Falcon (Edison Scientific) deep research report: Functional annotation of spt16
(UniProt O94267), the FACT complex large subunit, in Schizosaccharomyces pombe.
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Synthesis confirms Spt16 (O94267) is the essential large subunit of the FACT
histone chaperone complex (Spt16/Pob3 heterodimer plus the HMGB protein Nhp6);
spt16+ is essential whereas pob3+ and nhp6+ are nonessential.
"The target protein **Spt16** in *S. pombe* is the large, essential subunit of the conserved **FACT (FAcilitates Chromatin Transactions)** histone-chaperone complex."
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FACT/Spt16 is an ATP-independent histone chaperone that reorganizes nucleosomes
(notably H2A-H2B dimer handling) to promote transcription and other chromatin
transactions while preserving chromatin integrity, rather than acting as a
peptidase enzyme.
"now broadly understood as an **ATP-independent histone chaperone** that promotes transcription and other chromatin transactions by reorganizing nucleosomes while preserving chromatin integrity."
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Although classified in the peptidase M24 family, the Spt16 N-terminal region is a
peptidase-like fold repurposed for protein/histone interactions, not a catalytic
protease, consistent with the UniProt CAUTION that it lacks conserved active-site
residues.
"the relevant fission-yeast literature describes this region primarily as a **peptidase-like fold used for protein/histone interactions**, not as a characterized protease that catalyzes peptide hydrolysis in vivo."
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In S. pombe heterochromatin, the Spt16 peptidase-like domain directly binds the
dimerized chromo-shadow domain of Swi6/HP1 (RKDD-loop dependent), providing a
molecular route for FACT recruitment to heterochromatin.
"The **peptidase-like domain** of Spt16 **directly binds** the **dimerized chromo-shadow domain (CSD)** of **Swi6** (but not the related HP1-family protein Chp2), establishing a molecular recruitment route for FACT to heterochromatin."
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At heterochromatin FACT suppresses histone turnover and supports establishment and
maintenance of silenced chromatin; in pob3-delta, Spt16 occupancy at heterochromatin
falls to ~50% of wild type, indicating partial Pob3 dependence plus a Swi6-dependent
recruitment route.
"In **pob3Δ** strains, **Spt16 binding to heterochromatin is reduced to ~50% of wild type**, indicating that Pob3 supports—but is not strictly required for—Spt16 recruitment, consistent with additional Swi6-dependent recruitment routes."
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Recent 2024 work integrates FACT/Spt16 into replication-coupled parental histone
recycling at the replisome, with Mcl1 (Ctf4 ortholog) required to connect the
replisome to FACT.
"**Mcl1** (Ctf4 ortholog) is required to connect the replisome to FACT, implicating FACT as a histone-capture/redeposition factor at replication forks."
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Spt16 acts on nuclear chromatin in both transcribed euchromatin and H3K9-methylated
heterochromatin, and during S phase is linked to replication forks; no evidence for
extracellular or membrane localization.
"Spt16 acts on **nuclear chromatin** in both **transcribed euchromatin** and **H3K9-methylated heterochromatin**, and during S phase it is also linked to **replication forks/replisome-enriched heterochromatin domains**."