SPM1

SPM1 (PF3D7_0909500) is a subpellicular microtubule-associated protein of Plasmodium that contributes to the structural reinforcement of the subpellicular microtubule (SPMT) cytoskeleton (PMID:36625655, PMID:36869034). In P. falciparum gametocytes, overexpressed PfSPM1 colocalizes with the microtubule marker TubulinTracker and with the SPMT protein PfSPM3 along the SPMTs, distributing more basally than PfSPM3 (PMID:36625655), and it is recovered as a high-confidence component of the PfSPM3 proxiome by proximity biotinylation, confirming its association with the SPMT network (PMID:41358875). In situ cryo-electron tomography of P. falciparum sporozoites showed that SPMTs are reinforced by an interrupted luminal helix built from TrxL1- and SPM1-like proteins; this luminal structure is present in the motile mosquito stages (ookinetes and sporozoites) but absent from merozoites and gametocytes, and SPM1-GFP is highly expressed in sporozoites (PMID:36869034). In the rodent parasite P. berghei, spm1 is a direct transcriptional target of the ookinete transcription factor AP2-O (PMID:26018192), and its deletion causes a partial (~57%) reduction in salivary gland sporozoite counts (PMID:37708854) but is otherwise dispensable for transmission through the mosquito (PMID:39908102). Beyond its role as a luminal SPMT-reinforcing protein, the precise molecular contribution of SPM1 to microtubule mechanics has not been resolved in the available corpus.

1. 2015 Medium

   Establishing that spm1 is a defined transcriptional target of AP2-O placed SPM1 within the ookinete-stage gene expression program before its protein function was known.

   Evidence ChIP-seq mapping AP2-O binding to the spm1 upstream region in P. berghei ookinetes

   PMID:26018192

   Open questions at the time

   • Does not demonstrate that AP2-O binding drives functional SPM1 expression
   • No protein-level or localization data from this study
   • Regulation in other stages or species not addressed
2. 2023 Medium

   Colocalization of PfSPM1 with tubulin and PfSPM3 first assigned SPM1 to the subpellicular microtubule cytoskeleton, defining it as an SPMT-associated protein.

   Evidence Fluorescence colocalization of overexpressed mCherry-PfSPM1 with TubulinTracker and PfSPM3-GFP in P. falciparum gametocytes

   PMID:36625655

   Open questions at the time

   • Relied on overexpressed tagged protein with cytosolic background
   • Does not resolve whether SPM1 binds microtubules directly or via other MAPs
   • Functional consequence of the SPMT localization untested
3. 2023 High

   Cryo-ET revealed the structural role of SPM1 as part of an interrupted luminal helix reinforcing SPMTs specifically in motile mosquito stages, explaining a stage-specific microtubule specialization.

   Evidence In situ cryo-electron tomography with subvolume averaging and SPM1-GFP imaging in P. falciparum sporozoites

   PMID:36869034

   Open questions at the time

   • Exact stoichiometry and periodicity of SPM1 within the luminal helix not fully defined
   • Whether SPM1 loss abolishes the luminal helix not directly tested
   • Contribution of SPM1 versus TrxL1 to helix assembly unresolved
4. 2023 Medium

   Genetic disruption in P. berghei tested whether SPM1 is functionally required, showing a partial defect in salivary gland sporozoite colonization rather than an essential role.

   Evidence Clonal gene knockout and salivary gland sporozoite counts plus barcode sequencing screen in P. berghei

   PMID:37708854

   Open questions at the time

   • Mechanism linking SPM1 loss to reduced salivary gland sporozoites unknown
   • Single lab, single method for the quantitative phenotype
   • Ultrastructural effect of knockout on SPMTs not examined
5. 2025 Medium

   A dedicated transmission knockout confirmed that SPM1 is dispensable for the full mosquito-to-vertebrate transmission cycle, framing its role as modulatory rather than essential.

   Evidence Single gene deletion and mosquito transmission assays in P. berghei

   PMID:39908102

   Open questions at the time

   • Does not exclude subtle motility or fitness defects
   • No P. falciparum knockout to confirm dispensability in the human parasite
6. 2025 Medium

   Proximity biotinylation independently validated the SPM1-SPMT network association by recovering SPM1 as a high-confidence PfSPM3 proxiome member in a physiologically tagged setting.

   Evidence BioID with PfSPM3-BirA*-GFP bait and mass spectrometry across three replicates in P. falciparum gametocytes

   PMID:41358875

   Open questions at the time

   • Proximity does not establish direct SPM1-SPM3 binding
   • Single lab and single bait
   • Gametocyte context, while SPMT luminal helix is absent from gametocytes

• How SPM1 mechanically stabilizes or specializes the SPMT lumen and why its loss selectively impairs salivary gland invasion remain unresolved.
• No structure-function dissection of the SPM1 microtubule-binding surface
• No test of whether SPM1 deletion disrupts the interrupted luminal helix in vivo
• Mechanistic basis for the salivary gland sporozoite defect unknown