RSP9

RSP9 (PF3D7_1215800) is a radial spoke protein required for axoneme assembly during male gametogenesis in Plasmodium; deletion of the single RSP9 orthologue in P. berghei abolishes axoneme formation, prevents male gamete release, and severely reduces fertilisation and mosquito-stage life-cycle progression (PMID:37288670). This role is specific to flagellar axoneme construction rather than general microtubule biology, and cross-species comparison shows divergent function, as the trypanosomatid orthologues are needed for flagellar beating but not axoneme assembly (PMID:37288670). RSP9 protein levels are regulated by the CRL4WIG1 ubiquitin ligase pathway, and RSP9 is required for 9+2 axoneme symmetry rather than for microtubule nucleation (PMID:39207167). RSP9 also co-purifies with the mitotic kinase NEK1 during early gametogenesis, placing it in the NEK1 interaction network at the stage of basal body and axoneme formation (PMID:39255311). Beyond these findings, no further mechanistic detail on RSP9 has been characterized in the available corpus.

1. 2023 High

   Whether Plasmodium RSP9 is required for axoneme assembly was unknown; knockout established it as necessary for flagellar axoneme formation and male gamete transmission, and revealed lineage-divergent roles relative to trypanosomatids.

   Evidence Gene deletion in P. berghei with structural analysis and cross-species orthologue comparison

   PMID:37288670

   Open questions at the time

   • Does not define the specific radial-spoke substructure or partner proteins RSP9 contacts within the axoneme
   • No molecular mechanism for how RSP9 loss blocks axoneme assembly
   • Phenotype established in P. berghei, not directly in P. falciparum PF3D7_1215800
2. 2024 Low

   Whether RSP9 associates with the gametogenesis kinase machinery was unknown; a NEK1-GFP pulldown placed RSP9 in the NEK1 interaction network during early male gametogenesis.

   Evidence Proteomic cross-linking / co-IP of NEK1-GFP with mass spectrometry in P. berghei

   PMID:39255311

   Open questions at the time

   • Single co-purification without reciprocal validation or RSP9-specific functional follow-up
   • Direct versus indirect nature of the NEK1 association is undetermined
   • Functional consequence of the NEK1-RSP9 association is not tested
3. 2024 Low

   How RSP9 abundance is controlled and which structural feature it supports was unclear; WIG1 disruption showed RSP9 is downregulated among ciliary proteins and required for axoneme symmetry rather than microtubule nucleation.

   Evidence Proteomics of WIG1-disrupted P. berghei plus U-ExM ultrastructure, with the symmetry phenotype cited from prior RSP9 deletion work

   PMID:39207167

   Open questions at the time

   • RSP9 downregulation from a single proteomics dataset without direct RSP9-specific validation in this study
   • Whether RSP9 is a direct CRL4WIG1 ubiquitination target is not shown
   • Mechanism linking RSP9 level to 9+2 symmetry is not resolved

• The direct radial-spoke binding partners, structural placement, and the molecular basis by which RSP9 enforces 9+2 axoneme symmetry in Plasmodium remain undefined.
• No reconstituted or structural model of RSP9 within the radial spoke
• No validated direct interaction partners for RSP9
• Function not directly demonstrated for the P. falciparum PF3D7_1215800 gene itself