| 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. | (pqac-00000006, pqac-00000005, pqac-00000000) |
| 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. | (pqac-00000004, pqac-00000006, pqac-00000007) |
| 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. | (pqac-00000003, pqac-00000007, pqac-00000008, pqac-00000009) |
| 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. | (pqac-00000000, pqac-00000005, pqac-00000006, pqac-00000004) |
| 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. | (pqac-00000012, pqac-00000018) |
| 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. | (pqac-00000017, pqac-00000014, pqac-00000016) |
| 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. | (pqac-00000017, pqac-00000014) |
| 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. | (pqac-00000008, pqac-00000009, pqac-00000010, pqac-00000013) |


*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.*