Unfiltered reconstructions for TRPV1 (EMD-5778), γ-secretase (EMD-3061) and β-galactosidase (EMD-2984) were obtained from the EMDB model challenge website (
http://challenges.emdatabank.org). Initial map targets for coordinate refinement were generated by applying uniform filtering to the original EMDB entries. The deposited map for TRPV1 (EMD-5778) was globally sharpened using an additional B-factor of −100 Å
2. Coordinate refinement was performed using real-space refinement against the initial map as implemented in cctbx/PHENIX (Adams et al., 2010 (
link)) with additional restraints on secondary structure. Residue-grouped atomic B-factors were refined in reciprocal space. For TRPV1 and β-galactosidase, non-crystallography symmetry (NCS) restraints were employed to account for rotational and dihedral symmetry. Reference restraints for PETG were obtained with
phenix.elbow from the crystal structure of 2-phenylethyl 1-thio-β-D-galactopyranoside (Brito et al., 2011 (
link)). Building of glycans used glycan modeling tools available in Coot. Local resolution for all maps was estimated by local FSC computations using an in-house Python program
locres.py. Local resolution mapped to the coordinate models using the
measure_mapValues function in UCSF Chimera (Pettersen et al., 2004 (
link)), and the residue-averaged resolution was used to assign local resolution-scaled reference restraints. Model rebuilding was performed in Coot (Emsley and Cowtan, 2004 (
link)). Half-set reconstructions for cross validation were obtained from the EMDB model challenge. Model maps were generated by inverse Fourier transform using B-factor-weighted electron form factors (Colliex et al., 2006 ). The FSC
ref between model map and half-set 3D reconstructions was used to assess over-fitting. To this end, coordinates were first refined against the full map. Coordinates were then randomly displaced by a maximum of 0.5 Å and subjected to three cycles of real-space refinement against one of the half maps (work map) using the same protocol outlined above. The other half map (test map) was used as the test map for cross-validation. Following refinement, we computed the FSC of the refined model against the work map (FSC
work) and the cross-validated FSC between refined model and the test map (FSC
test). All FSCs were computed using a structure mask obtained from the respective EMDB entry that was low-passed filtered to 60 Å.
To compare model refinement against deposited EMDB entries and LocScale maps, PDB-deposited coordinate models associated with the respective EMDB entry were randomly perturbed by applying atom shifts of up to 0.4 Å to serve as starting models. Five iterations of local and global real-space coordinate refinement against the respective EMDB or LocScale map were each followed by refinement of atomic B-factors in reciprocal space. As above, secondary structure restraints, resolution-dependent weights and NCS restraints were employed. EMRinger scores were computed using
phenix.em_ringer (Barad et al., 2015 (
link)); all other validation scores were obtained using MOLPROBITY (Chen et al., 2010 (
link)). All cross-validation FSC and real-space correlations were computed against the original reconstruction and the deposited EMDB map, respectively.
Jakobi A.J., Wilmanns M, & Sachse C. (2017). Model-based local density sharpening of cryo-EM maps. eLife, 6, e27131.