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Structural and functional studies of SF1B Pif1 from Thermus oshimai reveal dimerization-induced helicase inhibition.
Nucleic Acids Res, 49(7):4129-4143.
Pif1 is an SF1B helicase that is evolutionarily conserved from bacteria to humansand plays multiple roles in maintaining genome stability in both nucleus andmitochondria. Though highly conserved, Pif1 family harbors a large mechanisticdiversity. Here, we report crystal structures of Thermus oshimai Pif1 (ToPif1)alone and complexed with partial duplex or single-stranded DNA. In the apo stateand in complex with a partial duplex DNA, ToPif1 is monomeric with its domain2B/loop3 adopting a closed and an open conformation, respectively. When complexedwith a single-stranded DNA, ToPif1 forms a stable dimer with domain 2B/loop3shifting to a more open conformation. Single-molecule and biochemical assays showthat domain 2B/loop3 switches repetitively between the closed and openconformations when a ToPif1 monomer unwinds DNA and, in contrast with othertypical dimeric SF1A helicases, dimerization has an inhibitory effect on itshelicase activity. This mechanism is not general for all Pif1 helicases butillustrates the diversity of regulation mechanisms among different helicases. Italso raises the possibility that although dimerization results in activation forSF1A helicases, it may lead to inhibition for some of the other uncharacterizedSF1B helicases, an interesting subject warranting further studies.
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