and K6 = 80 +/- 8 microM. A salient feature of this mechanism is the presence of a kinetically trapped long-lived tight nucleotide binding state, E'-ADP-Pi. In the context of our "subunit switching" model for Rep dimer translocation during processive DNA unwinding [Bjornson, K. B., Wong, I., & Lohman, T. M. (1996) J. Mol. Biol. 263, 411-422], this state may serve an energy storage function, allowing the energy from the binding and hydrolysis of ATP to be harnessed and held in reserve for DNA unwinding.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
authors.forename: I
authors.initials: I
authors.lastname: Wong
authors.forename: T M
authors.initials: TM
authors.lastname: Lohman
mesh_terms: D000244:Adenosine Diphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D004265:DNA Helicases / Q000737:chemistry / Q000378:metabolism; D004277:DNA, Single-Stranded / Q000378:metabolism; D019281:Dimerization; D004926:Escherichia coli / Q000201:enzymology; D029968:Escherichia coli Proteins; D006868:Hydrolysis; D007700:Kinetics; D008956:Models, Chemical; D009942:Organometallic Compounds / Q000494:pharmacology*; D010710:Phosphates / Q000494:pharmacology; D013379:Substrate Specificity
chemical_list: D004277:DNA, Single-Stranded; D029968:Escherichia coli Proteins; D009942:Organometallic Compounds; D010710:Phosphates; C105164:adenosine diphosphate tetrafluoroaluminate; C506744:rep protein, E coli; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D004265:DNA Helicases