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, GDT_TS scores, 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 6vr4, please see SARS-CoV-2 protein ORF8. 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 Free Modeling Results More than 100 domains were provided as prediction targets in CASP 14. 14 of these were in the most difficult category, free modeling ("FM"), meaning that no informative homology modeling templates existed. For 8 of these (57%), AlphaFold2's predictions achieved GDT_TS scores of 87-93 (median 88.5). For those 8, GDT_TS of the second best predictions were 43-76 (median 66). Two cases will be analyzed below. First, SARS-CoV-2 ORF8, a 92-residue FM domain where AlphaFold2's GDT_TS was 87, and the second best was 43 (by the group of Xian Ming Pan), the largest difference between 1st and 2nd predictions among the FM targets. It is further unusual because two independently-determined X-ray crystallographic structures were subsequently published. Inspiration for this case came from the discussion by Rubiera. Second, the longest domain in the FM category, 404 residues. This domain is part of the 2,180-residue RNA polymerase of a bacteriophage, some of whose group members are prevalent in the human gut. Eight of the CASP 14 FM target domains are parts of this protein, 6vr4. For the 404-residue domain, AlphaFold2 achieved GDT_TS of 88, and the second best prediction, 63 (by Seok-refine). Among the 14 FM targets, the second-longest has 276 residues, the median 132, and the shortest, 92. SARS-CoV-2 ORF8 Our first example is 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 superposed 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). 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 prediction was largely accurate regarding salt bridges and cation-pi interactions (see Tables III and IV below). Table I. ORF8 Predictions Superposed With Chain A of 7jx6 Model GDT_TS Disulfide Bonds Cα RMSD, Å Cα Superposed RMSD Including Sidechains, Å Atoms Superposed 7jtl:A 88 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%) Dali top hit 5a2f 53 na 3.2 1.95 92/92 (100%) 48/92 (52%) na na 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%) † Superpositions by "Magic Fit" of Swiss-PdbViewer 4.1. Superpositions by "Iterative Magic Fit" of Swiss-PdbViewer 4.1. na: Not Applicable. *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 superpose. ‡ 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. This prediction was largely incorrect regarding salt bridges and cation-pi interactions (see Tables III and IV below). 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 superposed, the remainder superposes 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 several incorrectly predicted salt bridges. The structural superposition 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 superposition is very poor and is not shown. Phage RNA polymerase T1037 Our second example is a 404-residue domain within a 2,180 residue RNA polymerase, 6vr4, gp66 from a bacteriophage of the crAss-like group, some members of which are prevalent in the human gut. One known host for the target-containing phage is gram-negative, aerobic bacteria Cellulphaga baltica isolated from marine microalgae. CASP 14 classified this domain, coded T1037, as a free modeling ("FM") target, meaning that no informative templates existed in the PDB. X-ray crystal structure The crystallographic structure, not available to the prediction teams during the CASP competition, is 6vr4, with resolution 3.5 Å, and an Rfree "reliability" of "much better than average for this resolution" (according to FirstGlance in Jmol). The termini of the chain are far apart from each other (~85 Å), and there are no disulfide bonds. The CASP 14 target T1037 sequence of 404 residues begins at sequence number 337 and ends at 901, a span of 565 residues. The target sequence is 404 residues because it excludes residues 370-530 (length 161), which form a different domain. Here is the full 2,180 residue chain with 337-901 (565 residues) opaque. Here is the 404-residue target sequence with the intervening domain (excluded from the CASP target) highlighted within the full 2,180-residue chain. The X-ray structure of CASP 14 domain T1037 (length 404 residues) consists of residues 337-369 + 531-901 of 6vr4. It is an alpha/beta domain with secondary structure 45% helices,  19% beta strands , and 37% loops and turns. The N- and C-termini are 10 Å apart, and there are no cysteines (thus no disulfide bonds). AlphaFold2 prediction for T1037 AlphaFold2 predicted the structure of T1037 with high accuracy (GDT_TS 88; see Table II below for details). Table II. T1037 Predictions Superposed With Sub-Domain of 6vr4 Chain B Model GDT_TS Cα RMSD, Å Cα Superposed RMSD Including Sidechains, Å Atoms Superposed T1037 of 6vr4:A XX 0.25 0.25 404/404 (100%) 404/404 (100%) 0.58 0.24 3157/3157 (100%) 1616/3157 (51%)

ORF8 Sidechain Accuracy

AlphaFold2's predictions for sidechain positions seem fairly good, while sidechain positions in the 2nd best prediction seem poor. This conclusion is based on three types of observations:

1. Table I gives RMSD values for all atoms, which is one indication of sidechain accuracy.
2. Prediction of homology modeling and empirical models.
3. Visualization of the distributions of charges on the surfaces.

Salt Bridges and Cation-Pi Interactions

• 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 III. 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.
• Italics: erroneous prediction.

Table IV. 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.

Visualization of Surface Charge Distributions

GDT_TS Calculations

GDT_TS values for predictions are taken from CASP 14 results. GDT_TS values for 7JTL and 5A2F vs. 7JX6 chain A were calculated using the AS2TS server of Adam Zemla. See instructions for 7jtl. CASP 14 reported GDT_TS 86.96 for the AlphaFold2 prediction, while the AS2TS server calculated GDT_TS 86.41 vs. 7jx6 chain A, and 88.59 vs. 7JTL chain A.

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Eric Martz