AlphaFold2 examples from CASP 14

This page is under construction. Eric Martz 01:03, 22 February 2021 (UTC)

Prediction of protein structures from amino acid sequences, homology modeling, has been extremely challenging. In 2020, breakthrough success was achieved by AlphaFold2, a project of DeepMind. For an overview of this breakthrough, documented by the bi-annual prediction competition empirical models, please see 7jtl. Below are illustrated some examples of predictions from that competition.

spinqualitylabelsall models Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D SARS-CoV-2 ORF8 Following the discussion by Rubiera, our first example will be SARS-CoV-2 protein ORF8, a protein that contributes to virulence in COVID-19. CASP 14 classified ORF8 as a "free modeling" (FM) target, meaning that there were no adequate empirical templates for homology modeling. This was easily confirmed. When the amino acid sequence of ORF8 is submitted to Swiss Model, it reports the best templates for homology modeling. When the two empirical models that were not available during CASP 14 are excluded (7jtl and 7jx6), the best template offered, chain B of 3afc, covers only 36% of the length of ORF8 at 13.2% sequence identity, with a 4-residue untemplated gap in the sequence alignment. This template would not be adequate for constructing a useful model. X-Ray Structures for ORF8 The quality of predictions for the structure of ORF8 are judged by comparison with X-ray crystallographic empirical models which were not available to the groups making predictions. Shortly after the CASP 14 competition (summer 2020), two X-ray crystal structures were reported for ORF8: 7jtl released August 26, 2020, and 7jx6, released September 23, 2020. The resolutions are 2.0 and 1.6 Å respectively, and both have worse than average Rfree values. Click the green links below to change the molecular scene. Drag to rotate. Zoom the molecule with your mouse wheel, or Shift-Drag up/down. Here is one chain of ORF8 from the higher resolution X-ray structure, 7jx6. These chains form disulfide-linked dimers, and the dimers form higher order multimers (not shown). Notice that the amino and carboxy ends of the chain come together to form two parallel beta strands of a beta sheet. Also notice that there are 3 disulfide bonds. An accurate prediction would include both of these features. The two X-ray structures agree very well. The only substantial disagreement is for a large surface loop, sequence range 48-57. See the Table I below for RMSD values. ORF8 is not a novel fold Less than 2% of new empirically-determined structures have novel folds; that is, folds not aready represented in the PDB. When chain A of 7jx6 was submitted to Dali (February, 2021), the top hit was the N-terminal domain of the two domains in 5a2f, the CD166 human cell surface receptor involved in activation of T lymphocytes. The Z-score was 7.1, and 88 alpha carbons aligned with RMSD 3.2 Å. Swiss-PdbViewer obtained RMSD 1.95 Å for 48 alpha carbons. Dali reported the identity as 6% in its structure-based sequence alignment. Sequence alignment by MAFFT obtained 18% sequence identity using more and larger gaps. The structural similarity between Dali's top hit and 7jx6 is not as close as for AlphaFold2's prediction, but is closer than the 2nd best prediction (see Table I below). Dali's top hit has a single disulfide bond (compare with Table I). In conclusion, ORF8 does not have a novel fold. AlphaFold2 Prediction for ORF8 The quality of a prediction in CASP is judged, in large part, by the Global Distance Test Total Score, GDT_TS. AlphaFold2's predicted structure has a GDT_TS score of 87. (A score of 0 is meaningless, and a score of 100 means perfect agreement with an X-ray crystal structure.) 87 means the model is close to the accuracy of an X-ray crystal structure. The structure predicted by AlphaFold2 is almost as close to the X-ray crystallographic model 7jx6 as is the independently-determined X-ray structure 7jtl. AlphaFold2 predicted the positions of 92 amino acids. (CASP 14 excluded residues 48-59, a 12-residue surface loop, from the target residues.) See Table I below for RMSD values. The only intra-chain salt bridge (4.0 Å cutoff) that occurred in all 4 chains of the two X-ray structures was Arg101:Asp113. It also occurred in the AlphaFold2 prediction (Arg86:Asp98). Table I. ORF8 Predictions Aligned With Chain A of 7jx6 Model GDT_TS Disulfde Bonds Cα RMSD, Å Cα Aligned RMSD Including Sidechains, Å Atoms Aligned 7jtl:A 3 4.02 0.66 102/102 (100%) 87/102 (85%) 4.3 1.58 829/829 (100%) 709/829 (86%) AlphaFold2 87 3 2.58 1.25 92/92 (100%) 83/92* (90%) 3.23 1.91 747/748 (100%) 679/748 (91%) 2nd Best* 43 0 5.33 1.71 92/92 (100%) 38/92 (41%) 6.54 5.86 747/748 (100%) 324/748 (43%) 3rd Best§ 33 0 13.37 † 92/92 (100%) † 14.50 † 747/748 (100%) † Zhang-TBM Server 27 0 14.90 † 92/92 (100%) † 15.61 † 747/748 (100%) † Rosetta Server 26 (2‡) 14.99 † 92/92 (100%) † 16.07 † 747/748 (100%) † Alignments by "Magic Fit" of Swiss-PdbViewer 4.1. Alignments by "Iterative Magic Fit" of Swiss-PdbViewer 4.1. *Second best: Group of Xian Ming Pan, Tsinghua University, Beijing. §Third best: Group of Alberto Perez, University of Florida, Gainsville. † Iterative Magic Fit was unable to align. ‡ Neither disulfide bond is correct. Second Best Prediction for ORF8 In CASP 14, 70 research groups and 42 automated servers predicted structures for ORF8. The median GDT_TS score for all 112 predictions was 26. AlphaFold2 made the best prediction (GDT_TS 87). The second best prediction was by the group of Xian Ming Pan, with GDT_TS 43 (see Table I above). The fold and topology were predicted correctly, but the details are far less accurate than those in AlphaFold2's prediction. The 2nd best prediction has no disulfide bonds. The salt bridge Arg86:Asp98 is correctly predicted, along with two incorrectly predicted salt bridges. Third Best Prediction for ORF8 The third best prediction for ORF8 was by the Perez Lab, with GDT_TS 33 (see Table I above). It correctly predicted the parallel beta strands formed by the amino and carboxy terminal ends of the chain. When the 2-stranded parallel beta strands formed by the ends of the chains are aligned, the remainder aligns poorly. This prediction has no disulfide bonds. The salt bridge Arg86:Asp98 is correctly predicted, along with two incorrectly predicted salt bridges. Top Prediction by an Automated Server Among predictions by automated servers for all ~100 CASP 14 targets, the top ranking server was QUARK from the Yang Zhang group (Univ. Michigan). For ORF8, the Zhang-TBM server made the best server prediction with a GDT_TS of 27. (The prediction by QUARK was almost as good, GDT_TS 26.) The prediction has the two chain termini not parallel, and the amino terminus is not a beta strand, differing in both respects from the X-ray model. Also, no disulfide bonds are predicted. The salt bridge Arg86:Asp98 is correctly predicted, along with two incorrectly predicted salt bridges. The structural alignment is very poor and is not shown. Baker Rosetta Server Prediction for ORF8 Among predictions for all ~100 CASP 14 targets, the group of David Baker ranked second. The Rosetta Server of the Baker group ranked 18th overall, but was the 4th ranked server. For ORF8, the Rosetta Server prediction GDT_TS was 26, a bit better than the median of 23. The Rosetta Server's prediction for ORF8 has the two termini far apart (Cα 13 Å or farther apart), a substantial difference from the X-ray structure (Cα mostly ~5 Å apart). It predicts two disulfide bonds, but neither matches the pairs of Cys residues in the actual disulfide bonds. The salt bridge Arg86:Asp98 is correctly predicted, along with one incorrectly predicted salt bridge. The structural alignment is very poor and is not shown.

ORF8 Sidechain Accuracy

Table I gives RMSD values for all atoms, which is one indication of sidechain accuracy. Another is prediction of 7jx6 and 3afc. As detailed in Tables II and III:

• AlphaFold2's prediction was correct for 4/5 interactions, with one incorrect interaction.
  • AlphaFold2's prediction was correct for one of two salt bridges, and predicted no incorrect salt bridges.
  • AlphaFold2's prediction was correct for three of three cation-pi interactions, but predicted one incorrect interaction.
• The 2nd best prediction was correct for 1/5 interactions, with 2 incorrect interactions.
  • The 2nd best prediction was correct for one of two salt bridges, but predicted two incorrect salt bridges.
  • The 2nd best prediction failed to predict any of the three cation-pi interactions, predicting zero interactions.

Table II. Salt Bridge Prediction Accuracy

7JX6	7JTL	AlphaFold2	2nd Best
R101:D112 (AB)	R101:D113 (AB)	R86:D98	R86:D98
R115:D119 (AB)	R115:D119 (AB)	–	R100:E4
K44:E59 (AB)	K44:E59 (AB)	K29:E44	–
–	–	–	K78:E77

• Bridges in the same row are identical (except for red residues). Subtract 15 from the sequence numbers in the X-ray structures for the equivalent sequence numbers in the predictions.
• Black: Shortest sidechain nitrogen to sidechain oxygen distance ≤4.0 Å.
• Gray: Shortest sidechain nitrogen to sidechain oxygen distance 4.4 to 4.8 Å.
• –: Shortest sidechain nitrogen to sidechain oxygen distance 6 to 16 Å.
• (AB): The two chains in each X-ray model.

Table III. Cation-Pi Prediction Accuracy

7JX6	7JTL	AlphaFold2	2nd Best
R101:Y46+Y108 (AB)	R101:Y46+Y108 (AB)	R86:Y31+Y96	–
K44:F108 (B)	K44:F108 (AB)	K29:F93	–
–	–	K79:F105	–

• All interactions listed are deemed energetically significant by the CaPTURE Server.
• Interactions in the same row are identical. Subtract 15 from the sequence numbers in the X-ray structures for the equivalent sequence numbers in the predictions.
• Italics: erroneous prediction.
• The 2nd best prediction has no cation-pi interactions.
• (AB): The two chains in each X-ray model.

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Eric Martz