All families / PF07754
PF07754: Ribosomal_aS21
Ribosomal protein aS21
- Curation status
- SEEDED
- Seed status
- EX DUF
- Characterization
- UNSCORED
Grounding
PF07754: Ribosomal protein aS21
IPR011668: integrated entry for Ribosomal protein aS21
Source description
This entry represents ribosomal protein aS21, an archaea-specific component of the small ribosomal subunit (SSU). The protein was first characterised as HVO_2753 (Small CPxCG-related zinc finger protein) from Haloferax volcanii [InterPro reference PUB00096940] and has been identified in ribosome structures from Pyrococcus abyssi [InterPro reference PUB00114374] and Pyrobaculum calidifontis [InterPro reference PUB00161770], where it is located at a position corresponding to eS21 in eukaryotic ribosomes. NMR 3D structure analysis revealed four C(P)XCG motifs, suggesting the presence of two zinc-binding pockets (ZBPs). While some members of this entry bind two zinc ions [InterPro reference PUB00161688], in HVO_2753, only C(P)XCG motifs 2 and 4 (comprising Cys-32 to Cys-35 and Cys-50 to Cys-53) form a ZBP and bind one zinc atom, while C(P)XCG motifs 1 and 3 (comprising Cys-12 to Cys-15 and Cys-39 to Cys-42) form a four-Cys cluster that do not bind zinc. The four C(P)XCG motifs are critical for protein stability, folding and functionality [InterPro reference PUB00096940]. Despite structural topology resembling eukaryotic eS21, sequence identities in the structurally superimposed regions are limited, and the archaeal protein contains two zinc knuckles not observed in the eukaryotic version [InterPro reference PUB00114374]. The widespread conservation of this protein in archaeal genomes supports its role as a core ribosomal component [InterPro reference PUB00114374,InterPro reference PUB00161770].
Evidence and references
- InterPro entry and its literature references. PUB identifiers are internal InterPro references, not PubMed identifiers. Unresolved citations link to the originating entry.
- Frozen description source: InterPro/Pfam API
Source descriptions and links are provenance, not independent functional validation.
Evidence counts: not scored.
Curated functional assertions
No curated functional assertions supplied.
Protein sequence examples
1 verified sequence example. FASTA
| UniProt | Protein / organism | Sequence | Provenance |
|---|---|---|---|
| P81308 | Small ribosomal subunit protein aS21 Methanocaldococcus jannaschii (strain ATCC 43067 / DSM 2661 / JAL-1 / JCM 10045 / NBRC 100440) | 57 aa; version 1 UniProt 2026_03 | PTM REGISTRY MIRROR |
Pfam membership is not functional evidence. Whole-protein sequences do not establish domain boundaries. A coverage gap here is not absence from UniProt.
Sequences and metadata: UniProt Consortium, CC BY 4.0.
Domain matches
No residue-level domain match evidence is available in this build. Structure and AlphaFold counters do not supply coordinates or establish function.
Snapshot counters
- proteins
- 557
- matches
- 557
- proteomes
- 551
- taxa
- 1,013
- structures
- 33
- alphafold models
- 526
- domain architectures
- 1
Classification basis
pfam previous unknown name
No scoring reasons supplied.
Related Mech records
This family joined the worklist after the frozen cross-Mech scan, so other Mechs were not searched for it. Missing links here are not evidence that other Mechs lack it; rows shown come from mentions of a former DUF/UPF name.
No cross-Mech examples are present in the supplied snapshot.
Curation history and reviews
No retained curation events supplied.
No retained review supplied.
Record source
Status and evidence
- Seed status
- A name-based classification of frozen InterPro/Pfam metadata. UNKNOWN CANDIDATE means the metadata explicitly says the function is unknown. KNOWN HISTORICAL DUF means a DUF name matched without that wording; this heuristic is not proof of a known function and does not establish experimental characterization. EX DUF means Pfam renamed the family away from a DUF/UPF name, as recorded in Pfam's previous identifiers; a rename usually follows published work on some members but is naming history, not proof of function for every member. Some EX DUF families' current names or descriptions still say the function is unknown; their classification basis lists this, and scoring does not treat them as characterized. FALSE POSITIVE TEXT HIT means the naming rules did not match.
- Characterization
- A separate evidence-scoring result. UNSCORED means no score has been calculated; it is not evidence that the family lacks a known function. Missing scores are not negative evidence. PARTIALLY CHARACTERIZED has partial support; KNOWN HISTORICAL DUF can inherit the seed label or reflect a known-function signal. Neither label implies experimental validation.
- Evidence counts
- Source-specific signals, not unique proteins, publications, or confidence scores. Known counts Rhea reactions and experimentally supported GO molecular-function annotations. Partial counts functional/domain assignments. Context counts structural, environmental, orthology, interaction and neighborhood signals; context alone does not assign function. Repeated annotations may not be independent.
- Curation status
- Reported separately from seed classification and characterization. SNAPSHOT ONLY means no curated metadata was supplied for this build. SEEDED denotes a source-derived record, not an expert-reviewed functional claim.
- Missing counts
- Not available means the source did not supply that counter. A zero is a reported count. Not scored means evidence counts are unavailable.