SAS4

PF3D7_1458500 (SAS4) is a basal body component of the outer, cytoplasmic centriolar microtubule-organizing center in Plasmodium male gametocytes, where it contributes to de novo basal body assembly (PMID:35077503). In P. berghei, SAS4 occupies a distinct structural subdomain, forming a toroid within the lumen of the basal body microtubule wall that surrounds centrin and SAS6 at the proximal and central cores, and basal bodies assemble from a deuterosome-like precursor in a SAS4-dependent manner (PMID:35077503). SAS4 co-localizes and associates with the basal body kinesin-8B but not with the kinetochore protein NDC80, placing it at the cytoplasmic centriolar MTOC rather than the intranuclear mitotic MTOC (PMID:35550346). Despite this structural role, deletion of SAS4 in P. berghei produces no detectable defect in male gamete formation or mosquito transmission, indicating it is dispensable for these processes (PMID:35550346). Beyond these findings in the P. berghei ortholog, no further mechanistic detail for PF3D7_1458500 has been characterized in the available corpus.

1. 2022 Medium

   To resolve where SAS4 sits within the parasite MTOC architecture, its subcellular position was mapped, establishing it as a distinct structural subdomain of the basal body required for de novo basal body assembly.

   Evidence Ultrastructure expansion microscopy (U-ExM) and Pan-ExM with 3D reconstruction and protein localization in P. berghei

   PMID:35077503

   Open questions at the time

   • The molecular composition and binding partners forming the SAS4-toroid are not defined
   • How SAS4 mediates de novo basal body assembly from the deuterosome-like structure is not resolved
   • Findings rest on the P. berghei ortholog; direct validation in P. falciparum is absent
2. 2022 Medium

   To determine which MTOC compartment SAS4 belongs to, its association with candidate partners was tested, distinguishing the cytoplasmic centriolar MTOC from the intranuclear mitotic MTOC.

   Evidence Fluorescence co-localization and genetic association studies in P. berghei male gametocytes

   PMID:35550346

   Open questions at the time

   • The kinesin-8B association is shown by co-localization, not direct biochemical binding or reciprocal validation
   • Whether SAS4 functionally requires kinesin-8B is not established
3. 2022 Medium

   To test whether SAS4's structural role is functionally required, the gene was deleted, showing it is dispensable for male gamete formation and transmission despite its basal body localization.

   Evidence Gene deletion (knockout) with phenotypic readout of male gamete formation and mosquito transmission

   PMID:35550346

   Open questions at the time

   • The absence of phenotype leaves the physiological function of the SAS4-toroid unexplained
   • Possible redundancy with other basal body components was not tested
   • No phenotypic assay beyond gamete formation and transmission was examined

• The molecular function of SAS4 and the physiological consequence of its basal body toroid remain unresolved, given that its structural presence is not matched by any deletion phenotype.
• No biochemical activity or direct interaction partner has been defined for SAS4
• The reason a structurally organized component is dispensable is unexplained
• No data on the P. falciparum gene itself as opposed to the P. berghei ortholog