SPIRE1

SPIRE1 is a WH2-domain actin nucleator that assembles actin filaments at organelle membranes to power membrane trafficking, organelle dynamics, and cytoskeletal organization across diverse cellular contexts (PMID:21983562, PMID:32661310). Its central biochemical activity is cooperative actin nucleation with formin family members: the SPIRE KIND domain adopts a protein kinase fold whose acidic groove electrostatically engages the positively charged FSI motif of Formin-2, an interaction that couples and reciprocally tunes the nucleation activities of the two proteins (PMID:21705804). SPIRE1 acts as a Rab27a effector recruited to melanosome membranes, where it builds actin tracks together with formin-1 that support myosin-Va-dependent long-range melanosome transport (PMID:32661310); the same SPIRE–formin–myosin logic organizes vesicle-connecting actin networks for directional, microtubule-independent transport and drives asymmetric meiotic spindle positioning and polar body extrusion in oocytes (PMID:21620703, PMID:21983562). SPIRE1 nucleates actin patches on early endosomes downstream of Annexin A2 to control endosome biogenesis and ATG9A trafficking into autophagosomes (PMID:19289089, PMID:26289944), organizes actin rings at Weibel-Palade body fusion sites to promote VWF secretion (PMID:35084586), and through its mitochondria-targeted Spire1C splice variant cooperates with the ER-anchored formin INF2 at ER–mitochondria contacts to drive mitochondrial fission (PMID:36744380). Beyond its trafficking roles, SPIRE1 functions as a virus restriction factor that enhances RIG-I/MDA-5-dependent IRF3 and NF-κB signalling, an activity directly antagonized by the vaccinia virus K7 protein binding to a SPIRE1 diphenylalanine motif (PMID:35148361).

1. 2001 Medium

   Established SPIRE1 as a membrane-associated regulator of vesicle transport, defining where it acts before its actin-nucleating function was understood.

   Evidence Co-localization of GFP-Rab GTPases with Spir-1, motif-disruption mutants, and a VSV-G transport assay in NIH 3T3 cells

   PMID:11747823

   Open questions at the time

   • Does not establish actin nucleation as the molecular mechanism
   • FYVE and Spir-box contributions to membrane targeting not separated functionally
2. 2009 Medium

   Placed SPIRE1 in an Annexin A2 endosomal pathway, showing it nucleates actin patches that drive early-to-late endosome transport alongside Arp2/3.

   Evidence siRNA knockdown, co-localization imaging, and endosome transport assays

   PMID:19289089

   Open questions at the time

   • Relationship between SPIRE1 nucleation and Arp2/3 nucleation not resolved
   • Direct interaction with Annexin A2 not shown
3. 2011 High

   Defined the core SPIRE–formin actin machine in vivo and structurally, showing SPIRE1/2 cooperate with Fmn2 to build actin networks for spindle positioning, polar body extrusion, and myosin-Vb-dependent vesicle transport.

   Evidence Oocyte depletion with live imaging and Fmn2 epistasis; vesicle actin-track imaging with myosin-Vb inhibition; KIND–FSI crystal structure with interface mutagenesis

   PMID:21620703 PMID:21705804 PMID:21983562

   Open questions at the time

   • Mechanism of SPIRE/formin recruitment to specific membranes not defined
   • How nucleation activity is switched on at the organelle surface unclear
4. 2013 Medium

   Extended SPIRE1's actin-nucleating role to disease-relevant contexts, linking it to invadosome-driven matrix degradation and to neuronal circuits governing fear behavior.

   Evidence Co-IP and co-localization with Src/mDia1/Rab3A plus matrix degradation assays; gene-trap mouse with fear conditioning and dendritic spine analysis

   PMID:24213528 PMID:24345451

   Open questions at the time

   • Invadosome partner interactions are Co-IP based without reciprocal validation
   • Molecular basis of the neuronal phenotype not mapped to actin nucleation
5. 2015 Medium

   Connected SPIRE1 actin function to autophagy by showing it routes ATG9A from endosomes to autophagosomes downstream of Annexin A2.

   Evidence siRNA knockdown, ATG9A trafficking and autophagosome formation assays, epistasis with Annexin A2 and ARP2

   PMID:26289944

   Open questions at the time

   • Direct actin requirement at ATG9A vesicles not visualized
   • Selectivity for starvation-induced versus basal autophagy unclear
6. 2016 Medium

   Identified SPIRE1 (distinct from SPIRE2) as a non-redundant host factor for Salmonella invasion at the bacterial binding step.

   Evidence Genome-wide siRNA screen, gentamicin protection assays, and stage-specific analysis in SPIRE1/SPIRE2 knockout fibroblasts

   PMID:27627128

   Open questions at the time

   • Mechanistic detail at the binding step limited
   • Whether actin nucleation underlies the binding defect not established
7. 2020 High

   Resolved how SPIRE1 is targeted to organelles, identifying it as a Rab27a effector that builds formin-1-dependent actin tracks for myosin-Va melanosome transport.

   Evidence Reciprocal Co-IP, organelle fractionation, live imaging, and knockdown/knockout of SPIRE1 and Rab27a in melanocytes

   PMID:32661310

   Open questions at the time

   • Direct structural basis of Rab27a–SPIRE1 binding not defined
   • Generality of Rab-effector recruitment to other organelles not tested here
8. 2021 Medium

   Revealed post-transcriptional control of SPIRE1, showing lnc-SMaRT and DHX36 repress its translation via an mRNA G-quadruplex to time skeletal muscle differentiation.

   Evidence RNA immunoprecipitation, translation reporter assays, G-quadruplex mutagenesis, and SPIRE1 modulation during myoblast differentiation

   PMID:34863993

   Open questions at the time

   • Actin-dependent mechanism in muscle differentiation not defined
   • Physiological trigger for translational repression unclear
9. 2022 High

   Broadened SPIRE1's roles to regulated secretion and innate antiviral immunity, organizing actin rings for VWF release and enhancing RIG-I/MDA-5 signalling targeted by vaccinia K7.

   Evidence WPB co-localization with knockdown and VWF ELISA; SPIRE1 KO cells, K7 direct binding with diphenylalanine motif mutagenesis, IRF3/NF-κB reporters, and viral titres

   PMID:35084586 PMID:35148361

   Open questions at the time

   • Whether antiviral signalling depends on SPIRE1 actin nucleation not resolved
   • Mechanism linking the diphenylalanine motif to IRF3 activation unclear
10. 2023 High

    Established a mitochondria-specific SPIRE1 isoform function, showing Spire1C polymerizes actin with INF2 at ER–mitochondria contacts to enable Myo19-driven fission.

    Evidence Spire1C depletion, super-resolution imaging, split-luciferase ER–mitochondria contact assay, and epistasis with INF2 and Myo19

    PMID:36744380

    Open questions at the time

    • How the Spire1C splice variant is targeted to mitochondria not defined
    • Coordination of fission with the broader fission machinery (Drp1) not addressed
11. 2023 Low

    Provided a first link between MEK/PKA signalling and SPIRE1, indicating its cortical localization in oocytes is regulated post-translationally during maturation.

    Evidence Pharmacological MEK and PKA inhibition during in vitro oocyte maturation with immunofluorescence and western blot

    PMID:38158690

    Open questions at the time

    • No direct phosphorylation site mapping or kinase identification
    • Inhibitor-based evidence only, no genetic confirmation

• How SPIRE1's single core actin-nucleation activity is selectively deployed and regulated across its many membrane and signalling contexts remains unresolved.
• No unified model of how Rab/effector recruitment, splice variants, and post-translational signals direct SPIRE1 to specific organelles
• Whether antiviral signalling and bacterial-invasion roles require actin nucleation is untested
• Direct structural basis of organelle-targeting interactions beyond KIND–FSI not defined