Cited by 569Protein Structure and DynamicsReceptor Mechanisms and SignalingMachine Learning in Bioinformatics

Sampling alternative conformational states of transporters and receptors with AlphaFold2

Open access
del Alamo, Diego, Sala, Davide, Mchaourab, Hassane S, Meiler, Jens·2022·eLife

Published version of a preprint

Abstract

Equilibrium fluctuations and triggered conformational changes often underlie the functional cycles of membrane proteins. For example, transporters mediate the passage of molecules across cell membranes by alternating between inward- and outward-facing states, while receptors undergo intracellular structural rearrangements that initiate signaling cascades. Although the conformational plasticity of these proteins has historically posed a challenge for traditional de novo protein structure prediction pipelines, the recent success of AlphaFold2 (AF2) in CASP14 culminated in the modeling of a transporter in multiple conformations to high accuracy. Given that AF2 was designed to predict static structures of proteins, it remains unclear if this result represents an underexplored capability to accurately predict multiple conformations and/or structural heterogeneity. Here, we present an approach to drive AF2 to sample alternative conformations of topologically diverse transporters and G-protein-coupled receptors that are absent from the AF2 training set. Whereas models of most proteins generated using the default AF2 pipeline are conformationally homogeneous and nearly identical to one another, reducing the depth of the input multiple sequence alignments by stochastic subsampling led to the generation of accurate models in multiple conformations. In our benchmark, these conformations spanned the range between two experimental structures of interest, with models at the extremes of these conformational distributions observed to be among the most accurate (average template modeling score of 0.94). These results suggest a straightforward approach to identifying native-like alternative states, while also highlighting the need for the next generation of deep learning algorithms to be designed to predict ensembles of biophysically relevant states.

AI snapshot

Written from the paper’s full text.

TL;DR

Reducing the depth of input MSAs by stochastic subsampling drives AlphaFold2 to sample alternative, native-like conformations of transporters and GPCRs whose structures were absent from its training set. The approach recovers experimentally observed inward/outward or inactive/active states, but it is a methodological “hack” with limited success for proteins already represented in the AF2 training set.

Key findings

  • For all eight benchmark targets—five transporters (LAT1, ZnT8, MCT1, STP10, ASCT2) and three GPCRs (CGRPR, PTH1R, FZD7)—accurate models of at least one conformation were obtained (TM score ≥0.9), and both conformations could be predicted with shallow MSAs.
  • Models at the extremes of the first principal component (PC1) were among the most accurate: average TM score was 0.94 for the top 1, top 3, and top 10 PC1 models; PC1 captured 64.9% ± 16.1% of structural variation among models generated with MSAs of 32 or more sequences.
  • Predicted structural fluctuations correlated with experimental conformational changes for ASCT2, LAT1, CGRPR, and MCT1 with templates (R² ≥ 0.75); Pearson correlation between PC1 and TM score exceeded 0.8 for all transporters.
  • MCT1 was an exception: over 99% of template-free models had TM ≥0.9 to the inward-facing structure and <0.9 to the outward-facing structure regardless of MSA depth; accurate outward-facing models required templates and MSAs of 16–32 sequences.
  • For four proteins with at least one conformation in the AF2 training set (CCR5, SERT, PfMATE, MurJ), conformational diversity was far more limited, and none adopted the alternative conformer.

Methods

  • AF2 v2.0.1 with modified ColabFold was used; MSAs were generated with MMseqs2, template search was disabled except for explicit template-based tests, recycles were set to one, and final MD refinement was skipped.
  • For template-free predictions, all five neural networks were used, with 10 predictions per network per MSA size, yielding 50 models per protein per MSA depth; MSA depth was controlled via `max_msa_clusters` and `max_extra_msa`.
  • Benchmark set: eight membrane proteins absent from the PDB before April 30, 2018 and determined in two or more conformations, plus four proteins with at least one conformation in the AF2 training set; structural similarity was assessed by TM-align, and PCA/RMSF were calculated in CPPTRAJ with loop residues omitted from PCA.

Limitations

  • Stated: the approach is a methodological “hack,” not an explicit AF2 objective; AF2 likely does not learn energy landscapes, does not account for the lipid environment, and showed limited success for proteins whose structures were in the training set.
  • Stated: optimal MSA depth and template choice varied by protein, with no one-size-fits-all solution; the GPR114/ADGRG5 predictions require experimental validation.
  • Evident from the text: the benchmark is small (eight targets plus four training-set proteins), validation depends on already-known experimental structures, and the authors skipped the final MD simulation to reduce computational cost.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.7554/elife.75751

Cites

64 of 64 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 30 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    The first principal component (PC1) captured 64.9% ± 16.1% of the structural variations among models generated using MSAs with 32 or more sequences.

    “the first principal component (PC1) captured 64.9% ± 16.1% of the structural variations among the models generated using MSAs with 32 or more sequences”
    Quote in the paperSubstantial claim
  • Findingnone given

    This approach showed limited success when applied to transporters whose structures were used to train AF2.

    “this approach showed limited success when applied to transporters whose structures were used to train AF2”
    Quote in the paperSubstantial claim
  • Findingown data

    In every case, a correlation was evident between each model's PC1 values and their TM scores.

    “In every case, a correlation was evident between each model’s PC1 values and their TM scores.”
    Quote in the paperSubstantial claim
  • Findingown data

    No general pattern was readily apparent regarding the ideal MSA depth required to balance diversity and native folding, even when accounting for sequence length of the target.

    “No general pattern was readily apparent regarding the ideal MSA depth required to achieve this balance, even when accounting for sequence length of the target”
    Quote in the paperNot a substantial claim
  • Findingown data

    Correlation between each residue's Cα distance between two superimposed experimental structures and its RMSF among all 50 models was observed in most cases and was notable for ASCT2, LAT1, CGRPR, and MCT1 with templates (R2 ≥ 0.75).

    “Correlation between these two measures was observed in most cases and was notable for ASCT2, LAT1, CGRPR, and MCT1 with templates (R2 ≥ 0.75).”
    Quote in the paperSubstantial claim
  • May combine two claims. 16
  • Classified as probably not a substantial claim. 9
  • The quote only partly states this claim. 1
  • May combine two claims.

    Findingown data

    Models with the most extreme PC1 values were among the most accurate: average TM scores were 0.94 for the top 1, top 3, and top 10 PC1 models.

    “Indeed, the models with the most extreme PC1 values were also among the most accurate: average TM scores were 0.94 for the top 1, top 3, and top 10 PC1 models”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    Similar results were observed when modeling PTH1R using either inactive or active templates, as well as LAT1 using either OF or IF templates.

    “Similar results were observed when we modeled PTH1R using either inactive or active templates, as well as LAT1 using either OF or IF templates”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    Increasing the depth of subsampled MSAs eliminated misfolded models while also limiting the extent to which alternative conformations were sampled.

    “Increasing the depth of subsampled MSAs had the desirable effect of eliminating these models, while also limiting the extent to which alternative conformations were sampled.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    MCT1 was exclusively modeled by AF2 in either inward-facing (IF) or fully occluded conformations; over 99% of the models had TM scores ≥0.9 to the IF structure and <0.9 to the outward-facing (OF) structure, regardless of MSA depth.

    “One target, MCT1, was exclusively modeled by AF2 in either inward-facing (IF) or fully occluded conformations; over 99% of the models had TM scores of ≥0.9 and <0.9 to the IF and outward-facing (OF) structures, respectively, regardless of MSA depth.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    Misfolded models virtually never coclustered and could thus be identified and excluded from further analysis.

    “misfolded models virtually never coclustered and could thus be identified and excluded from further analysis”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    The inclusion of templates restored the correlation in MCT1 but contributed negligibly to those of PTH1R and LAT1.

    “The inclusion of templates restored this correlation in MCT1 but contributed negligibly to those of PTH1R and LAT1”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingown data

    In the benchmark, the predicted conformations spanned the range between two experimental structures of interest, with models at the extremes of these conformational distributions observed to be among the most accurate (average TM score 0.94).

    “In our benchmark, these conformations spanned the range between two experimental structures of interest, with models at the extremes of these conformational distributions observed to be among the most accurate (average template modeling score of 0.94).”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    Providing templates of homologs in exclusively OF conformations alongside MSAs of various sizes yielded accurate OF models only with MSAs containing 16-32 sequences, and these constituted a minor population in an ensemble dominated by IF models.

    “Accurate OF models were obtained only with MSAs containing 16–32 sequences and constituted a minor population in an ensemble dominated by IF models.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S4 (based).

    Findingnone given

    Models of most proteins generated using the default AF2 pipeline are conformationally homogeneous and nearly identical to one another.

    “Whereas models of most proteins generated using the default AF2 pipeline are conformationally homogeneous and nearly identical to one another”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingnone given

    Models of CCR5 were less biased toward the training set conformation, while deep MSAs reduced conformational diversity.

    “while models of CCR5 were less biased toward the training set conformation, deep MSAs reduced conformational diversity.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    The optimal MSA depth and choice of templates varied for each protein.

    “because the optimal MSA depth and choice of templates varied for each protein”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingcitation

    AF2 modeled the multidrug transporter LmrP (CASP14 target T1024) in multiple conformations, two of which were individually consistent with published experimental data.

    “Upon examining the list of CASP14 targets and corresponding models, we found that AF2 modeled the multidrug transporter LmrP (target T1024) in multiple conformations, two of which were individually consistent with published experimental data”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Jumper et al., 2021b

      Not checked in the free run.

    • cites Del Alamo et al., 2021a

      Not checked in the free run.

    • cites Debruycker et al., 2020 source

      Not checked in the free run.

  • The quote only partly states this claim.

    Findingown data

    Reducing the depth of the input multiple sequence alignments by stochastic subsampling led to the generation of accurate models in multiple conformations for topologically diverse transporters and GPCRs absent from the AF2 training set.

    “reducing the depth of the input multiple sequence alignments by stochastic subsampling led to the generation of accurate models in multiple conformations.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data

    Although conformational diversity was demonstrated to a limited extent by the generation of occluded models of MurJ and PfMATE, none of the models observed adopted the alternative conformer.

    “Although conformational diversity was demonstrated to a limited extent by the generation of occluded models of MurJ and PfMATE, none of the models observed adopted the alternative conformer.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationown data

    Accurate representatives of distinct conformers were generally obtainable with exhaustive sampling and could be identified by performing PCA and selecting models at the extreme positions of PC1.

    “Accurate representatives of distinct conformers were generally obtainable with exhaustive sampling and could be identified by performing PCA and selecting models at the extreme positions of PC1.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim.

    Interpretationown data

    Traditional methods may still be required to capture alternative conformers for transporters whose structures were used to train AF2.

    “hinting at the possibility that traditional methods may still be required to capture alternative conformers”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Crawley et al., 2011

      Not checked in the free run.

    • cites Ollikainen et al., 2013

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The resulting models from traditional pipelines are unnaturally compact and frequently distorted, preventing critical questions about ligand and/or drug binding modes from being addressed.

    “The resulting models are unnaturally compact and frequently distorted, preventing critical questions about ligand and/or drug binding modes from being addressed (Ovchinnikov et al., 2015; Nicoludis and Gaudet, 2018).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Ovchinnikov et al., 2015

      Not checked in the free run.

    • cites Nicoludis and Gaudet, 2018

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    The physiological relevance of the proposed structural movements requires experimental validation.

    “The physiological relevance of these proposed structural movements nonetheless requires experimental validation.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: AF2 does not directly account for the lipid environment.
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    For GPCRs, ligand binding on the extracellular side triggers structural rearrangements on the intracellular side that initiate downstream signaling.

    “For G-protein-coupled receptors (GPCRs), ligand binding on the extracellular side triggers structural rearrangements on the intracellular side that initiate downstream signaling (Wang et al., 2020; Gusach et al., 2020).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Wang et al., 2020

      Not checked in the free run.

    • cites Gusach et al., 2020

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    For transporters, vectorial translocation of substrates is mediated by movements that open and close extra- and intracellular gates.

    “the vectorial translocation of substrates by transporters is mediated by movements that open and close extra- and intracellular gates (Drew and Boudker, 2016; Kazmier et al., 2017).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Drew and Boudker, 2016

      Not checked in the free run.

    • cites Kazmier et al., 2017

      Not checked in the free run.

  • Classified as probably not a substantial claim. May combine two claims.

    Study design and conductnone given

    All templates were manually inspected, and those with structures similar to the desired conformation of interest were retained.

    “All templates were manually inspected, and those with structures similar to the desired conformation of interest were retained.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim.

    Premisecitation

    AlphaFold2 achieved remarkably accurate de novo structure prediction.

    “A performance breakthrough was unveiled during CASP14 by AlphaFold2 (AF2) (Jumper et al., 2021a; Tunyasuvunakool et al., 2021; Pereira et al., 2021), which achieved remarkably accurate de novo structure prediction.”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Jumper et al., 2021a

      Not checked in the free run.

    • cites Tunyasuvunakool et al., 2021 source

      Not checked in the free run.

    • cites Pereira et al., 2021 source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    MSA subsampling was carried out randomly by AF2.

    “MSA subsampling was carried out randomly by AF2”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    No manual intervention was carried out to fine-tune the composition of the MSAs used for prediction.

    “No manual intervention was carried out to fine-tune the composition of these alignments.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Interpretationcitation

    The structural model of GPR114/ADGRG5 deposited in the AF2 database likely depicts an active conformation that diverges from the structure of the homolog GPR97.

    “The structural model deposited in the AF2 database, which likely depicts an active conformation that diverges from the structure of the homolog GPR97 (Ping et al., 2021), could be recapitulated by using deep MSAs.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Ping et al., 2021

      Not checked in the free run.

25 of 25
  • The paper relies on this without support.

    AlphaFold2 was designed to predict static structures of proteins.

    “Given that AF2 was designed to predict static structures of proteins, it remains unclear if this result represents an underexplored capability to accurately predict multiple conformations and/or structural heterogeneity.”
  • The paper relies on this without support.

    The experimental structures used as references are accurate representations of the native conformations.

    “we used the active conformation of FZD7 and the inactive conformation of the nearly identical FZD4”
  • The paper relies on this without support.

    The two experimental structures of each target bracket the relevant conformational ensemble.

    “In our benchmark, these conformations spanned the range between two experimental structures of interest”
  • The paper relies on this without support.

    AF2 does not directly account for the lipid environment.

    “AF2 does not directly account for the lipid environment”
  • The paper relies on this without support.

    The structure of GPR114/ADGRG5 has not been experimentally determined.

    “GPR114/ADGRG5, a class B2 adhesion GPCR whose structure has not been experimentally determined.”
5 of 5

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • del Alamo et al modified AF2 by reducing recycles and limiting MSA depth to sample functionally relevant alternative conformations of transporters and GPCRs.

    “del Alamo et al modified the AF2 pipeline by reducing the number of recycles and restricting the depth of randomly subsampled MSAs to sample functionally relevant alternative conformations of transporters and G-protein-coupled receptors46”
  • Initial efforts to enhance conformational sampling have involved altering the number of sequences used to generate shallower MSA representations, masking coevolutionary information from MSAs, and splitting conflicting coevolutionary signals by clustering MSAs.

    “Initial efforts to enhance sampling of different conformations have involved altering the number of sequences used to generate shallower MSA representations, masking coevolutionary information provided by MSAs, and splitting conflicting coevolutionary signals by clustering MSAs46,47,105.”
  • Subsampling the input MSA enables AF2 to predict known conformational changes of transporters.

    “Despite these demonstrations of shortcomings, it was shown that subsampling the input MSA enables AF2 to predict known conformational changes of transporters14.”

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • FZD7 does not meet the stated two-conformation selection criterion

    Minor

    The paper says the benchmark is restricted to proteins experimentally determined in two or more conformations. For FZD7, it instead uses an active FZD7 structure and an inactive structure of the related protein FZD4.

1 of 1

Cites

Flagged first, then by citations

2 of the 66 references couldn’t be matched to a known paper.

Research works printed but not linked: 1

  1. Del Alamo D. 2021b. Prediction of alternative conformations using AlphaFold 2. Software Heritage. swh:1:rev:d60db86886186e80622deaa91045caccaf4103d3. https://archive.softwareheritage.org/swh:1:dir:732ab369a1b141030fbc5728d9ab3f50e222b3ea;origin=https://github.com/delalamo/af2_conformations;visit=swh:1:snp:cb0ae8b45df2c5f548867364bbfe52debb482bda;anchor=swh:1:rev:d60db86886186e80622deaa91045caccaf4103d3.

Cited by

Sorted by citations

Authors
del Alamo, D., Sala, D., Mchaourab, H. S., & Meiler, J.
Affiliations
  • Department of Molecular Physiology and Biophysics, Vanderbilt University
  • Leipzig University (DE)
  • Vanderbilt University (US)
Corresponding author
  • Mchaourab, Hassane S
  • Meiler, Jens
Venue
eLife
Published
2022-03-03
Volume
11
ISSN
2050-084X (ISSN-L)
OpenAlex
W4214847177
Cited by
511
References
66
Citations by year
2026: 115 · 2025: 121 · 2024: 138 · 2023: 93 · 2022: 42 · 2021: 2
FWCI
32.09
Type
journal article
Language
en
Open access
Locations
  • eLife (publishedVersion, cc-by)
  • bioRxiv (Cold Spring Harbor Laboratory) (acceptedVersion, cc-by-nc-nd)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • Florence Research (University of Florence) (submittedVersion, other-oa)
Funders
  • National Institutes of Health (GM 128087)
  • Deutsche Forschungsgemeinschaft (CRC 1423, project number 421152132, subproject Z04)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Protein Structure and Dynamics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Receptor Mechanisms and Signaling
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Machine Learning in Bioinformatics
Keywords
AlphaFold2, conformational sampling, membrane transporters, G protein-coupled receptors, structural heterogeneity, protein conformational plasticity, protein structure prediction, conformational ensembles
MeSH terms
Algorithms, Furylfuramide, Protein Conformation, Sequence Alignment, Membrane Transport Proteins

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