Abstract
FtsK protein contains a fast DNA motor that is involved in bacterial chromosome dimer resolution. During cell division, FtsK translocates double-stranded DNA until both dif recombination sites are placed at mid cell for subsequent dimer resolution. Here, we solved the 3.6-Å resolution electron cryo-microscopy structure of the motor domain of FtsK while translocating on its DNA substrate. Each subunit of the homo-hexameric ring adopts a unique conformation and one of three nucleotide states. Two DNA-binding loops within four subunits form a pair of spiral staircases within the ring, interacting with the two DNA strands. This suggests that simultaneous conformational changes in all ATPase domains at each catalytic step generate movement through a mechanism related to filament treadmilling. While the ring is only rotating around the DNA slowly, it is instead the conformational states that rotate around the ring as the DNA substrate is pushed through.
Highlights
FtsK protein contains a fast DNA motor that is involved in bacterial chromosome dimer resolution
Previous structures of DNA- or RNAhelicase complexes have suggested that nucleic acids, to peptidic substrate in unfoldases, are generally recognized by a more-or-less asymmetrical ring through motifs organized into a partial helix, akin to a spiral staircase [16,17,18,19,20,21,22]
The proposed translocation mechanisms for helicases are based on singlestranded nucleic acids, and it remains to be seen if similar mechanisms apply to motors translocating on more rigid double-stranded DNA substrates
Summary
Purified FtsKαβ mixed with 45-bp dsDNA displays ATPase activity that is too fast for EM grid preparation and 1D 31P NMR (Fig. 1A and SI Appendix, Fig. S1), as expected from its previously determined turnover rate of 2,600 ATP s−1 per hexamer [23]. FtsKαβ hydrolyzes ATPγS (but not AMPPNP) at a much lower rate (at least three orders of magnitude lower) (Fig. 1A and SI Appendix, Fig. S1). Cryo-EM with DNA, in the absence or presence of the nucleotides ADP, AMPPNP, and ATPγS, confirmed that FtsKαβ forms homo-hexameric rings without the γ-module (Fig. 1B and SI Appendix, Fig. S2A), as observed previously [13]. Its translocation mechanism is poorly characterized, and there is currently no structural data of an active doublestranded DNA translocase bound to its substrate. This article is a PNAS Direct Submission.
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