Equipment

Linux workstation (Dell) running a Linux distribution Ubuntu 18.04.5 LTS (GNU/Linux 4.15.0-118-generic x86_64)
Access to a scalar supercomputer (petaflop performance) for molecular dynamics simulations

Software

PTools (Saladin et al., 2009) is a python/C++ library dedicated to the manipulation, assembly, and analysis of macromolecular complexes. It is freely available in the GitHub repository at the address https://github.com/ptools/ptools/tree/develop; it is licensed under the GNU General Public License v3.0. The Heligeom module relates protomer-protomer binding geometries to the geometry of large oligomeric assemblies (Boyer et al., 2015). Heligeom basic functions consist of analyzing protomer interfaces in terms of the helical parameters (Figure 1A) of associated screw objects – pitch (P), number of monomers per turn (N), handedness, see Figure 1A, right – and constructing helices with the desired number of protomers. The helix can optionally be aligned to the Z axis at the origin. Figure 1B shows an example of the workflow of PTools/Heligeom functions that are commonly used in this work. These include selection functions (chains, residue numbers or types, atoms numbers or types), the superposition function that outputs a transformation matrix, and the mat_trans_to_screw function that produces the screw object corresponding to the transformation matrix.
The NAMD software (Scalable Molecular Dynamics, Mackerell et al., 2004; Phillips et al., 2005; http://www.ks.uiuc.edu/Research/namd/) is used to perform energy minimization and molecular dynamics (MD) simulations.
VMD (Humphrey et al., 1996) is a molecular visualization program: VMD can display and analyze large biomolecular systems and their MD trajectories using 3-D graphics and built-in scripting https://www.ks.uiuc.edu/Research/vmd/script_library/.
NAMD and VMD are both developed at the NIH Center for Macromolecular Modeling & Bioinformatics, University of Illinois at Urbana-Champaign, by the Theoretical and Computational Biophysics Group. Both are distributed free of charge with source code.

Figure 1. PTools/Heligeom. (A) Left: scheme of a screw transformation between two proteins A and B, with structurally similar core regions coreA and core2; right: calculation of the number of monomers per turn (nb), the pitch and the handedness (R: right, L: left) from the screw parameters θ and trans. (B) Typical PTools operations that enable the combination of available binding geometries. This includes creating Rigidbody objects from the .pdb files ("import .pdb files…"), defining structurally superposable regions with identical sizes using the select_resid_range attribute ("select a protein region"), defining mat, the transformation matrix that superposes the defined regions using the superpose function ("superpose selected regions…"), and defining the screw transformation hp equivalent to mat; example of how to use the screw object hp to create a new Rigidbody object according to the desired screw transformation is given under "apply transformation matrix…"

Procedure

Filament construction
1. Preparing building blocks
  1. Binding geometries are defined by pairs of interacting RecA monomers extracted from the RecA crystal structures with PDB codes 2REB (RecA bound to ADP molecules) and 3CMW (RecA bound to ATP and two DNA strands at site I). From each of these structure files, manually extract two neighboring monomers to define the corresponding binding geometry (monomers with different binding geometries are schematized in Figure 2A). In the case of 3CMW, in which binding geometries slightly differ along the 5-monomer filament, select central monomers (position 3 and 4) to avoid possible end effects. Make sure that the two monomers have the same number of residues and are structurally close (RMSD on CA atoms < 3 Å); otherwise, select structurally similar regions using the commands given in reference (Saladin et al., 2009) and Figure 1B. The pair of monomers extracted from the 2REB structure will be denoted R_A and R_B, and those extracted from 3CMW, C_A and C_B(Figure 2A). The corresponding new .pdb files should be named RA.pdb, RB.pdb, CA.pdb, CB.pdb (see Note 1).
  2. Building blocks are formed by monomer C_A (resp. C_B) with bound ATP/Mg²⁺ together with one, two, or three bound DNA segments. To define the DNA segments that are part of the building blocks, proceed as follows: for each of the two strands at site I, extract the three nucleotides that are bound to C_A(resp. C_B) in the 3CMW structure (the stoichiometry of RecA/DNA association is 3 nucleotides/monomer); note that determination of the residue numbers of the target nucleotides may require visual inspection of the 3CMW structure using VMD. The strand at site II is taken from our model published in Yang et al. (2015) (.pdb file is available upon request): select one RecA monomer and the bound DNA. Then, using PTools commands (Figure 1B), superpose the RecA monomer onto C_A(resp. C_B), calculate the transformation matrix associated with the superposition, and apply this transformation matrix to the DNA segment. Finally, for each number of strands, generate a .pdb file, named CA_x.pdb where x = 1, 2, or 3, that contains the C_Amonomer and its associated ATP molecule, magnesium ion, and DNA segment(s).
1. Assembling building blocks
  1. Construct a 6-monomer filament segment with geometry characteristic to 3CMW using PTools/Heligeom (see Figure 2B) via the command line:
    $ ptools heligeom -n 6 -o BLOCK1.pdb -Z CA_x.pdb CB_x.pdb
  2. In a python script into which the PTools package has been imported, construct the Rigidbody objects corresponding to R_A, R_B,and BLOCK1 (resp. RA, RB, and BL1). Select chain A of BL1 and store it in a Rigidbody object B1. Select chain F of BL1 and store it in a Rigidbody object B6. Superpose RA onto RB and output the corresponding transformation matrix mat1 (Figure 2A). Apply mat1 onto B6, thereby generating a monomer B7 that adjoins B6 following the binding geometry characteristic to 2REB (Figure 2C). Superpose B1 onto B7 and calculate the transformation matrix mat2 (Figure 2D); apply mat2 onto BL1, thus generating a new 6-monomer filament segment BL2 adjacent to BL1, with the binding geometry between the last unit of BL1 and the first unit of BL2 being 2REB type (Figure 2E and 2F).
Figure 2. Construction of a composite filament. (A) Schematic representation of the building blocks that define two RecA-RecA binding modes: (top) the C_A-C_B type and (bottom) the R_A-R_B type; calculation of the transformation matrix mat1 that relates Rigidbody objects RA and RB (arrow). (B) Applying the heligeom command line to building blocks C_A and C_B produces a 6-unit helix with interfaces of the C_A-C_B type (Step A2a). (C) Generation of Rigidbody B7 from B6 by applying mat1; B6 and B7 interact via a RA-RB interface. (D) Calculation of mat2 that transforms B1 into B7. (E) Applying mat2 to the BL1 block. (F) Final result (see text in A.2.b). PTools object names are written in blue when used and in red when defined. The process is illustrated in the accompanying video at Figure2_video.mp4.
1. Refining the junction between building blocks
  1. Adjust the interface flexible groups by replacing the N-terminal domain (one helix and associated linker, residues 1 to 37) of Rigidbody B7 with the equivalent region from RA. To this end, create an empty Rigidbody object BL2_adjusted. Select the Cα atoms of residue numbers 38 to 156, 165 to 193, and 213 to 333 of RA (non-flexible regions) and store them in the Rigidbody SRA. Perform the same selection for B7, stored in SB7; superpose SRA onto SB7 and output the transformation matrix mat3. Select the N-terminal domain of RA, residues 1 to 37, and store the selection in NT; store residues 38 to 333 of B7 in CB7; store chains B, C, D, E, and F of BL2 in BL2_BtoF. Apply mat3 to NT and store the result in NT7. Successively concatenate NT7, CB7, and BL2_BtoF to BL2_adjusted; concatenate BL2_adjusted to BL1. Output the structure in PDB format, named FINAL_x.pdb, where x = 1, 2, 3 is the number of DNA strands. Three final structures have now been generated.
  2. Process the final structures in such a way as to group the DNA fragments, label them with a specific chain name, group the ATP molecules and change their chain name, and name the protein chains from A to L (Figure 2C, right); replace the ATP/Mg²⁺bound to chain F with ADP. This can be done within the python/PTools script before outputting the .pdb files in the previous paragraph or manually by processing the .pdb files using a text editor.
2. Closing DNA chains with restrained minimization for each file FINAL_x.pdb, x = 1, 2, 3
  1. Using Recipe 1, roughly reposition the trinucleotide DNA segment T3_F from strand 3 associated with monomer F. This segment presents numerous steric clashes with monomer G following the change in filament axis orientation, consecutive to the modification of the F-G interface geometry. The change in orientation also produces an important rupture of site II continuity between T3_Fand the DNA segment T3_G of strand 3 associated with monomer G.
  2. Generate a topology .psf file using the «Automatic PSF builder» VMD extension.
  3. Create a constraint file from the FINAL_x.pdb file for NAMD minimization by adding «1.00» to column 62 of the restrained atoms (P for the DNA chains and CA for proteins); the script given as Recipe 2 below can be used to this end. The resulting file FINAL_x_cstr.pdb needs to be modified in order to free the phosphate atoms situated at the junction between chains F and G for energy minimization, where the helix symmetry has been broken, by replacing «1.00» with «0.00» for these specific phosphate atoms (three phosphate atoms are freed per DNA chain).
  4. Perform 5000 steps of conjugate gradient energy minimization using NAMD2.10. This enables closure of the DNA backbones by restoring the distances between covalently linked atoms according to the topology file and releasing small steric clashes if any.
Stability and evolution of the filament model: molecular dynamics simulations
1. Simulation preparation for each file FINAL_x.pdb, x = 1, 2, 3
  1. Immerse the structure in a water box using the VMD extension «Add Solvation Box»; use a TIP3P water model and a box padding value of 10 Â.
  2. Add Na⁺ and Cl- ions to the water box using the VMD extension «Add Ions»; select the option «Neutralize and set NaCl concentration to 0.15 mol/L».
2. Simulation runs for each of the three systems using NAMD2.10
  Given the large size of the systems (about 450,000 atoms), this step needs to be performed on distributed computer nodes, for example, in a computer center. The simulation follows standard NAMD protocols.
  1. Simulation conditions: use the CHARMM 27 force field including CMAP corrections; use periodic boundary conditions, with the particle mess Ewald method to account for long range interactions and a smooth switch of van der Waals interactions between 10 and 12 Å; set time steps to 2 fs using the SHAKE algorithm; control the temperature and pressure using a Langevin dynamics scheme and a Nose-Hoover Langevin piston.
  2. Simulation run: perform 5000 steps of conjugate gradient minimization followed by a progressive heating stage up to 300K, a long equilibration phase of 30 ns, and a 100 ns production phase. During heating and equilibration, set restraints on the P and CA atoms of the files, with the force constant decreasing from 0.5 to 0.05 kcal·mol^-1·Å^-2. Do not use restraints during the production phase. Perform three independent simulations for the three-stranded system using identical conditions.

Data analysis

Results of the following analysis steps can be found in the Supplemental Information (SI) of Boyer et al. (2015).

Conventional MD analysis. Check the stability of the energy and temperature during the molecular dynamics simulation trajectories using VMD tools. Reduce the size of the trajectory files by eliminating the water molecules. Plot the time evolution of root-mean-square fluctuation (RMSF) or deviation (RMSD) values of the system or selected regions of the system, taken on the Cα atoms: whole filament, filament center, DNA center (Boyer et al., 2015, Figure SI-7). To separate the fluctuations of each monomer from those of the entire filament, calculate the RMSD and RMSF values along the trajectory for each individual RecA monomer, after superposition restricted to this monomer; plot the evolution of the average value of the resulting RMSDs, taken on all monomers (Boyer et al., 2015, Figure SI-7); individually plot the RMSF corresponding to each monomer (Boyer et al., 2015, Figure SI-5). Visually inspect the final structures of trajectories using VMD to select atoms or groups of atoms of interest that modify their contacts with other groups as a result of the interface modification; plot the time evolution of these distances; calculate the corresponding distances in regular filaments and incorporate this value into the plot as a horizontal line as a reference (Boyer et al., 2015, Figure SI-3).
Specific MD analysis for a filamentous assembly. Calculate the width of the groove entrance at the level of each monomer, as described in Boyer et al. (2015) (Materials and Methods); plot the time evolution of the width value for each monomer and then plot the value measured in regular filaments as in Boyer et al. (2015), Figure SI-2. Using PTools utilities, calculate the evolution, along the trajectory, of the f_NAT values for each non-terminal interface, where f_NAT is the fraction of pair contacts from the starting structure that are conserved in a given snapshot (a contact being defined as a pair of residues from each interacting monomer that are distant by less than 5 Å, and pair contacts being restricted to the non-flexible regions of the interface, as defined above in Step A3a) (Boyer et al., 2015, Figure SI-6).
DNA analysis. Evolution of the interaction between DNA strands is monitored in two ways: by measuring the shortest phosphate-phosphate inter-strand distances between each pair of strands for each snapshot (see Boyer et al., 2015, Figure SI-9 for details) or by calculating the time evolution of pairing distances between three base pairs, selected by visual inspection using VMD (Boyer et al., 2015, Figure 4).

Notes

In the 3CMW .pdb file, the residues are numbered from 1 to 333 for the first monomer, from 1001 to 1333 for the second, from 2001 to 2333 for the third, etc. It is advisable to edit the files CA.pdb and CB.pdb in order to have their residue numbering within the range of 1-333. Care must be taken that all modifications of .pdb files respect the .pdb file format.
The models generated in this work were deposited into ModelArchive https://www.modelarchive.org/ with codes ma-900pk, ma-l1kfl and ma-eaaa9, and are publicly available at the following URLs: https://www.modelarchive.org/doi/10.5452/[code] (where [code] is either ma-900pk, ma-l1kfl or ma-eaaa9).

Recipes

The T3_Ftrinucleotide segment defined in Step A4a can be repositioned by following the commands below, where the chain index for strand 3 is "P," the T3_Fnucleotides are numbered from 16 to 18 in the 5’-3’ direction, nucleotide number 15 is associated with monomer E, and nucleotide 19 is the first nucleotide of T3_G in the 5’ direction. The backbone discontinuity occurs between nucleotides 18 and 19. The script below defines three points, O, A, and B, where O is positioned on atom P of nucleotide 15, A on atom O3' of nucleotide 19, and B on atom P of nucleotide 18. T3_F will be rotated by 40° around the axis (O,u), where u is the normalized cross product between vectors OA and OB:

$ python
>>> from ptools import *
>>> fila = Rigidbody("FINAL_3.pdb")
>>> strP = fila.select_chain_id("P").create_rigid()
>>> t3f = strP.select_res_range(15,18).create_rigid()
>>> atom=(strP.select_res_range(15,15) & \
strP.select_atom_type("P")).create_rigid()
>>> O = atom.get_coords(0)
>>> atom=(strP.select_res_range(19,19) & \
strP.select_atom_type("O3'")).create_rigid()
>>> A = atom.get_coords(0)
>>> atom=(strP.select_res_range(18,18) & \
strP.select_atom_type("P")).create_rigid()
>>> B = atom.get_coords(0)
>>> OA = A - O
>>> OB = B - O
>>> u = Coord3D()
>>> u.x = OA.y * OB.z - OA.z * OB.y
>>> u.y = OA.z * OB.x - OA.x * OB.z
>>> u.z = OA.x * OB.z - OA.z * OB.x
>>> u.normalize()
>>> newt3f = Rigidbody(t3f)
>>> newt3f.ab_rotate(O,O+u,40*3.14/180.)
>>> write_pdb(newt3f,"new_T3F.pdb")

The file FINAL_3.pdb can then be edited to incorporate new_T3F.pdb in place of T3_F.
The following Linux command can be used to generate a constraint file for NAMD in order to restrain the displacement of P and CA atoms during energy minimization, heating, and the beginning of equilibration (see Step A4c):

$ awk '$3 == "P" || $3 == "CA" { \
> printf "%s 1.00%s\n",substr($0,1,61), substr($0,67)}; \
> $3 != "P" && $3 != "CA" ' FINAL_x.pdb > FINAL_x_cstr.pdb

This command labels all "P" and "CA" atoms of the file FINAL_x.pdb. In order for the minimization process to be able to close the DNA backbones, phosphate atoms from the junction regions must be released by replacing the "1.00" in columns 63 to 66 with "0.00". For example, for strand 3, if one keeps the notations from Recipe 1, phosphates 15 to 20 of chain "P" will be released.

Acknowledgments

The authors wish to acknowledge the ‘Initiative d’Excellence’ program of the French State for funding [DYNAMO, ANR-11-LABX-0011-01]. The original research paper from which this protocol is derived was published by Boyer et al. (2019). We thank Raquel de Miranda for assistance in preparing the video describing RecA filament preparation.

Competing interests

The authors certify that they have no competing interests.

Ethics

The protocol presented here did not use any human or animal subjects.

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