SFI1

SFI1 is a conserved centrin-binding scaffold that builds and partitions the cellular structures that organize spindle poles and centrioles, acting through arrays of internal tryptophan-anchored repeats that bind centrin (PMID:25031431, PMID:26779587). In budding yeast Sfi1 is an essential gene required for mitotic spindle assembly and G2-M progression (PMID:10455233), where it templates spindle pole body (SPB) duplication: its N-terminus, together with the first Cdc31 (centrin) binding sites, directly recruits the SPB components Spc29 and Spc42 to nucleate daughter SPB assembly at the distal end of the bridge (PMID:33523111), while its C-terminal region, anchored to the nuclear envelope by Kar1, governs bridge splitting and timely SPB separation (PMID:26076691, PMID:17392514). SPB duplication is licensed once per cycle by Cdk1 phosphorylation of Sfi1, which is opposed by Cdc14 dephosphorylation; phosphorylation of the N-terminus and of centrin (Cdc31 S15) is inhibitory and times daughter SPB biogenesis and array disassembly to the cell cycle (PMID:33523111, PMID:25340401, PMID:25736294). The structural Sfi1–centrin interaction is mediated by repeat tryptophans whose mutation mispartitions Sfi1 between daughter poles and abolishes assembly of the deficient pole (PMID:25031431), and is disrupted by CK2 phosphorylation of centrin (PMID:24918055). In human cells SFI1 localizes to the distal centriole lumen as part of a ring network with centrin-2, C2CD3, CEP135 and NA14 that scaffolds centriolar architecture [PMID:36125182, PMID:bio_10.1101_2025.06.17.660204]; there it is required not for centriole duplication itself but for distal Centrin recruitment, CEP164 distribution, CP110 removal and ciliogenesis (PMID:36125182), while during S phase it recruits the deubiquitylase USP9X to the centrosome to stabilize the duplication factor STIL (PMID:31197030). A divergent role is seen in the protist Spirostomum, where a reconstituted centrin–Sfi1 repeat complex undergoes calcium-dependent compaction underlying ATP-independent myoneme contractility (PMID:42213749).

1. 1999 Medium

   Established that SFI1 is an essential gene whose loss blocks mitosis, framing it as a core component of spindle/pole biogenesis rather than a dispensable factor.

   Evidence Conditional galactose-regulated yeast allele with nuclear staining and microscopy showing G2 arrest and absent spindle

   PMID:10455233

   Open questions at the time

   • Molecular function and binding partners undefined
   • No distinction between roles in duplication versus separation
2. 2007 Medium

   Separated SFI1's duplication-initiation role from a distinct requirement in SPB bridge splitting, showing the C-terminal region governs separation of duplicated poles.

   Evidence mad1 synthetic-lethal genetic screen with C-terminal alleles plus light and electron microscopy showing unseparated duplicated SPBs

   PMID:17392514

   Open questions at the time

   • Molecular partner anchoring the C-terminus not identified
   • Regulation of the splitting step unknown
3. 2014 High

   Defined the cell-cycle licensing logic of SPB duplication by showing Cdk1 phosphorylation of Sfi1 restricts duplication to once per cycle, opposed by Cdc14.

   Evidence Non-phosphorylatable phosphosite mutagenesis with yeast genetics and epistasis to Cdc14

   PMID:25340401

   Open questions at the time

   • Direct demonstration of Cdc14 dephosphorylating Sfi1 not shown
   • Which phosphosites control which step not fully resolved
4. 2014 Medium

   Showed that conserved repeat tryptophans control equal partitioning of Sfi1 to daughter SPBs, linking repeat architecture to balanced pole inheritance.

   Evidence Live imaging and Trp-to-Arg mutagenesis (sfi1-M46) with light/electron microscopy in fission yeast

   PMID:25031431

   Open questions at the time

   • Mechanism of symmetric partitioning unresolved
   • Single-organism evidence
5. 2014 Medium

   Identified CK2 phosphorylation of centrin as a switch that abolishes centrin–Sfi1 binding, revealing a second kinase input on the complex.

   Evidence In vitro CK2 kinase assay, ITC binding, and phosphomimetic centrin mutagenesis

   PMID:24918055

   Open questions at the time

   • In vivo relevance of CK2 regulation not established
   • Cellular context of phosphorylation unknown
6. 2015 High

   Mapped how Sfi1 is anchored and stabilized within the SPB bridge, showing Kar1 tethers the Sfi1 C-terminus to the nuclear envelope to ensure timely SPB separation.

   Evidence PALM super-resolution localization, direct binding assays, and kar1Δ CDC31-16 suppressor genetics in budding yeast

   PMID:26076691

   Open questions at the time

   • No mammalian counterpart to Kar1 tethering established
   • Structure of the assembled bridge not resolved
7. 2015 Medium

   Demonstrated that centrin phosphorylation regulates Sfi1–centrin array stability, with Cdc31 S15 phosphorylation driving Sfi1 dissociation at mitotic onset.

   Evidence Cdc31 phosphosite mutagenesis with fluorescence microscopy and cell-cycle analysis in fission yeast

   PMID:25736294

   Open questions at the time

   • Single method set from one lab
   • Kinase responsible in vivo not pinpointed
8. 2016 Medium

   Defined the structural basis of the centrin–Sfi1 repeat interaction and showed human centrin lacks the centrin–centrin contacts seen in yeast Cdc31.

   Evidence Site-directed mutagenesis of Sfi1 repeat tryptophans and centrin residues with thermal denaturation/CD and complex purification

   PMID:26779587

   Open questions at the time

   • High-resolution structure of full complex not determined
   • In vitro stability not linked to in-cell function
9. 2019 High

   Connected human SFI1 to centriole duplication through a recruitment axis, showing it brings USP9X to the centrosome to deubiquitylate and stabilize STIL.

   Evidence Reciprocal Co-IP, centrosome localization, in vitro deubiquitylation assay, and USP9X patient cell analysis with siRNA

   PMID:31197030

   Open questions at the time

   • Direct SFI1–USP9X interaction interface not mapped
   • Whether SFI1 itself is a USP9X substrate unclear
10. 2022 High

    Placed human SFI1 at the distal centriole with a dedicated Centrin pool and showed its function is in centriolar architecture and ciliogenesis rather than duplication.

    Evidence Expansion microscopy, siRNA depletion, immunofluorescence, and functional ciliogenesis assays

    PMID:36125182

    Open questions at the time

    • How SFI1 controls CP110 removal and CEP164 distribution mechanistically unresolved
    • Relationship between distal pool and the USP9X/STIL role unclear
11. 2025 Medium

    Embedded human SFI1 in a defined distal-centriole luminal ring network, clarifying its scaffolding partners and dependence on C2CD3.

    Evidence U-ExM, iterative U-ExM, cryo-electron tomography, siRNA depletion and Co-IP (preprint)

    PMID:bio_10.1101_2025.06.17.660204

    Open questions at the time

    • Preprint; SFI1-specific data limited within larger study
    • Stoichiometry and assembly order of the ring not defined
12. 2026 Medium

    Revealed an actomyosin-independent contractile function for a centrin–Sfi1 complex, broadening the protein's mechanistic repertoire beyond pole/centriole scaffolding.

    Evidence Immunofluorescence, immunogold EM, and in vitro reconstitution with calcium-dependent compaction assay in Spirostomum

    PMID:42213749

    Open questions at the time

    • Relevance of contractile mechanism to mammalian SFI1 unknown
    • Structural basis of calcium-driven compaction not resolved

• How the mammalian distal-centriole scaffolding role, the USP9X–STIL duplication axis, and the cell-cycle phospho-licensing logic established in yeast are integrated into a single regulatory model remains open.
• No structure of the human luminal ring complex
• Cdk1/Cdc14 licensing not demonstrated for mammalian SFI1
• Mechanistic link between distal Centrin pool and ciliogenesis effectors unresolved