Bio Remote Homology

Version Compatibility

Reference examples tested with: NCBI BLAST+ 2.15+, HMMER 3.4+, MMseqs2 15+, DIAMOND 2.1+, HH-suite3 3.3+, Foldseek 9+

Before using code patterns, verify installed versions match. If versions differ:

• CLI: <tool> --version then <tool> --help to confirm flags
• Python: pip show <package> then introspect signatures

If a flag is unrecognized or behavior changes, introspect with --help and adapt the example to match the installed version rather than retrying.

Remote Homology

"Find homologs my BLAST missed" -> Standard BLAST detects similarity reliably down to ~35% pairwise identity (the "twilight zone", Rost 1999 Protein Eng 12:85). Below that, profile methods (PSSMs, HMMs) and structure-aware methods (Foldseek) recover homologs that pairwise alignment misses.

This skill covers the decision: which method, when, against what database. The competition has shifted substantially since 2015: PSI-BLAST is no longer the de-facto standard; MMseqs2 and DIAMOND have replaced BLAST in most large-scale workflows; Foldseek (van Kempen et al. 2024 Nat Biotechnol 42:243) detects homologs no sequence method can reach by searching with a 3Di structural alphabet derived from AlphaFold/ESMFold predictions.

• CLI: psiblast, jackhmmer, hmmsearch, hhblits, mmseqs, diamond, foldseek
• Python: Bio.SearchIO for output parsing; tool-specific clients exist but subprocess is preferred
• Web: HHpred (HHblits webserver), Foldseek webserver, ColabFold for paired structure search

Required Setup

# Install via conda
conda install -c bioconda hmmer mmseqs2 diamond hhsuite foldseek
# BLAST+ (separate)
conda install -c bioconda blast

# Verify
hmmsearch -h | head -3       # HMMER 3.4+
mmseqs version               # MMseqs2 15+
diamond --version            # DIAMOND 2.1+
hhblits -h | head -3         # HH-suite3 3.3+
foldseek --version           # Foldseek 9+

Decision matrix: which method when

Foldseek: the 2024 revolution

Foldseek (van Kempen, Kim, Tumescheit et al. 2024 Nat Biotechnol 42:243) searches protein structures by representing each residue's local geometry as a 21-letter "3Di" alphabet, then running BLAST-style alignment in this alphabet. Two consequences:

• 4-5 orders of magnitude faster than DALI (the previous gold-standard structure aligner).
• Finds homologs that sequence methods cannot: when sequence has diverged past detection but structure is preserved, Foldseek recovers the homology. Reported sensitivity vs traditional structure search is comparable; sensitivity vs sequence methods is dramatically higher at low identity.

Two access modes:

1. Have a structure (PDB or AlphaFold): foldseek easy-search query.pdb db_dir result.m8 tmp_dir
2. Sequence only, no structure: use ProstT5 (Heinzinger et al. 2024) to embed sequence to 3Di alphabet directly, skipping AF2 entirely: foldseek databases ProstT5 prostt5_db tmp then foldseek easy-search seq.fa db result.m8 tmp --prostt5-model prostt5_db

The major prebuilt Foldseek databases (AlphaFoldDB, PDB100, ESMAtlas) are downloadable via foldseek databases.

PSI-BLAST: still useful, but watch the drift

PSI-BLAST (Altschul et al. 1997 Nucleic Acids Res 25:3389) builds a PSSM iteratively: each iteration includes hits below -inclusion_ethresh (default 0.005) in the next PSSM. Convergence is when no new hits cross the threshold. Stopping at convergence is often the wrong call -- iterations 2-3 are usually optimal; iterations 4+ frequently drift into paralog inclusion, contaminating the PSSM.

PSI-BLAST is also non-deterministic in detail: different input order or DB version can produce different PSSMs. For reproducibility, save the PSSM (-out_pssm pssm.asn) and re-use with -in_pssm.

HMMER 3 (hmmsearch, jackhmmer)

HMMER 3 (Eddy 2011 PLoS Comput Biol 7:e1002195) is profile HMM search. Two main workflows:

• hmmsearch profile.hmm seqdb: search a database with a known HMM (Pfam, custom).
• jackhmmer query.fa seqdb: iterative search like PSI-BLAST but with full HMM math. Typically higher sensitivity than PSI-BLAST at the same number of iterations.

For domain assignment, hmmscan query.fa Pfam-A.hmm is the canonical pipeline. Pfam HMMs come with calibrated gathering thresholds (-gathering) -- use them instead of arbitrary E-value cutoffs.

# Build domain database once
wget https://ftp.ebi.ac.uk/pub/databases/Pfam/current_release/Pfam-A.hmm.gz
gunzip Pfam-A.hmm.gz
hmmpress Pfam-A.hmm

# Annotate query against Pfam-A with gathering threshold (calibrated cutoff)
hmmscan --cut_ga --domtblout query.domtbl Pfam-A.hmm query.fa

HHblits / HHsearch (HH-suite3)

HHblits (Remmert et al. 2012 Nat Methods 9:173; HH-suite3: Steinegger et al. 2019 BMC Bioinformatics 20:473) is profile-profile alignment. The most sensitive method when both query and target have an HMM representation. Standard pipeline:

1. Build query MSA with hhblits against UniRef30 (or HHblits' default DB).
2. Convert MSA to query HMM.
3. Search against a profile DB (PDB70 for structure, Pfam-A for domains) with hhsearch.

Output is in HHM format. For very deep homology (the structural twilight zone), HHsearch vs PDB70 is still the gold standard.

MMseqs2 (the modern protein search workhorse)

MMseqs2 (Steinegger & Soding 2017 Nat Biotechnol 35:1026) replaces BLAST in nearly all large-scale workflows.

Key advantages:

• Speed: 400-10,000x faster than blastp at similar sensitivity.
• Sensitivity: At -s 7.5, matches HMMER sensitivity (but on raw sequence, not profiles).
• Iterative profile search: mmseqs search --num-iterations 3 matches PSI-BLAST behavior at much higher speed.

# Build target DB
mmseqs createdb target.fasta targetDB
mmseqs createindex targetDB tmp

# Sensitive search
mmseqs easy-search query.fa targetDB results.m8 tmp -s 7.5 --num-iterations 3

# All-vs-all clustering at 50% sequence identity
mmseqs easy-cluster all_proteins.fa cluster tmp --min-seq-id 0.5 -c 0.8

The -s parameter trades sensitivity for speed: 1.0 (fast), 4.0 (default), 7.5 (HMMER-like sensitivity).

DIAMOND (the modern blastp replacement)

DIAMOND (Buchfink et al. 2015 Nat Methods 12:59; v2: Buchfink et al. 2021 Nat Methods 18:366) is the de-facto replacement for blastp on large-scale workflows.

# Build DIAMOND DB
diamond makedb --in nr.fa -d nr

# Sensitive search
diamond blastp -d nr -q query.fa -o results.tsv \
        --more-sensitive -e 1e-10 -p 16 \
        --outfmt 6 qseqid sseqid pident length qcovhsp evalue bitscore stitle

# Long-read frameshift-aware (for nanopore/PacBio metagenomics)
diamond blastx -d nr -q longreads.fa --frameshift 15 -o reads.tsv --outfmt 6

For protein remote homology in 2026, DIAMOND --ultra-sensitive or MMseqs2 -s 7.5 should be the default before reaching for BLAST.

Iterative HMMER (jackhmmer)

# 3 iterations, save checkpoint HMM at each iteration
jackhmmer -N 3 --chkhmm iter.hmm --tblout hits.tbl query.fa uniref90.fa

# After convergence (no new hits below threshold) use the final HMM for downstream searches
hmmsearch iter-3.hmm target.fa > hits.txt

Code patterns

Foldseek search against AlphaFoldDB

Goal: Find structural homologs of a protein structure (or sequence via ProstT5) in AlphaFold's predicted structure database.

Approach: Download AlphaFoldDB structure DB (or ProstT5 for sequence-only); search with foldseek easy-search; parse m8 tabular output.

Reference (Foldseek 9+):

#!/bin/bash
# Reference: foldseek 9+ | Verify API if version differs

mkdir -p foldseek_dbs tmp
# Download the AlphaFoldDB Swiss-Prot subset (~few GB; full AFDB is much larger)
foldseek databases Alphafold/Swiss-Prot afdb_sp foldseek_dbs/tmp

# Structure-vs-structure search
foldseek easy-search query.pdb foldseek_dbs/afdb_sp results.m8 tmp \
         --format-output query,target,fident,alnlen,evalue,bits,prob,qtmscore,ttmscore

head -5 results.m8
# qtmscore/ttmscore are TM-score equivalents from local Foldseek alignment.
# Hits with prob > 0.9 are confidently structurally homologous.

Sequence-only Foldseek via ProstT5

foldseek databases ProstT5 prostt5_model tmp
foldseek easy-search query.fa foldseek_dbs/afdb_sp seq_results.m8 tmp \
         --prostt5-model prostt5_model --threads 8

This is the path to take when only a protein sequence is available -- ProstT5 (a protein language model) predicts the 3Di alphabet directly from sequence.

PSI-BLAST with saved PSSM

Goal: Build a position-specific scoring matrix iteratively, then re-use it for downstream searches.

Approach: 3 iterations against UniRef90 (or nr); save ASN.1 + ASCII PSSM; subsequent searches use -in_pssm.

Reference (NCBI BLAST+ 2.15+):

psiblast -query distant_protein.fa -db uniref90 \
         -num_iterations 3 \
         -inclusion_ethresh 0.002 \
         -evalue 0.01 \
         -num_threads 8 \
         -out_pssm distant.pssm.asn \
         -out_ascii_pssm distant.pssm.txt \
         -out psiblast_results.txt

# Reuse saved PSSM in subsequent searches against a different DB
psiblast -in_pssm distant.pssm.asn -db swissprot \
         -out swissprot_via_pssm.txt

MMseqs2 sensitive iterative search

Goal: PSI-BLAST-equivalent iterative profile search, but 100x faster.

Approach: mmseqs search --num-iterations 3 -s 7.5.

Reference (MMseqs2 15+):

mmseqs createdb query.fa queryDB
mmseqs createdb uniref90.fa uniref90DB
mmseqs createindex uniref90DB tmp

mmseqs search queryDB uniref90DB resultDB tmp \
       --num-iterations 3 \
       -s 7.5 \
       -e 1e-5 \
       --threads 16

mmseqs convertalis queryDB uniref90DB resultDB results.m8 \
       --format-output query,target,fident,alnlen,evalue,bits

Pfam domain annotation (canonical)

# One-time prep
hmmpress Pfam-A.hmm

# Annotate
hmmscan --cut_ga --domtblout query.domtbl --cpu 8 Pfam-A.hmm query.fa

# Filter: gathering threshold passes are already significance-validated
awk '!/^#/ {print $1, $2, $4, $5, $7, $8, $13}' query.domtbl | head
# columns: target_name, accession, query_name, accession, full_evalue, full_score, i_evalue

HHsearch against PDB70 (deepest homology to PDB)

# Build query MSA via HHblits vs UniRef30
hhblits -i query.fa -d uniref30 -oa3m query.a3m -n 3 -cpu 8

# Search PDB70 with the query profile
hhsearch -i query.a3m -d pdb70 -o query.hhr -cpu 8

head -30 query.hhr   # Top hits with probability + alignment statistics

DIAMOND ultra-sensitive on a metagenome

diamond makedb --in uniref90.fa -d uniref90
diamond blastp -d uniref90 -q metagenome_proteins.fa -o hits.tsv \
        --ultra-sensitive -e 1e-5 -p 32 \
        --outfmt 6 qseqid sseqid pident length qcovhsp evalue bitscore stitle

Failure modes

PSI-BLAST profile drift

• Trigger: Iterating to convergence (5+ iterations).
• Mechanism: Each iteration includes hits below threshold; eventually paralogs and divergent family members contaminate the PSSM.
• Symptom: Later iterations return many implausible hits; functional inference goes wrong.
• Fix: Cap at 3 iterations; inspect the saved PSSM and the included sequence set; use stricter -inclusion_ethresh 0.001.

Foldseek "structure but no homology" hits

• Trigger: Searching small fragments or highly conserved folds (TIM barrels, Rossmann folds).
• Mechanism: Structural fold is preserved across deep divergence; superfamily hits exist without true homology.
• Symptom: High structural similarity to functionally unrelated proteins.
• Fix: Combine Foldseek hits with sequence-based evidence; check shared catalytic residues; consider that fold-level similarity is necessary but not sufficient for homology.

MMseqs2 default sensitivity

• Trigger: mmseqs easy-search without -s.
• Mechanism: Default -s 4.0 is fast but misses remote homologs.
• Symptom: Equivalent to a fast BLAST; misses what HMMER would find.
• Fix: Set -s 7.5 for distant homology; -s 5.7 is a middle ground.

DIAMOND default mode lossy

• Trigger: diamond blastp without --more-sensitive or --ultra-sensitive.
• Mechanism: Default mode trades ~5% sensitivity for speed vs blastp.
• Symptom: Hits BLAST would find are missing.
• Fix: Use --more-sensitive for general work, --ultra-sensitive for remote homology.

Profile method on a low-complexity query

• Trigger: Query has signal peptide, coiled-coil, or repeat region.
• Mechanism: Profile is dominated by low-complexity columns; false hits to other low-complexity proteins.
• Symptom: Many high-scoring hits to unrelated low-complexity proteins.
• Fix: Mask the low-complexity region (SEG: segmasker -infmt fasta -in query.fa) before building the profile.

HHblits database version drift

• Trigger: Using a UniRef30 database from a different release than the PDB70 search DB.
• Mechanism: Profile statistics depend on the DB's amino acid distribution.
• Symptom: Hit probabilities are miscalibrated.
• Fix: Use UniRef30 and PDB70 from the same MMseqs2 / HH-suite release.

Foldseek without ProstT5 for sequence-only query

• Trigger: No structure available; tried to predict with AF2 first (slow).
• Mechanism: ProstT5 predicts 3Di alphabet directly from sequence, skipping AF2.
• Symptom: Days-long AF2 prediction step for a query that could be Foldseek'd in seconds.
• Fix: Use foldseek databases ProstT5 ... and --prostt5-model.

Common errors

References

• Altschul SF, Madden TL, Schaffer AA, Zhang J, Zhang Z, Miller W, Lipman DJ. (1997) Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res 25:3389-3402.
• Rost B. (1999) Twilight zone of protein sequence alignments. Protein Eng 12:85-94.
• Eddy SR. (2011) Accelerated profile HMM searches. PLoS Comput Biol 7:e1002195.
• Remmert M, Biegert A, Hauser A, Soding J. (2012) HHblits: lightning-fast iterative protein sequence searching by HMM-HMM alignment. Nat Methods 9:173-175.
• Buchfink B, Xie C, Huson DH. (2015) Fast and sensitive protein alignment using DIAMOND. Nat Methods 12:59-60.
• Steinegger M, Soding J. (2017) MMseqs2 enables sensitive protein sequence searching for the analysis of massive data sets. Nat Biotechnol 35:1026-1028.
• Steinegger M, Meier M, Mirdita M, Vohringer H, Haunsberger SJ, Soding J. (2019) HH-suite3 for fast remote homology detection and deep protein annotation. BMC Bioinformatics 20:473.
• Buchfink B, Reuter K, Drost HG. (2021) Sensitive protein alignments at tree-of-life scale using DIAMOND. Nat Methods 18:366-368.
• van Kempen M, Kim SS, Tumescheit C, Mirdita M, Lee J, Gilchrist CLM, Soding J, Steinegger M. (2024) Fast and accurate protein structure search with Foldseek. Nat Biotechnol 42:243-246.
• Heinzinger M, Weissenow K, Sanchez JG, Henkel A, Mirdita M, Steinegger M, Rost B. (2024) Bilingual language model for protein sequence and structure. NAR Genom Bioinform 6:lqae150.

Related Skills

• blast-searches - Remote BLAST against NCBI; baseline for closer homologs
• local-blast - Local BLAST+ for moderate-scale workflows
• ortholog-inference - Orthology calls (RBH, OrthoFinder, OMA, Compara)
• alignment/multiple-alignment - Build MSAs for HMM profiles
• structural-biology/alphafold-predictions - Predict structures for Foldseek input
• structural-biology/modern-structure-prediction - ESMFold, ColabFold pipelines