The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Signal recognition particle subunit SRP68 (gene SRP68, UniProt Q9UHB9) is a core protein component of the mammalian signal recognition particle (SRP), a ribonucleoprotein that mediates co-translational targeting of nascent secretory and membrane proteins to the endoplasmic reticulum (ER). SRP68 is not an enzyme or transporter; rather, it is a structural/coordinating SRP subunit that forms a stable heterodimer with SRP72, binds/remodels 7SL (SRP) RNA, and contributes substantially to ribosome binding during targeting. Multiple recent studies (2023–2024) emphasize SRP68’s participation in a nucleolar/nuclear phase of SRP biogenesis and highlight open mechanistic questions about SRP68/72’s precise role in later targeting steps. A strong, direct clinical association exists with severe congenital neutropenia caused by biallelic SRP68 loss-of-function variants, supported by patient-derived functional assays showing reduced SRP68 protein, impaired granulopoiesis, ER stress signaling, and p53-pathway activation. (wild2019reconstitutionofthe pages 1-2, kellogg2022signalrecognitionparticle pages 1-2, kellogg2023unravelingsrpbiogenesis pages 26-29, issa2024thenucleolarphase pages 1-2, schmaltzpanneau2021identificationofbiallelic pages 6-10)
Target confirmed: The literature evidence explicitly connects human SRP68 to UniProt accession Q9UHB9 as a subunit of mammalian SRP. In particular, a reconstitution/biophysics study of the human SRP system lists SRP68 (UniProt Q9UHB9) as one of the six SRP proteins assembled on 7SL RNA. (wild2019reconstitutionofthe pages 1-2)
Organism confirmed: The cited SRP68 evidence is for human/mammalian SRP (e.g., “human SRP system,” “mammalian SRP,” human cell lines). (wild2019reconstitutionofthe pages 1-2, issa2024thenucleolarphase pages 1-2)
Family/domain consistency: SRP68 is consistently described as an SRP68/SRP72 heterodimeric subunit of the SRP S-domain, with SRP68 featuring a TPR-like RNA-binding/remodeling role in 7SL RNA positioning, consistent with SRP68-family annotations. (kellogg2023unravelingsrpbiogenesisa pages 26-29, kellogg2023unravelingsrpbiogenesis pages 26-29)
The signal recognition particle is a conserved ribonucleoprotein complex that recognizes hydrophobic signal sequences on nascent polypeptides as they emerge from the ribosome and delivers translating ribosomes to the ER membrane, enabling co-translational translocation or membrane insertion. In mammals, SRP is composed of 7SL RNA and six proteins (SRP9, SRP14, SRP19, SRP54, SRP68, SRP72). (kellogg2022signalrecognitionparticle pages 1-2, issa2024thenucleolarphase pages 1-2)
SRP is organized into:
- an Alu domain (SRP9/SRP14) that contacts the ribosomal factor-binding site and contributes to elongation arrest, and
- an S domain (SRP19, SRP54, SRP68, SRP72) that contains the major signal-sequence recognition and receptor-docking machinery. (kellogg2022signalrecognitionparticle pages 1-2, wild2019reconstitutionofthe pages 1-2)
SRP68 is an S-domain SRP subunit that forms a heterodimer with SRP72 and contributes to SRP assembly and ribosome binding. The primary signal-sequence recognition and GTPase-based docking with the SRP receptor are attributed to SRP54 (signal-binding M-domain; NG-domain for receptor interaction), while SRP68/72 are best supported as structural/coordinating factors that shape SRP RNA and stabilize SRP–ribosome interactions. (kellogg2022signalrecognitionparticle pages 1-2, kellogg2023unravelingsrpbiogenesisa pages 26-29, kellogg2023unravelingsrpbiogenesis pages 26-29, wild2019reconstitutionofthe pages 1-2)
Evidence supports SRP68 as an SRP subunit that:
1. Heterodimerizes with SRP72 and helps incorporate SRP72 into the SRP complex. (kellogg2023unravelingsrpbiogenesis pages 26-29, kellogg2023unravelingsrpbiogenesis pages 23-26)
2. Binds and remodels 7SL RNA, bending it to enable specific RNA loop positioning that supports ribosome contacts (e.g., facilitating the 7SL RNA “5f loop” contacting the ribosome). (kellogg2023unravelingsrpbiogenesisa pages 26-29, kellogg2023unravelingsrpbiogenesis pages 26-29)
3. Promotes ribosome binding of SRP via multivalent/avidity interactions together with SRP72. Quantitatively, the full-length SRP68/72 heterodimer binds the human 80S ribosome with ~160 ± 20 nM affinity and apparent ultrasensitivity (Hill coefficient ~2.3), consistent with multiple interaction sites rather than a single-point contact. (wild2019reconstitutionofthe pages 5-5)
Collectively, SRP68 can be annotated as an RNA-binding SRP structural subunit that helps organize the SRP ribonucleoprotein for efficient ribosome engagement and targeting.
A widely cited model of the mammalian SRP cycle is:
- SRP recognizes N-terminal signal sequences on nascent chains and can induce a transient elongation pause.
- SRP–ribosome complexes engage the SRP receptor on the ER membrane.
- The ribosome is transferred to the SEC61 translocon, and translation resumes with co-translational translocation/insertion.
- GTP hydrolysis resets the SRP/SR receptor cycle.
SRP68 contributes to this process as part of the SRP S-domain that engages the ribosome and positions SRP RNA/protein components, even though SRP54 is the key signal-sequence and receptor-docking subunit. (kellogg2022signalrecognitionparticle pages 1-2, wild2019reconstitutionofthe pages 1-2, kellogg2023unravelingsrpbiogenesis pages 26-29)
SRP68 is implicated in a nuclear/nucleolar phase of SRP assembly and in export of SRP assembly intermediates:
- A 2024 study reports that SRP biogenesis occurs “in part in the nucleolus,” with SRP proteins associating with nucleolar proteins involved in ribosome biogenesis and nucleolar structure; and that an intact nucleolus is required for proper SRP protein localization. (issa2024thenucleolarphase pages 1-2)
- A 2023 synthesis describes an ordered SRP assembly pathway on 7SL RNA in which SRP19 binds first, followed by SRP68/72 and then SRP9/14, creating a “pre-SRP” complex. SRP68/72 are described as required for pre-SRP export in yeast, with a similar role assumed in mammals via Exportin-5; SRP54 joins later in the cytoplasm to yield mature SRP. (kellogg2023unravelingsrpbiogenesis pages 23-26)
- The 2024 nucleolar biogenesis study also observed SRP proteins (including SRP72 and SRP19 in the reported excerpt) in Cajal bodies, suggesting additional (previously underappreciated) assembly/trafficking steps. (issa2024thenucleolarphase pages 10-13)
Issa et al. (Life Science Alliance; June 2024; https://doi.org/10.26508/lsa.202402614) used quantitative proteomics and localization perturbations to argue that SRP assembly is coordinated with nucleolar organization and ribosome biogenesis. The work reports extensive associations of SRP proteins with nucleolar factors and shows that controlled nucleolar disruption alters SRP protein localization, concluding that an intact nucleolus is required for proper SRP protein localization. This strengthens the “SRP biogenesis is nucleolar” view and provides a modern framework for annotating SRP68 as participating not only in cytosolic targeting but also in upstream assembly and localization control. (issa2024thenucleolarphase pages 1-2, issa2024thenucleolarphase pages 10-13)
A 2023 synthesis on SRP biogenesis and quality control summarizes mechanistic ideas and open questions. It reiterates that SRP68/72 bind 7SL RNA helices 5/6/8 during pre-SRP formation, that SRP68 promotes SRP72 binding, and that Exportin-5 is implicated in mammalian pre-SRP export. It also emphasizes that SRP68’s role includes remodeling/bending 7SL RNA to support ribosome interactions while noting that “little is currently known” about SRP68/72’s exact roles in later steps like SRβ binding and translocon transfer. This is an important expert-level caution: SRP68 is essential and structurally influential, but many mechanistic details remain unresolved. (kellogg2023unravelingsrpbiogenesis pages 26-29, kellogg2023unravelingsrpbiogenesis pages 23-26)
SRP68 is actively used as a handle for studying SRP assembly and localization through:
- GFP-tagged SRP component cell lines and nucleolar perturbations to dissect SRP assembly pathways (e.g., nucleolar disruption by low-dose actinomycin D or ribosomal protein depletion, which re-distributes SRP proteins). (issa2024thenucleolarphase pages 10-13)
- Quantitative proteomics interactome mapping of SRP components to identify nucleolar and quality-control associated proteins linked to SRP biogenesis. (issa2024thenucleolarphase pages 1-2, issa2024thenucleolarphase pages 10-13)
A direct real-world implementation is in rare disease diagnosis: identification of pathogenic biallelic SRP68 variants as a cause of severe congenital neutropenia with marrow maturation arrest, placing SRP68 among SRP-pathway genes relevant to hematologic diagnostic pipelines. (schmaltzpanneau2021identificationofbiallelic pages 1-6)
Across reviews and mechanistic studies, SRP54 is framed as the central signal-sequence recognition and SR receptor GTPase docking subunit, while SRP68/72 are described as contributing to SRP architecture, RNA remodeling, and ribosome binding. This division of labor is relevant for functional annotation: SRP68 is best categorized as a non-enzymatic RNP structural organizer whose disruption compromises SRP function and triggers downstream stress/quality control. (kellogg2022signalrecognitionparticle pages 1-2, kellogg2023unravelingsrpbiogenesisa pages 26-29, wild2019reconstitutionofthe pages 1-2)
The 2024 nucleolar assembly study explicitly argues that SRP and ribosomes, which collaborate in the cytoplasm, are assembled in the same condensate (nucleolus), and that nucleolar integrity is required for proper SRP localization. This supports an expert-level interpretation that SRP68 function should be annotated in two coupled processes: (i) SRP biogenesis/assembly (nuclear/nucleolar) and (ii) SRP-mediated co-translational targeting (cytosol/ER interface). (issa2024thenucleolarphase pages 1-2)
A 2021 Haematologica report (April 2021; https://doi.org/10.3324/haematol.2020.247825) describes a single sporadic pediatric case with biallelic SRP68 variants ([c.184+2T>C];[exon 1 deletion]) presenting at 6 weeks with:
- WBC 6.1×10^9/L, neutrophils 0.2×10^9/L, monocytes 1.7×10^9/L,
- hemoglobin 7.5 g/dL, platelets 149×10^9/L,
- serial monitoring (n=41) showing persistent profound neutropenia (0.200×10^9/L, range 0–1.800). (schmaltzpanneau2021identificationofbiallelic pages 1-6)
In patient-derived granulocytic differentiation cultures, SRP68 protein was decreased to 68% of control in immature granulocytic cells and 39% of control in more mature granulocytic cells, with ~6-fold lower granulocytic proliferation. The study also reports increased spliced XBP1 (indicative of IRE1 arm activation of the unfolded protein response) and increased expression of p53-pathway target genes (e.g., BAX, NOXA1, P21, MDM2), consistent with increased p53-dependent apoptosis. (schmaltzpanneau2021identificationofbiallelic pages 6-10)
These findings provide a mechanistic link between SRP68 loss-of-function and ER proteostasis stress in granulopoiesis, consistent with an SRP-mediated translocation defect model. (schmaltzpanneau2021identificationofbiallelic pages 6-10, kellogg2023unravelingsrpbiogenesis pages 46-51)
Biochemical reconstitution of the human SRP system provides quantitative evidence that SRP68/72 is a major determinant of SRP–ribosome interaction:
- SRP68/72 heterodimer binds the 80S ribosome with KD ~160 ± 20 nM and apparent Hill coefficient ~2.3, supporting multivalent/avidity binding. (wild2019reconstitutionofthe pages 5-5)
- Visual schematics and tables in the same work summarize SRP architecture (Alu vs S-domain) and the SRP68/72 ribosome contact with KD on the order of ~200 nM. (wild2019reconstitutionofthe media 8320e838)
The following cropped figures/tables from Wild et al. visually summarize SRP architecture and SRP68/72 binding contributions. (wild2019reconstitutionofthe media 8320e838, wild2019reconstitutionofthe media b811f4ad, wild2019reconstitutionofthe media eee3cd7f)
The table below links major annotation claims to their strongest supporting sources.
| Topic | Key points | Best supporting citations (pqac IDs) |
|---|---|---|
| Identity/domains | Human SRP68 corresponds to UniProt Q9UHB9; it is an S-domain SRP subunit that forms a heterodimer with SRP72 on 7SL RNA. Reviews describe an SRP68 TPR-containing RNA-binding/remodeling region that bends 7SL RNA to help position the 5f loop for ribosome contact. | (wild2019reconstitutionofthe pages 1-2, kellogg2023unravelingsrpbiogenesis pages 26-29, kellogg2023unravelingsrpbiogenesisa pages 26-29) |
| Complex membership | Mammalian SRP contains 6 proteins + 7SL RNA; SRP68 and SRP72 are the larger S-domain proteins, distinct from the Alu-domain pair SRP9/14. SRP68 is therefore a structural/core SRP component rather than a standalone enzyme or transporter. | (kellogg2022signalrecognitionparticle pages 1-2, wild2019reconstitutionofthe pages 1-2, issa2024thenucleolarphase pages 1-2) |
| Localization/biogenesis | SRP68 participates in a nuclear/nucleolar phase of SRP assembly with 7SL RNA and is implicated in pre-SRP export to the cytoplasm; intact nucleoli are required for proper localization of SRP proteins, and Cajal bodies may host additional assembly steps. | (kellogg2023unravelingsrpbiogenesis pages 23-26, issa2024thenucleolarphase pages 1-2, issa2024thenucleolarphase pages 10-13) |
| Mechanistic role in SRP targeting | SRP68 is best supported as a structural/coordinating subunit: it promotes SRP72 association, remodels 7SL RNA, and contributes to ribosome engagement during co-translational ER targeting. Primary signal-sequence recognition and SRP receptor GTPase docking remain centered on SRP54, so SRP68 is not the primary signal-binding catalyst. | (kellogg2023unravelingsrpbiogenesisa pages 26-29, kellogg2023unravelingsrpbiogenesis pages 26-29, wild2019reconstitutionofthe pages 1-2, kellogg2022signalrecognitionparticle pages 1-2) |
| Quantitative binding data | Reconstituted human SRP studies showed the full-length SRP68/72 heterodimer binds the 80S ribosome with ~160 ± 20 nM affinity and apparent ultrasensitivity (Hill coefficient ~2.3), consistent with multivalent/avidity-based binding; schematic summaries place SRP68/72 ribosome contact in the ~200 nM range. | (wild2019reconstitutionofthe pages 5-5, wild2019reconstitutionofthe media 8320e838) |
| Disease association/variant | The clearest direct human disease link is severe congenital neutropenia (SCN) from biallelic SRP68 variants: c.184+2T>C plus exon 1 deletion, causing aberrant splicing and residual but markedly reduced SRP68 protein. Reviews also summarize SRP68 loss as causing ER-stress-related translocation defects. | (schmaltzpanneau2021identificationofbiallelic pages 1-6, schmaltzpanneau2021identificationofbiallelic pages 6-10, kellogg2023unravelingsrpbiogenesis pages 46-51) |
| Key quantitative clinical/functional data | In the reported SRP68-SCN case: onset at 6 weeks; initial counts WBC 6.1×10^9/L, neutrophils 0.2×10^9/L, monocytes 1.7×10^9/L, Hb 7.5 g/dL, platelets 149×10^9/L; serial counts (n=41) confirmed profound neutropenia. Patient-derived granulocytic cells showed SRP68 protein reduced to 68% in immature and 39% in mature granulocytic cells, with ~6-fold lower granulocytic proliferation and increased spliced XBP1 plus p53-pathway targets. | (schmaltzpanneau2021identificationofbiallelic pages 1-6, schmaltzpanneau2021identificationofbiallelic pages 6-10) |
| Recent 2023-2024 developments | 2024 work strengthened the concept that SRP68 functions within a nucleolar SRP assembly program connected to ribosome biogenesis and possibly Cajal bodies. 2023 synthesis emphasized open mechanistic questions: how SRP68/72 regulate export, ribosome transfer, and quality control, even though their structural role in RNA remodeling and ribosome engagement is increasingly clear. | (issa2024thenucleolarphase pages 1-2, issa2024thenucleolarphase pages 10-13, kellogg2023unravelingsrpbiogenesis pages 23-26, kellogg2023unravelingsrpbiogenesisa pages 23-26) |
Table: This table summarizes the main functional annotations, mechanistic evidence, quantitative data, and disease relevance for human SRP68 (UniProt Q9UHB9). It is useful as a compact evidence map linking SRP68 biology to the strongest available primary and review sources.
References
(wild2019reconstitutionofthe pages 1-2): Klemens Wild, Keven D Juaire, Komal Soni, Vivekanandan Shanmuganathan, Astrid Hendricks, Bernd Segnitz, Roland Beckmann, and Irmgard Sinning. Reconstitution of the human srp system and quantitative and systematic analysis of its ribosome interactions. Nucleic Acids Research, 47:3184-3196, Jan 2019. URL: https://doi.org/10.1093/nar/gky1324, doi:10.1093/nar/gky1324. This article has 43 citations and is from a highest quality peer-reviewed journal.
(kellogg2022signalrecognitionparticle pages 1-2): Morgana K. Kellogg, Elena B. Tikhonova, and Andrey L. Karamyshev. Signal recognition particle in human diseases. Frontiers in Genetics, Jun 2022. URL: https://doi.org/10.3389/fgene.2022.898083, doi:10.3389/fgene.2022.898083. This article has 30 citations and is from a peer-reviewed journal.
(kellogg2023unravelingsrpbiogenesis pages 26-29): MK Kellogg. Unraveling srp biogenesis and quality control: implications for human disease pathogenesis. Unknown journal, 2023.
(issa2024thenucleolarphase pages 1-2): Amani Issa, Florence Schlotter, Justine Flayac, Jing Chen, Ludivine Wacheul, Manon Philippe, Lucas Sardini, Lalia Mostefa, Franck Vandermoere, Edouard Bertrand, Céline Verheggen, Denis LJ Lafontaine, and Séverine Massenet. The nucleolar phase of signal recognition particle assembly. Life Science Alliance, 7:e202402614, Jun 2024. URL: https://doi.org/10.26508/lsa.202402614, doi:10.26508/lsa.202402614. This article has 8 citations and is from a peer-reviewed journal.
(schmaltzpanneau2021identificationofbiallelic pages 6-10): Barbara Schmaltz-Panneau, Anne Pagnier, Séverine Clauin, Julien Buratti, Caroline Marty, Odile Fenneteau, Klaus Dieterich, Blandine Beaupain, Jean Donadieu, Isabelle Plo, and Christine Bellanné-Chantelot. Identification of biallelic germline variants of srp68 in a sporadic case with severe congenital neutropenia. Haematologica, 106:1216-1219, Apr 2021. URL: https://doi.org/10.3324/haematol.2020.247825, doi:10.3324/haematol.2020.247825. This article has 15 citations.
(kellogg2023unravelingsrpbiogenesisa pages 26-29): MK Kellogg. Unraveling srp biogenesis and quality control: implications for human disease pathogenesis. Unknown journal, 2023.
(kellogg2023unravelingsrpbiogenesis pages 23-26): MK Kellogg. Unraveling srp biogenesis and quality control: implications for human disease pathogenesis. Unknown journal, 2023.
(wild2019reconstitutionofthe pages 5-5): Klemens Wild, Keven D Juaire, Komal Soni, Vivekanandan Shanmuganathan, Astrid Hendricks, Bernd Segnitz, Roland Beckmann, and Irmgard Sinning. Reconstitution of the human srp system and quantitative and systematic analysis of its ribosome interactions. Nucleic Acids Research, 47:3184-3196, Jan 2019. URL: https://doi.org/10.1093/nar/gky1324, doi:10.1093/nar/gky1324. This article has 43 citations and is from a highest quality peer-reviewed journal.
(issa2024thenucleolarphase pages 10-13): Amani Issa, Florence Schlotter, Justine Flayac, Jing Chen, Ludivine Wacheul, Manon Philippe, Lucas Sardini, Lalia Mostefa, Franck Vandermoere, Edouard Bertrand, Céline Verheggen, Denis LJ Lafontaine, and Séverine Massenet. The nucleolar phase of signal recognition particle assembly. Life Science Alliance, 7:e202402614, Jun 2024. URL: https://doi.org/10.26508/lsa.202402614, doi:10.26508/lsa.202402614. This article has 8 citations and is from a peer-reviewed journal.
(schmaltzpanneau2021identificationofbiallelic pages 1-6): Barbara Schmaltz-Panneau, Anne Pagnier, Séverine Clauin, Julien Buratti, Caroline Marty, Odile Fenneteau, Klaus Dieterich, Blandine Beaupain, Jean Donadieu, Isabelle Plo, and Christine Bellanné-Chantelot. Identification of biallelic germline variants of srp68 in a sporadic case with severe congenital neutropenia. Haematologica, 106:1216-1219, Apr 2021. URL: https://doi.org/10.3324/haematol.2020.247825, doi:10.3324/haematol.2020.247825. This article has 15 citations.
(kellogg2023unravelingsrpbiogenesis pages 46-51): MK Kellogg. Unraveling srp biogenesis and quality control: implications for human disease pathogenesis. Unknown journal, 2023.
(wild2019reconstitutionofthe media 8320e838): Klemens Wild, Keven D Juaire, Komal Soni, Vivekanandan Shanmuganathan, Astrid Hendricks, Bernd Segnitz, Roland Beckmann, and Irmgard Sinning. Reconstitution of the human srp system and quantitative and systematic analysis of its ribosome interactions. Nucleic Acids Research, 47:3184-3196, Jan 2019. URL: https://doi.org/10.1093/nar/gky1324, doi:10.1093/nar/gky1324. This article has 43 citations and is from a highest quality peer-reviewed journal.
(wild2019reconstitutionofthe media b811f4ad): Klemens Wild, Keven D Juaire, Komal Soni, Vivekanandan Shanmuganathan, Astrid Hendricks, Bernd Segnitz, Roland Beckmann, and Irmgard Sinning. Reconstitution of the human srp system and quantitative and systematic analysis of its ribosome interactions. Nucleic Acids Research, 47:3184-3196, Jan 2019. URL: https://doi.org/10.1093/nar/gky1324, doi:10.1093/nar/gky1324. This article has 43 citations and is from a highest quality peer-reviewed journal.
(wild2019reconstitutionofthe media eee3cd7f): Klemens Wild, Keven D Juaire, Komal Soni, Vivekanandan Shanmuganathan, Astrid Hendricks, Bernd Segnitz, Roland Beckmann, and Irmgard Sinning. Reconstitution of the human srp system and quantitative and systematic analysis of its ribosome interactions. Nucleic Acids Research, 47:3184-3196, Jan 2019. URL: https://doi.org/10.1093/nar/gky1324, doi:10.1093/nar/gky1324. This article has 43 citations and is from a highest quality peer-reviewed journal.
(kellogg2023unravelingsrpbiogenesisa pages 23-26): MK Kellogg. Unraveling srp biogenesis and quality control: implications for human disease pathogenesis. Unknown journal, 2023.