id: Pfam:PF07754
pfam_id: PF07754
name: Ribosomal protein aS21
short_name: Ribosomal_aS21
seed_status: EX_DUF
characterization_status: UNSCORED
curation_status: SEEDED
source_url: https://www.ebi.ac.uk/interpro/api/entry/pfam/PF07754
counters:
  proteins: 557
  matches: 557
  proteomes: 551
  taxa: 1013
  structures: 33
  alphafold_models: 526
  domain_architectures: 1
provenance:
  snapshot_id: interpro-pfam-duf-2026-10-10
  path: data/worklists/interpro-pfam-duf-2026-10-10.json
  sha256: 17fb625876c9326ed284263a47bcd84e0d7844a31c040dc06cd2260c36f1c383
  generated_at: '2026-10-10T07:44:21.614711Z'
  source_url: https://www.ebi.ac.uk/interpro/api/entry/pfam/
interpro_id: InterPro:IPR011668
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 [[cite:PUB00096940]] and has been identified in ribosome structures
  from Pyrococcus abyssi [[cite:PUB00114374]] and Pyrobaculum calidifontis [[cite: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 [[cite: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 [[cite: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 [[cite:PUB00114374]]. The widespread conservation of this protein in archaeal genomes supports
  its role as a core ribosomal component [[cite:PUB00114374],[cite:PUB00161770]].