SAS6

In Toxoplasma gondii, TgSas-6 is a cartwheel component of the centriole that participates in the parasite's centriole/basal body assembly machinery (PMID:23687115, PMID:26855772). It localizes both to the centriole and to the outer core of the bipartite Toxoplasma centrosome, a compartment positioned distal from the nucleus that also contains TgCentrin1/TgSfi1 and a novel Aurora-related kinase and is functionally dedicated to cytokinesis; a temperature-sensitive TgSfi1 mutant that eliminates outer cores blocks daughter parasite budding, establishing the outer core as the driver of budding (PMID:25734885). During S phase the TgSas-6-containing outer core duplicates before the inner core, ensuring each daughter inherits one copy of each core type (PMID:25734885). The distinct SAS6-like paralog SAS6L localizes above the conoid rather than to the centriole, marking it as functionally separate from centriole biogenesis (PMID:23687115). Beyond these localization and cell-cycle findings, direct biochemical characterization of TgSas-6 (its cartwheel assembly partners, structural organization, and molecular activity) has not been characterized in the available corpus.

1. 2013 Medium

   Establishing where TgSas-6 acts required distinguishing it from its paralog; showing TgSas-6 at the centriole while SAS6L sits above the conoid defined two spatially and functionally separate family members.

   Evidence Fluorescence localization of SAS-6 versus SAS6L in Toxoplasma tachyzoites

   PMID:23687115

   Open questions at the time

   • No functional perturbation of TgSas-6 itself
   • Cartwheel assembly role inferred from localization, not tested biochemically
2. 2015 High

   It was unclear how the bipartite Toxoplasma centrosome partitions its functions; placing TgSas-6 in the distal outer core and linking outer-core loss to a budding block showed this compartment drives cytokinesis.

   Evidence Fluorescence localization plus genetic epistasis via a temperature-sensitive TgSfi1 mutant with a defined budding phenotype

   PMID:25734885

   Open questions at the time

   • Cytokinesis phenotype was demonstrated through TgSfi1, not by direct TgSas-6 depletion
   • Molecular mechanism linking outer core to budding not defined
3. 2015 Medium

   How the two centrosome cores are faithfully inherited was unknown; tracking duplication order showed the TgSas-6 outer core duplicates before the inner core, ensuring sequential inheritance of one of each core per daughter.

   Evidence Cell-cycle staging with fluorescence microscopy of centrosome core markers

   PMID:25734885

   Open questions at the time

   • Mechanism enforcing sequential duplication order unknown
   • Role of TgSas-6 in setting duplication timing not tested
4. 2016 Low

   Whether apicomplexans use canonical centriole factors was open; genomic survey showed SAS6, SAS4/CPAP, and BLD10/CEP135 are conserved while many other basal body/flagellar components are absent, framing a divergent biogenesis pathway.

   Evidence Comparative genomics of apicomplexan genomes

   PMID:26855772

   Open questions at the time

   • Computational inference only, no functional test of TgSAS6
   • Does not establish which conserved factors interact with TgSas-6

• The biochemical basis of TgSas-6 cartwheel assembly, its direct partners within the outer core, and its structural contribution to centriole/budding remain undefined.
• No direct TgSas-6 loss-of-function phenotype reported
• No structural or interaction data for the cartwheel