Structure-based sequence alignment

What is structure-based sequence alignment?

Structure-based sequence alignment is the process of arranging protein sequences by matching residues that occupy corresponding positions in three-dimensional structures. It is especially useful for remote homologs whose folds are conserved despite low sequence identity. In ProteinIQ, USAlign performs structural superposition and returns the resulting residue alignment, TM-scores, RMSD, sequence identity, and superposed coordinates.

Choose structure-based alignment when compatible experimental or predicted structures exist and sequence-only methods disagree or fail to recover a conserved fold. The method can clarify core correspondence across remote homologs, circular permutations, oligomers, and nucleic-acid structures, depending on the selected USAlign mode.

Structural alignment is not independent of structure quality. Missing residues, alternate conformations, domain movements, chain selection, prediction uncertainty, and different biological assemblies can all change the correspondence. Review coverage, both normalized TM-scores, RMSD, aligned length, sequence identity, and the superposed coordinates together.

When to use structure-based sequence alignment

Use this approach when the comparison scope is clear:

• Two structures are available despite weak sequence identity.
• Residue mapping depends on a conserved fold.
• A structural superposition will guide annotation review.

Strengths and limitations

A three-dimensional superposition can locate corresponding core residues after sequence divergence obscures them. US-align returns a residue mapping and score normalizations that can be inspected alongside the actual aligned coordinates.

A shared fold does not mean every loop or active-site residue corresponds. Flexible domains, different oligomeric states and incomplete models can distort a rigid superposition. TM-score and RMSD emphasize different aspects of the aligned region.

Structure-based sequence alignment methods

USAlign searches for a residue correspondence and rigid-body superposition that optimize a length-normalized TM-score. TM-score and RMSD summarize different aspects of the result and should not be treated as interchangeable.

FoldSeek can identify structurally similar candidates at scale, while USAlign is suited to detailed pairwise superposition. Sequence extraction may help compare structure-derived and sequence-only alignments without conflating them.

Structure-based sequence alignment applications

A structure-derived mapping can connect residues across remote homolog candidates or compare models with an experimental reference. It is particularly useful for a conserved core when ordinary sequence alignment places many gaps ambiguously.

Functional annotation transfer should use local geometry and chemistry around the proposed site. A global fold match can coexist with different ligands, mechanisms or evolutionary histories.

How to run structure-based sequence alignment online

The featured workflow gives two PDB structures to US-align and retains the superposition and residue correspondence.

1. Choose structures. Select the relevant chains, constructs and biological states.
2. Check coordinates. Note missing residues, low-confidence segments and major domain movement.
3. Run pairwise superposition. Use US-align on mobile and reference PDB inputs.
4. Inspect mappings. Read aligned length, both TM-score normalizations, RMSD and gaps with the superposed coordinates.
5. Review local sites. Check whether catalytic or binding residues truly correspond before transferring labels.
6. Export evidence. Keep structures, chain choices, alignment and normalization settings together.

How to interpret structure-based sequence alignment results

Inspect aligned length and both TM-score normalizations because unequal chain lengths can produce asymmetric values. RMSD should always be read with the number and fraction of aligned residues.

Do not transfer catalytic, binding, or numbering annotations across a gap or poorly superposed loop without local evidence. Conserved global folds can support very different biochemical roles.

How structure-based sequence alignment works

USAlign superposes mobile and reference structures and derives a residue alignment from their three-dimensional correspondence.

1. Choose structures. Choose structures representing the intended state, construct, and biological assembly.
2. Select chains. Set chains, molecule type, oligomer handling, and any circular-permutation mode.
3. Run USAlign. Run USAlign and preserve both score normalizations and aligned coordinates.
4. Inspect superposition. Inspect coverage, RMSD, gaps, flexible regions, and the superposed model.
5. Transfer cautiously. Transfer residue annotations only across well-supported structural correspondence.

Inputs and outputs

Check formats before running, then inspect and download the result from every workflow step.

Inputs

Alignment input

FASTAPDBmmCIF

Two protein, RNA, or DNA structures in PDB or mmCIF format.

Outputs

Alignment outputs

FASTACSVTSVPDBJSON

Structure-derived residue alignment, TM-scores, RMSD, aligned length, identity, and superposed coordinates.