SFI1

SFI1 (TgSFI1) is a centrosomal scaffold protein that localizes to the outer core of the bipartite Toxoplasma gondii centrosome, where it is specifically required for cytokinesis (daughter parasite budding) but dispensable for karyokinesis (PMID:25734885). At the outer core it forms a complex with TgCentrin1, the cartwheel protein TgSas-6, and a novel Aurora-related kinase, a compartment spatially and functionally distinct from the CEP250/C-Nap-containing inner core that independently drives nuclear division (PMID:25734885). Temperature-sensitive inactivation of TgSFI1 abolishes outer cores and blocks budding while the inner core and spindle pole continue to amplify, establishing its selective role in daughter formation (PMID:25734885). The outer core duplicates ahead of the inner core as parasites enter S phase, ensuring each daughter inherits one copy of each core type (PMID:25734885). TgSFI1 engages TgCentrin1 through multiple distinct SFI1-repeat motifs, with each repeat differing in affinity, Ca2+ sensitivity, and which lobe of TgCentrin1 it binds — a repeat-specific, Ca2+-regulated interaction that endows the complex with conformational plasticity (PMID:36009009).

1. 2015 High

   It was unknown which centrosomal component coordinates daughter parasite budding in Toxoplasma; localizing TgSFI1 to a discrete outer core and inactivating it established that this core drives cytokinesis independently of karyokinesis.

   Evidence Temperature-sensitive loss-of-function mutant, fluorescence localization, and co-localization of centrosome components in Toxoplasma gondii

   PMID:25734885

   Open questions at the time

   • The molecular activity of TgSFI1 within the complex (scaffolding vs. regulatory) is not defined
   • How TgSFI1 loss mechanistically prevents budding is not resolved
   • Roles of the associated Aurora-related kinase and TgSas-6 relative to TgSFI1 are not dissected
2. 2015 Medium

   How each daughter inherits both centrosome core types was unclear; time-resolved imaging showed the outer core duplicates before the inner core at S-phase entry, defining the temporal logic of bipartite centrosome inheritance.

   Evidence Time-resolved fluorescence microscopy of centrosome duplication across the cell cycle in Toxoplasma gondii

   PMID:25734885

   Open questions at the time

   • The trigger coupling outer-core duplication to S-phase entry is unknown
   • No independent replication of the sequential-duplication timing reported
3. 2022 High

   The physical basis of the TgSFI1–TgCentrin1 partnership was uncharacterized; biophysical dissection of individual repeat peptides showed each SFI1 repeat binds TgCentrin1 with distinct affinity, Ca2+ sensitivity, and lobe engagement, establishing a repeat-specific, Ca2+-regulated interaction.

   Evidence ITC, NMR, circular dichroism, and fluorescence spectroscopy with synthetic TgSFI1-repeat peptides and recombinant TgCentrin1 domains

   PMID:36009009

   Open questions at the time

   • Binding was measured with isolated synthetic repeat peptides, not full-length TgSFI1 in situ
   • The functional consequence of Ca2+-regulated repeat binding for cytokinesis is not tested
   • Stoichiometry of the assembled TgSFI1–TgCentrin1 array is not determined

• How the TgSFI1 scaffold mechanistically licenses daughter budding, and how the Aurora-related kinase and Ca2+ signaling regulate its assembly and duplication, remains open.
• No mechanism connecting outer-core assembly to the budding machinery
• Regulatory inputs to outer-core duplication timing are undefined
• Role of the Aurora-related kinase relative to TgSFI1 function is uncharacterized