1.1. Features visible at different resolutions

Here, we will systematically visualize electron density as a function of resolution, especially paying attention to the visibility of backbone carbonyl groups and thus of peptide orientation. Fig. 1 allows a comparison of this effect, and other features, at 4, 3, 2 and 1 Å resolution for a particular helix and a particular loop in apo T4 lysozyme crystal structures, with red `O's marking three specific carbonyl O atoms.

Figure 1 Comparing the same electron-density features at 4, 3, 2 and 1 Å resolution for crystal structures of apo T4 lysozyme. At the top is the helix Ile58–Gly77 and at the bottom is the loop Asn132–Trp138. The left panel is PDB entry 4gbr at 3.99 Å resolution (Zou et al., 2012), the second panel is PDB entry 5zbh at 3.0 Å resolution (Yang et al., 2018), the third panel is PDB entry 1lyi at 2.0 Å resolution (Bell et al., 1992) and the right panel is PDB entry 5jdt at 1.0 Å resolution (Consentius et al., 2016). All are wild type except for a Thr59→Asp mutation in PDB entry 1lyi. The σA-weighted 2Fo − Fc maps are contoured at 1.2σ (gray) and 3σ (purple).

In the well ordered parts of a 1 Å resolution structure, covalently bonded atoms show clearly resolved peaks, allowing bond angles and dihedral angles to be measured accurately, including the orientation of the carbonyls and the peptide planes. Even pairs of alternate conformations are often very clear, as seen for Asn68 on the right-hand side of the helix in the 1 Å panel in Fig. 1. However, resolution, or relative disorder, is a local property. Ultrahigh-resolution structures almost always have a few disordered stretches at termini or loops, and when model restraints have been greatly loosened or turned off, such parts have the worst geometry outliers in the PDB (Chen et al., 2011). In great contrast, at 4 Å resolution a helix has become a broad cylinder (see the 4 Å panel in Fig. 1), the β-sheet is confusing and only a few side chains such as those of tryptophan or selenomethionine can be directly identified. 3–4 Å is actually a difficult resolution range to model well because density connectivity is changing non-uniformly. For instance, a helix goes from a narrow spiral tube with zero density along the helix axis at 2 Å resolution to a wide tube with maximum density along the helix axis at 5 Å resolution. In between, false breaks and false connections are produced by local side-chain mass and hydrogen-bonding details.

In protein crystallography, a structure at 2 Å resolution is considered to be an excellent, workhorse standard. At 2 Å the backbone CO density is a high-contour, clearly directional protrusion out from the main-chain density, as seen for the red-circled O atoms in the 2 Å panel in Fig. 1. However, at 3 Å resolution there is no CO protrusion even at lower contour levels, the carbonyl O atom is out of density or at its edge, and that density is broadly smooth both along and around the main chain. Thus at 3 Å resolution neither visual inspection nor automated fitting can determine the orientation of the CO group, and thus of the peptide, directly from the experimental data. 2.5 Å is the approximate midpoint of that transition, with some fraction of interpretable nubbins.

By 3 Å resolution side chains have also become problematic, with some atoms outside the shortened, blobby density that no longer shows the characteristic amino-acid shape or its rotamer at all well. Lowering the contour level does not help much with side chains either. Ligand identity and conformation have also become unclear from the density alone, and waters are seldom identifiable.

1.2. Current tools

In general, at lower resolutions the fit to density of the model becomes increasingly ambiguous, with multiple distinctly different models giving an equally mediocre fit into the broad and sometimes misleading density, both locally and globally (Richardson, Williams, Videau et al., 2018). The familiar single-residue, empirical criteria such as Ramachandran, rotamers, ribose puckers etc. inherently have multiple minima that pure downhill refinement cannot move between. If multiple models are being compared in model building or rebuilding, then maximizing hydrogen bonding and minimizing some form of atom bumps helps, optimizing all-atom contacts with H atoms is better (Arendall et al., 2005), and adding Bayesian consideration of prior probabilities would be even better. However, only small local subsets of distinct conformations can feasibly be compared.

A special case is handling rotamers for exposed surface side chains with poor density and no good hydrogen-bond or even van der Waals contacts with anything. Such a side chain has nothing to hold it in an energetically unfavorable conformation (a rotamer outlier). If modeled at all, it should be placed in a favorable rotamer that does not clash with anything and preferably given an occupancy of <1 for the atoms that are poorly seen or unseen.

Recent validation tools aimed at lower resolution problems, such as CaBLAM (Williams, Videau et al., 2018), EMRinger (Barad et al., 2015), Pperp for ribose puckers (Chen et al., 2010) or pepflip in PDB-REDO (Joosten et al., 2014) help by operating on a scale larger than a single residue and/or by evaluating new, less classical parameters that are not yet being explicitly refined. Several new systems specifically tailored for various aspects of cryo-EM include likelihood-based fitting of predictions or fragments into maps (Read et al., 2023; Millán et al., 2023), ModelAngelo (Jamali et al., 2022) for maximum-likelihood (ML)-based initial model building and MEDIC (Reggiano et al., 2022) for ML-based validation of final models. These have not yet been broadly tested, but each has been shown to improve on previous methods for its test cases. Other new approaches will hopefully continue to emerge.

High-confidence parts of the new, unprecedentedly successful artificial intelligence (AI) predictions can also help with sparse-data entire structures or regions by providing very good starting hypotheses for the 3D structure, with few geometry or conformational problems. The low-confidence regions are usually bad but are sometimes useful.

2.1. Consequences of overfitting, especially peptide orientations

Using the well established Ramachandran, rotamer and other conformational preferences as targets in refinement sounds like a good idea, and some type of conformational restraints are necessary at lower resolution to keep secondary structures and well fitted loops from becoming distorted. However, refining inaccurate initial models against multiple-minimum targets turns out in practice to make the model worse (as shown in Fig. 2c), in addition to invalidating validation by giving perfect scores for very imperfect structures. As demonstrated and discussed in Section 1, the directionality of peptide CO groups cannot be seen at 3 Å or worse in either X-ray or cryo-EM. As a consequence, the most common error in a backbone trace at such lower resolutions is peptide orientations that are incorrect by large rotations (Prisant et al., 2020; Lawson et al., 2021), which means that the φ, ψ values of both surrounding residues will be wrong by as much as 90–180° and subsequently refine into the wrong local minimum in the Ramachandran plot (Croll et al., 2021). As well as being undefined when the backbone is a smooth tube, peptides are often misoriented at 3–4 Å because of the misleading local breaks or extra connectivity in backbone density described above or by trying to push the carbonyl O atom inside the density contour.

Figure 2 Fixups in PDB entry 6eyc from the misoriented peptides in PDB entry 3ja8. (a) Two misoriented CO groups at the end of a helix, flagged by CaBLAM outliers and clashes. (b) Corrected version, with no outliers, equal map fit and a standard helix C-cap conformation. (c) Ramachandran plot for eight residues neighboring misoriented peptides; green arrows point from the misfitted to the corrected positions.

A very useful example structure for the study of such effects is the large MCM2-7 heterohexamer helicase of PDB entry 3ja8 determined at 3.8 Å resolution by cryo-EM (Li et al., 2015), which had used Ramachandran restraints in refinement and was later carefully studied and corrected (to PDB entry 6eyc) as the test case for the ISOLDE user-guided molecular-dynamics (MD) rebuilding program (Croll, 2018). Those interactive corrections used the AMBER force field, map fit, user judgment and many validation flags, but were independent of the backbone markup in CaBLAM (Williams, Videau et al., 2018). Genuine cis nonproline peptides only occur in about one in 3000 residues (Croll, 2015; Williams, Headd et al., 2018), and all of the 116 in PDB entry 3ja8 were found to be incorrect, nearly all of them in very poor map density. Especially notably, a sequence misalignment was corrected and 32.5% (1229) of the residues moved a non-H atom by more than 2 Å and/or changed by >45° in φ, ψ or ω.

The first 135 residues in PDB entry 3ja8 (chain 2 residues 201–335) were chosen here as a workable-sized sample for detailed examination (and ended there because residues 340–373 are a weak-density region that was fitted rather differently in PDB entry 6eyc and is unsuitable for close comparisons). Residues 201–335 contain 15 peptide orientations that are wrong by >60°. Two (Leu234 and Thr302) are unjustified cis nonprolines. Every one of these 15 is flagged by CaBLAM, which validates CO–CO virtual dihedrals in the context of the surrounding five C^α atoms. One is a severe C^α geometry outlier, eight are at the 1% CaBLAM outlier level and six are at the 5% level, while only three are flagged by Ramachandran outliers. In that same region there are three other CaBLAM flags at the 5% level. Two of those peptide orientations needed no refitting and the third was rotated in PDB entry 6eyc by a marginal 50°. In this sample, then, all CaBLAM outliers were worth looking at, and even at the 5% level (purple markup) two thirds of them were found to be wrong.

Figs. 2(a) and 2(b) show before-and-after CO and peptide orientations for two cases at the end of a helix, both flagged graphically by CaBLAM outliers, three-part lines that follow adjacent CO orientations (Prisant et al., 2020). A misoriented peptide in an initial model changes the preceding ψ angle and the following φ angle by an amount similar to the misfitted peptide rotation angle, thus moving both surrounding φ, ψ positions by very large distances to locations usually within or close to a wrong Ramachandran region. Refinement of the Ramachandran values then pulls them further into the wrong minimum, often removing all Ramachandran outliers but making these model conformations worse rather than better. Fig. 2(c) shows the consequences on a Ramachandran plot for eight residues surrounding badly misoriented peptides, where each green arrow starts at the value in PDB entry 3ja8 and ends at the value in PDB entry 6eyc. All eight of these φ, ψ pairs in PDB entry 3ja8 had been refined into the wrong minimum in Ramachandran space rather than improved. In our anecdotal experience, the residues next to misoriented peptides end up in an incorrect Ramachandran region only somewhat less than 100% of the time.

A similar process happens when incorrect side-chain conformations are refined into the nearest valid rotamer or too slavishly into the map density. One of the most common side-chain errors at low local resolution is that both people and automated systems tend to scrunch side chains down into their nubbins of density, which is seldom the right answer. Fig. 3 shows two such examples: a helical valine and leucine in hemoglobin structures that are scrunched-down outliers at 3.5 Å and correct in unambiguous density at 1.25 Å resolution.

Figure 3 Two side chains pulled too close down into their nubbins of density. (a) In PDB entry 2qls at 3.5 Å, with backbone clashes (red spikes), a rotamer outlier (gold) and a Cβ deviation (magenta ball). (b) In PDB entry 2dn2 at 1.25 Å resolution, in clear 2Fo − Fc density with relaxed, spread-out rotamers and no outliers.

In addition to the examples above, we have thoroughly documented the many errors in 2.5–4 Å resolution structures and in disordered loops and termini at high resolution by assessments in the EMDB Cryo-EM Challenges (Richardson, Williams, Videau et al., 2018; Lawson et al., 2021), comparisons of low-resolution structures with later high-resolution versions (Moriarty et al., 2020), in our work correcting important structures (Croll et al., 2021; Dunkle et al., 2011; PDB entries 6vyo v.2.0, 6m71, 7btf, 7bv1, 7bv2 v.2.0, 5hut v.2.0 and 3q9v v.2.0) and even by local, carefully parallel maximum-likelihood-evaluated refinements between original and corrected versions (Richardson, Williams, Hintze et al., 2018). This can be a contentious issue because these structures have been made to look misleadingly good by well meaning but dubious procedures that were not obvious to their creators or end users, and no one likes to hear that, including us.

2.2. Remedies for overfitting peptide orientations and side chains

When you have an initial model, before refining it consult wider-scale validations including CaBLAM and PDB-REDO peptide flips for protein and Pperp sugar puckers for RNA, and try to fix the backbone outliers that they show before running refinement. The easiest common CaBLAM fixes are (i) idealize an outlier inside secondary structure and (ii) for two CaBLAM outliers in a row, try rotating the central CO group. Be aware of the prior improbabilities, such as one in 3000 for a cis nonproline (Williams, Videau et al., 2018) or other very rare conformations. It can also be quite helpful to run all of PDB-REDO (Joosten et al., 2014) or compare with a high-confidence AI prediction to identify candidates for fixing outliers. Accept changes if they are much more probable and/or correct outliers and are about as good a fit to the density. Do not try too hard to get rid of all outliers, and remember that a few outliers are genuine, strained conformations that are conserved by evolution for functional reasons. To prevent distortion in subsequent refinement, it is better to restrain hydrogen bonds rather than Ramachandran scores.

Then check for side chains that are scrunched down into map density as well as those with rotamer outliers, clashes or C^β deviation outliers (Lovell et al., 2003). Try all favorable rotamers to find one that does not clash, preferably makes good contact with something and can have one or two atoms (or even more for long side chains) out of density. Allow the backbone to move slightly when trying different rotamers, either as a backrub rotation around the axis between the n − 1 and n + 1 C^α atoms (Davis et al., 2006) or just by letting that large a region adjust in Coot (Emsley et al., 2010) or ISOLDE. There are of course other possible problems, but these are the most frequent and most fixable errors. At resolutions poorer than 3 Å, especially for very large structures, the ISOLDE user-guided MD rebuilding program with real-time density and validation display (Croll, 2018), a plug-in to ChimeraX (Pettersen et al., 2021), is the state of the art for correcting problems in either an initial or a post-refinement X-ray, cryo-EM or AlphaFold model.

2.3. Use of AI predictions at lower resolutions or in poor density

Another new set of useful tools are the highly successful AI structure predictions from AlphaFold2 (Jumper et al., 2021), RoseTTAFold2 (Baek et al., 2021), RoseTTAFoldNA (Baek et al., 2022), OpenFold (Ahdritz et al., 2022), ESMFold (Lin et al., 2023) and others, which bring a rich source of new information from large multiple sequence alignments (MSAs) or large language models. AlphaFold2 (AF), the earliest and with the huge, open AlphaFold Database (AFDB) of results (Varadi et al., 2022) at https://alphafold.ebi.ac.uk/ and the Colab notebook implementation (Mirdita et al., 2022) at https://bit.ly/alphafoldcolab, is the most used and studied so far and will provide almost all of our examples.

The AF confidence-level measure is called pLDDT (predicted local difference distance test, running from 0 to 100). Models or regions that are predicted with high confidence (pLDDT above 70–75) can very often make better starting models than de novo models built into the density at resolutions poorer than 2 Å, for either X-ray or cryo-EM. These should almost never have a stretch of misaligned sequence because estimated residue-pair distances are central to their data, and we have found that when they disagree with alignment in a PDB entry the AF version is always the correct one. They are of course exempt from scrunching side chains down into density since they do not see the density, and they are good at avoiding many other local errors common in experimental starting models. However, they have some new vulnerabilities, such as switching the positions of aliphatic side-chain branches or the positions of long opposed side chains, presumably because these choices are ambiguous in the distances predicted from MSA covariance. Properly pruned pieces of a predicted model (the best pLDDT region) can usually provide molecular-replacement solutions to solve the phase problem for crystal structures (Baek et al., 2021). All of the major software systems have automated utilities to superimpose a pruned model onto the optimal place in a cryo-EM map and some version of flexible fitting for either technique, as is almost always necessary between domains or for external loops. High-confidence, pruned AF predictions are good starting models for use in ISOLDE, and alternatively they can be used as restraints applied to a model from a different data source.

The next step is low-resolution validation and fixup as described in Section 2.2 for experimental starting models, followed by refinement and rebuilding, iterated several times. There is now a new way to improve even further before deposition by cycling back to give prediction implicit information from the experimental data by providing the rebuilt and refined model as a template, which is shown to improve both the predicted models and the models refined from them, converging in about three such cycles (Terwilliger et al., 2022, 2023).

A high-confidence prediction is definitely what one hopes for, but our laboratory is interested in helping less lucky structural biologists to sometimes be able to solve a structure when there is only a mid- or low-confidence prediction. In particular, we have found that even at a local pLDDT under 65 there can be `near-folded' parts that are reasonably compact and protein-like and are known to at least sometimes be good enough for a molecular-replacement solution (Fig. 4c). However, even more prevalent at pLDDT < 65 are the very different `barbed-wire' parts consisting of long, loopy strands that make essentially no contact with the rest of the model (Fig. 4a) and with an unprecedented concentration of dire local backbone geometry (Fig. 4b; Williams et al., 2022). We hypothesize that barbed-wire regions represent predictions that failed at an early stage, presumably because of unhelpful MSA patterns, and also that barbed wire is the reason that pLDDT < 50 was found to be an excellent predictor of intrinsically disordered regions (IDPs) in CASP14 (Ruff & Pappu, 2021). As is also true for many IDPs, some barbed-wire regions are known to fold when they find the right binding partner, which can often be shown by AlphaFold or RoseTTAFold multimer prediction (Drake et al., 2022). For anyone used to looking at protein 3D structures, barbed-wire segments are obvious in visualizations, especially with outliers turned on, but to enable automation we have developed a set of five specially tuned criteria (packing, ψ, ω, CaBLAM and geometry) that can identify and delete the barbed wire from low-pLDDT regions, leaving the near-folded parts that may have usable predictive value, as in Fig. 4(c). A preliminary version of this tool is available on the Phenix command line as barbed_wire_analysis and will be further tested and improved.

Figure 4 Barbed-wire regions versus near-folded regions in low-pLDDT AlphaFold predictions from the EBI AFDB. (a) Overview of the AF model for Saccharomyces cerevisiae UniProt P53076. (b) Close-up of a classic barbed-wire segment in the AF model for human UniProt Q1HG43, with MolProbity validation markup. (c) A near-folded model for Methanocaldococcus jannaschii UniProt Q57775 (pLDDT < 70 in yellow and pLDDT < 50 in orange).