SRP9 (P49458) review notes

Identity and overview

SRP9 is the 9 kDa subunit of the signal recognition particle (SRP). It is an 86-aa
protein (initiator Met removed; chain 2-86) belonging to the SRP9 family.

[file:human/SRP9/SRP9-uniprot.txt "Signal recognition particle 9 kDa protein"]

Core function: Alu domain / elongation arrest

SRP9 together with SRP14 and the Alu portion of the SRP RNA constitutes the
elongation-arrest domain of SRP. SRP9 binds RNA as a heterodimer with SRP14.

[file:human/SRP9/SRP9-uniprot.txt "SRP9 together with SRP14 and the Alu portion of the\nCC SRP RNA, constitutes the elongation arrest domain of SRP"]
[file:human/SRP9/SRP9-uniprot.txt "Heterodimer with SRP14; binds RNA as heterodimer"]
[file:human/SRP9/SRP9-uniprot.txt "The complex of SRP9 and SRP14 is required for SRP\nCC RNA binding"]

Review (PMID:34208095, full text available) describes the elongation arrest mechanism:
PMID:34208095
PMID:34208095
PMID:34208095
PMID:34208095

This supports:
- MF: SRP RNA / 7S RNA binding (GO:0008312) within SRP, as the SRP9/14 heterodimer
- BP: negative regulation of translational elongation (GO:0045900) — elongation arrest
- BP: SRP-dependent cotranslational protein targeting to membrane (GO:0006614)
- CC: signal recognition particle (GO:0005786 / GO:0048500)

Complex membership

SRP is a ribonucleoprotein of one 7SL RNA (~300 nt) and six proteins: SRP72, SRP68,
SRP54, SRP19, SRP14, SRP9. SRP9 forms the Alu-domain heterodimer with SRP14.
[file:human/SRP9/SRP9-uniprot.txt "Component of a signal recognition particle complex that\nCC consists of a 7SL RNA molecule of 300 nucleotides and six protein\nCC subunits: SRP72, SRP68, SRP54, SRP19, SRP14 and SRP9"]

ComplexPortal: CPX-2652 Signal recognition particle.
[file:human/SRP9/SRP9-uniprot.txt "ComplexPortal; CPX-2652; Signal recognition particle."]

Structural evidence: X-ray of SRP9 (2-86) in complex with SRP14 (PMID:11089964,
"Structure and assembly of the Alu domain of the mammalian signal recognition particle";
abstract NOT cached). Cryo-EM of SRP in complex with ribosome-nascent chain and SRP
receptor (PMID:34020957; abstract NOT cached). Both establish the Alu/elongation-arrest
geometry and SRP9 as an Alu-domain RNA-binding subunit — SRP9 itself is NOT a GTPase
(the GTPases in the pathway are SRP54, SRα, SRβ).
PMID:34208095

Original characterization (PMID:7730321, abstract only)

Hsu, Chang & Maraia 1995 characterized human SRP9; SRP9/14 = the Alu RNA-binding
protein (RBP); binds the Alu region of 7SL plus scAlu/scB1 RNAs with high affinity.
PMID:7730321
PMID:7730321
This GOA reference (PMID:7730321) underlies the TAS annotations RNA binding (GO:0003723)
and signal recognition particle binding (GO:0005047). It also was used (PINC) for a
GO:0005785 "signal recognition particle receptor complex" CC annotation — but the paper
is about SRP9 in SRP (the Alu RBP), not the SR (SRα/SRβ) receptor complex. SRP9 is a
component of SRP, not of the SRP receptor complex. GO:0005785 is likely a curation
mislabel (SRP vs SRP receptor confusion) and is an over/mis-annotation.

Subcellular location

UniProt: Cytoplasm. SRP9/14 colocalizes in the nucleolus during SRP biogenesis (review),
but the mature functional location is cytoplasm/cytosol.
[file:human/SRP9/SRP9-uniprot.txt "SUBCELLULAR LOCATION: Cytoplasm."]

protein binding (GO:0005515, IPI) annotations

Many IPI "protein binding" annotations exist. Several name SRP14 (P37108) as the partner
(PMID:28514442, 30021884, 32296183, 33961781, 35271311, 40205054) — this is the genuine,
biologically central SRP9-SRP14 Alu heterodimer interaction. UniProt INTERACTION block:
[file:human/SRP9/SRP9-uniprot.txt "P49458; P37108: SRP14; NbExp=12; IntAct=EBI-350743, EBI-353399;"]
Others name unrelated high-throughput partners: ACTN2/P35609 (PMID:25910212);
CDHR3/Q6ZTQ4, DPP9/Q86TI2-2, RYBP/Q8N488 (PMID:32814053). PMID:24965446 reports SRP9
co-purifying with the pestivirus Npro RNP complex (P19712-PRO_0000038050), an
incidental ribosomal/RNP capture. Bare "protein binding" is uninformative per curation
guidelines; keep as non-core (do not REMOVE experimental IPI annotations).

Conclusions for review actions

• Core MF: 7S/SRP RNA binding (GO:0008312) as the SRP9/14 Alu heterodimer; RNA binding
  (GO:0003723) is the correct but more general parent.
• Core BP: SRP-dependent cotranslational protein targeting to membrane (GO:0006614);
  negative regulation of translational elongation (GO:0045900, elongation arrest).
• Core CC: signal recognition particle (GO:0005786 / GO:0048500).
• GO:0005047 signal recognition particle binding (SRP9 binding to SRP) — acceptable but
  non-core (SRP9 IS part of SRP rather than an external SRP-binding factor; redundant with
  part_of SRP).
• GO:0005785 SRP receptor complex (part_of) — MODIFY/over-annotation: SRP9 is part of SRP,
  not the SRP receptor (SRα/SRβ) complex. Mark as over-annotated.
• GO:0006617 signal sequence recognition (NAS, ComplexPortal): signal-sequence recognition
  is SRP54's role, not SRP9's; SRP9 contributes elongation arrest. Keep as non-core
  (it is a complex-level NAS annotation; SRP9 is part of the complex that performs this).

Falcon deep-research findings (incorporated 2026-06)

• Structural basis of the SRP9/14 Alu-domain RNA clamp: 2.0 A crystal structure of the human Alu RNP (SRP9/14 + Alu RNA) in the ribosome-stalling conformation; the heterodimer clamps the RNA and docks at the ribosomal elongation-factor binding site PMID:26585389. Added (HIGH) and used to strengthen the core 7S-RNA-binding and elongation-arrest core_functions.
• SRP9 contributes ~700 A^2 of the ~1,820 A^2 SRP9/14-RNA interface; mutations weakening the interface beyond DDG > 3.5 kcal/mol abolish Alu retrotransposition PMID:26585389. Links SRP9/14 binding energy to Alu retroelement activity.
• Noncanonical nuclear role: SRP9/14 localizes substantially in the nucleus and transcriptionally regulates the Pol III Alu-like transcripts 7SL (RN7SL1) and BC200 (BCYRN1) by modulating Pol III occupancy rather than RNA stability PMID:37156570. Added (MEDIUM); not core SRP function.
• Noncanonical splicing role: SRP9/14 binds closed/compact Alu RNA folds and regulates inclusion of Alu-derived exons (RNA structure predicts inclusion better than sequence motifs) PMID:37309897. Added (MEDIUM).
• Nucleolar assembly phase: GFP-SRP9 is mostly nuclear; SRP proteins associate with many nucleolar/ribosome-biogenesis factors and nucleolar integrity is required for proper SRP localization PMID:38858088. Added (MEDIUM); supports a nuclear/nucleolar biogenesis compartment for SRP9.
• Abundance/affinity context: primate SRP9/14 is present at ~20-fold molar excess over assembled SRP and binds 7SL with sub-nanomolar affinity, enabling extensive extra-canonical Alu-RNA regulation PMID:39563162. Added (MEDIUM, review).
• Reconstitution work indicates SRP54 and SRP68/72 dominate measurable SRP-ribosome affinity while the Alu (SRP9/14) domain contributes little to affinity, consistent with SRP9/14's role being steric/regulatory at the factor-binding site rather than high-affinity docking (Wild et al. 2019, DOI 10.1093/nar/gky1324). Consistent with existing review framing; notes-only (not added).
• Biomarker/association: higher SRP9 nuclear staining associates with improved recurrence-free survival in resected pancreatic cancer, modulated by nutrient/amino-acid status PMID:38847231. Added (LOW); association-level, not core function.