Journal: bioRxiv / Year: 2025 Title: Distinct Quaternary States, Intermediates, and Autoinhibition During Loading of the DnaB-Replicative Helicase by the Phage λP Helicase Loader. Authors: Abhipsa Shatarupa / Dhanjai Brown / Paul Dominic B Olinares / Jillian Chase / Eta Isiorho / Brian T Chait / David Jeruzalmi Abstract: Replicative helicases require loader proteins for assembly at the origins of DNA replication. Multiple copies of the bacteriophage λP (P) loader bind to and load the DnaB (B) replicative helicase ...Replicative helicases require loader proteins for assembly at the origins of DNA replication. Multiple copies of the bacteriophage λP (P) loader bind to and load the DnaB (B) replicative helicase on replication-origin-derived single-stranded DNA. We find that the DnaB•λP complex exists in two forms: B P and B P . In the 2.66 Å cryo-EM model of B P , five copies of the λP loader assemble into a crown-like shape that tightly grips DnaB. In this complex, closed planar DnaB is reconfigured into an open spiral with a sufficiently sized breach to permit ssDNA to enter an internal chamber. The transition to the open spiral involves λP-mediated changes to the Docking Helix (DH)-Linker Helix (LH) interface. The loader directly stabilizes the open spiral. Unexpectedly, one λP chain in B P is bound across the breach, precluding entry of replication-origin-derived ssDNA into DnaB's central chamber. We suggest that the B P complex is an early intermediate in the helicase activation pathway wherein neither the DnaB helicase nor the λP loader has attained its final form. DnaB in this complex adopts a partially open planar configuration, termed ajar planar. The partially ordered λP loader assembly features a much looser interaction with DnaB. The ssDNA and ATP sites in both complexes are in a configuration ill-suited for binding or hydrolysis. Our work specifies the conformational changes required for the intermediate B P to transition to B P on the pathway to recruitment by the initiator protein complex to the replication origin.
Mass: 18.015 Da / Num. of mol.: 76 / Source method: isolated from a natural source / Formula: H2O
Has ligand of interest
Y
Has protein modification
Y
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Experimental details
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Experiment
Experiment
Method: X-RAY DIFFRACTION / Number of used crystals: 1
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Sample preparation
Crystal
Density Matthews: 1.92 Å3/Da / Density % sol: 35.97 % Description: Smooth rhombus; Trigonal pyramidal with one screw.
Crystal grow
Temperature: 298.15 K / Method: vapor diffusion, sitting drop Details: Crystals were prepared using the sitting drop vapor diffusion method by mixing either 0.1, or 0.2, or 0.4 uL of the protein solution and 0.2 uL of a series of commercially available ...Details: Crystals were prepared using the sitting drop vapor diffusion method by mixing either 0.1, or 0.2, or 0.4 uL of the protein solution and 0.2 uL of a series of commercially available crystallization screens (Qiagen). Optimized crystals for X-ray diffraction were grown at room temperature in a buffer consisting of 3 M to 4 M NaCl with 0.1 M HEPES-NaOH, pH 7 to 8 and required 7 days to form. PH range: 7 - 8
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Data collection
Diffraction
Mean temperature: 100 K / Ambient temp details: cold nitrogen gas stream / Serial crystal experiment: N
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