title: Processive translocation mechanism of the human Bloom's syndrome helicase along single-stranded DNA.
abstract: BLM, one of the human RecQ helicases, plays a fundamental role in homologous recombination-based error-free DNA repair pathways, which require its translocation and DNA unwinding activities. Although translocation is essential in vivo during DNA repair processes and it provides a framework for more complex activities of helicases, including strand separation and nucleoprotein displacement, its mechanism has not been resolved for any human DNA helicase. Here, we present a quantitative model for the translocation of a monomeric form of BLM along ssDNA. We show that BLM performs translocation at a low adenosine triphosphate (ATP) coupling ratio (1 ATP consumed/1 nucleotide traveled) and moderate processivity (with a mean number of 50 nucleotides traveled in a single run). We also show that the rate-limiting step of the translocation cycle is a transition between two ADP-bound enzyme states. Via opening of the helicase core, this structural change may drive the stepping of BLM along the DNA track by a directed inchworm mechanism. The data also support the conclusion that BLM performs double-stranded DNA unwinding by fully active duplex destabilization.
authors.affiliation: Department of Biochemistry, Eötvös University, Pázmány P. s. 1/c, H-1117 Budapest, Hungary.
authors.forename: Máté
authors.initials: M
authors.lastname: Gyimesi
authors.forename: Kata
authors.initials: K
authors.lastname: Sarlós
authors.forename: Mihály
authors.initials: M
authors.lastname: Kovács
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D001665:Binding Sites; D004277:DNA, Single-Stranded / Q000737:chemistry / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D021381:Protein Transport; D053484:RecQ Helicases / Q000378:metabolism*
chemical_list: D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; C097151:Bloom syndrome protein; D053484:RecQ Helicases