SFN

SFN (14-3-3σ/Stratifin) is a p53-inducible scaffold protein that enforces cell-cycle checkpoints and modulates cytoskeletal signaling by sequestering phosphorylated client proteins in the cytoplasm. Upon DNA damage, p53-driven transcription upregulates SFN, which sequesters the cyclin B1–Cdc2 complex in the cytoplasm to prevent mitotic entry; loss of both SFN alleles causes mitotic catastrophe, establishing it as essential for the G2/M DNA-damage checkpoint (PMID:9659898, PMID:10524633). SFN also binds and inhibits cyclin–CDK2 complexes to restrain S-phase entry and proliferation, and is itself targeted for proteasomal degradation by the RING-finger E3 ligase Efp, linking its turnover to estrogen-receptor signaling in breast cancer (PMID:10767298, PMID:12075357). Beyond cell-cycle control, SFN engages a broad phospho-dependent interactome that includes cytoskeletal regulators such as AKAP-Lbc (suppressing Rho activation), LIMK2/Cofilin (promoting actin remodeling), and MYH9 (promoting renal fibrosis), positioning it as a versatile adaptor coupling mitogenic and cytoskeletal pathways (PMID:15324660, PMID:37946061, PMID:38986830).

1. 1997 High

   Identifying SFN as a p53-responsive gene that arrests cells at G2/M answered how the p53 checkpoint reaches the mitotic machinery and established a direct transcriptional link between DNA damage and cell-cycle inhibition.

   Evidence Quantitative expression profiling in irradiated colorectal cancer cells, reporter assays mapping a p53-responsive element, overexpression causing G2 arrest

   PMID:9659898

   Open questions at the time

   • Direct binding partner through which G2 arrest is achieved was not identified
   • Relevance outside colorectal cancer cells untested
2. 1999 High

   Somatic knockout revealed that SFN is essential for G2/M checkpoint maintenance by sequestering cyclin B1–Cdc2 in the cytoplasm, resolving the molecular mechanism of the arrest first observed in 1997 and showing that loss leads to mitotic catastrophe.

   Evidence Both-allele knockout in HCT116 cells, subcellular fractionation of cyclin B1 and Cdc2, cell-cycle and viability analysis

   PMID:10524633

   Open questions at the time

   • Phospho-dependent binding site on Cdc2 or cyclin B1 not mapped
   • Whether other 14-3-3 isoforms compensate partially was not resolved
3. 2000 High

   Demonstration that SFN also binds and inhibits cyclin–CDK2 complexes extended its checkpoint role from G2/M into G1/S control and showed it can suppress anchorage-independent growth, broadening its tumor-suppressive function.

   Evidence Co-immunoprecipitation of SFN with cyclin–CDK2 in vitro and in vivo, CDK kinase assays, soft-agar colony formation in breast cancer lines

   PMID:10767298

   Open questions at the time

   • Specific phosphosite(s) mediating the SFN–cyclin-CDK2 interaction not mapped
   • Relative contribution of CDK2 inhibition versus Cdc2 sequestration to growth arrest unclear
4. 2002 High

   Identification of the RING-finger E3 ligase Efp as the enzyme targeting SFN for proteasomal degradation explained how SFN protein levels are kept low in proliferating cells and connected SFN turnover to estrogen-receptor signaling in breast cancer.

   Evidence In vitro ubiquitin ligase assay, Efp knockout MEFs, antisense knockdown, xenograft model

   PMID:12075357

   Open questions at the time

   • Ubiquitylation site(s) on SFN not determined
   • Whether Efp-mediated degradation is p53-dependent or independent not tested
5. 2004 High

   Proteomic cataloguing of ~200 14-3-3-associated proteins revealed a large cytoskeletal-regulatory clientele and defined a specific mechanism — phospho-dependent binding to AKAP-Lbc suppressing Rho activation — establishing SFN-family members as general adaptors linking kinase signaling to cytoskeletal remodeling.

   Evidence Mass spectrometry interactome in HEK293 cells, hierarchical clustering, PKA-dependent phosphosite validation, in vivo Rho activation assay

   PMID:15324660

   Open questions at the time

   • Contribution of the sigma isoform versus other 14-3-3 paralogs was not distinguished
   • Functional consequences for cell morphology not quantified at single-cell level
6. 2012 Medium

   Discovery of the SFN–SPARC complex in keratinocyte-conditioned media and its regulation of type I collagen synthesis in fibroblasts extended SFN function to extracellular matrix homeostasis and implicated it in dermal fibrosis.

   Evidence Co-immunoprecipitation from conditioned media, MS identification, 3D modeling, immunoblot in normal vs. fibrotic tissue

   PMID:22422640

   Open questions at the time

   • Binding interface and stoichiometry of SFN–SPARC complex undefined
   • Whether SFN acts intracellularly before secretion or extracellularly not resolved
   • Single-lab observation not independently replicated
7. 2023 Medium

   Placing SFN upstream of LIMK2/Cofilin phosphorylation in cervical cancer cells provided a defined signaling axis through which SFN promotes actin remodeling, invasion, and migration, revealing a pro-tumorigenic role distinct from its classical tumor-suppressive checkpoint function.

   Evidence SFN overexpression and silencing, transwell/scratch assays, actin-tracker staining, Western blot for LIMK2/p-LIMK2/Cofilin/p-Cofilin

   PMID:37946061

   Open questions at the time

   • Whether SFN directly binds LIMK2 or acts via an intermediary is unknown
   • Applicability beyond cervical cancer cells not tested
   • Single-lab study
8. 2024 Medium

   Identification of MYH9 as a cytoplasmic binding partner of SFN in renal tubular cells, with SFN knockdown alleviating fibrosis in a UUO model, provided in vivo evidence that the SFN–MYH9 axis drives organ fibrosis beyond the skin.

   Evidence LC-MS/MS interactome, co-IP, immunofluorescence co-localization, siRNA knockdown, UUO mouse model

   PMID:38986830

   Open questions at the time

   • Mechanism by which SFN binding maintains MYH9 expression/stability not defined
   • Whether the pro-fibrotic role involves canonical 14-3-3 phospho-binding unknown
   • Single-lab study

• A unifying structural and biophysical model explaining how SFN selectively engages its diverse clients — checkpoint kinases, cytoskeletal regulators, and extracellular matrix modulators — and how isoform-specific features distinguish it from other 14-3-3 paralogs remains to be established.
• No crystal structure of SFN bound to a client phosphopeptide has been reported in this timeline
• Isoform-specific residues conferring client selectivity are unmapped
• Relative contribution of SFN's tumor-suppressive vs. pro-fibrotic/pro-invasive functions in vivo is unresolved