SPM1

SPM1 is a microtubule inner protein (MIP) of Toxoplasma gondii that stabilizes the subpellicular cortical microtubule array (PMID:22021240, PMID:34031406). It localizes along the full length of the subpellicular microtubules but is excluded from conoid and spindle microtubules, and its association with microtubules requires four or more copies of its 32-amino-acid repeat (PMID:22021240). Cryo-EM of ex vivo cortical microtubules places SPM1 on the luminal surface, where together with TrxL1 and TrxL2 it forms a mesh that stabilizes the tubulin lattice in both longitudinal and lateral directions; loss of SPM1 abolishes microtubule resistance to detergent and chemical challenge and reduces parasite fitness (PMID:22021240, PMID:34031406). Within this complex SPM1 acts as the scaffold that recruits TrxL1, which does not bind microtubules directly (PMID:23873863). SPM1 is subject to post-translational control: it is deubiquitinated by TgJosephin, and its ubiquitination state governs parasite anti-IFN-γ virulence, as mutating the SPM1 ubiquitination sites restores virulence in TgJosephin-deficient parasites (PMID:41972766).

1. 2011 High

   Established SPM1 as a dedicated stabilizer of the subpellicular cortical microtubules and defined the repeat requirement for its microtubule association, distinguishing it from conoid and spindle microtubule factors.

   Evidence Gene deletion with detergent extraction phenotype, domain-deletion mutagenesis and fluorescence localization in Toxoplasma gondii

   PMID:22021240

   Open questions at the time

   • Molecular basis of lattice stabilization not resolved at the structural level
   • How the 32-aa repeats engage tubulin not defined
2. 2013 Medium

   Placed SPM1 within a cortical microtubule-coating complex and identified it as the scaffold that recruits TrxL1, which cannot bind microtubules on its own.

   Evidence Co-immunoprecipitation and fluorescence localization in Toxoplasma gondii

   PMID:23873863

   Open questions at the time

   • Interaction shown by Co-IP without structural mapping of the binding interface
   • Stoichiometry of the complex not determined
3. 2021 High

   Defined SPM1 structurally as a bona fide microtubule inner protein that, with TrxL1 and TrxL2, forms a luminal mesh stabilizing the tubulin lattice both longitudinally and laterally.

   Evidence Single-particle cryo-EM of ex vivo cortical microtubules plus gene deletion with chemical challenge in Toxoplasma gondii

   PMID:34031406

   Open questions at the time

   • Individual contribution of SPM1 versus the other MIPs to lattice stability not separated
   • In-cell assembly order of the mesh unknown
4. 2023 Low

   Proposed phospho-regulation of SPM1 by the phosphatase PPKL as an input controlling cortical microtubule rigidity and arrangement.

   Evidence Conditional PPKL knockdown with global phosphoproteomics (preprint) in Toxoplasma gondii

   PMID:37398039

   Open questions at the time

   • Correlative phosphoproteomics; not confirmed by SPM1 phosphosite mutagenesis
   • Whether SPM1 phosphorylation causally drives the microtubule phenotype is inferred, not demonstrated
5. 2026 Medium

   Established a ubiquitination-based regulatory axis on SPM1, showing that TgJosephin-mediated deubiquitination and SPM1 ubiquitination status control parasite anti-IFN-γ virulence.

   Evidence TgJosephin gene deletion, SPM1 ubiquitination-site mutagenesis and mouse virulence assays in IFNγR-knockout mice

   PMID:41972766

   Open questions at the time

   • Mechanism linking SPM1 ubiquitination to virulence not defined
   • Whether ubiquitination alters SPM1 microtubule association or stability not shown
   • Single lab

• How the distinct post-translational inputs (phosphorylation by PPKL, ubiquitination reversed by TgJosephin) integrate to tune SPM1's microtubule-stabilizing function and its role in virulence remains unresolved.
• No direct link established between phospho- and ubiquitin-regulation of SPM1
• Mechanistic coupling of lattice stabilization to virulence not defined