CEP250L1

TgCEP250L1 is an essential structural component of the bipartite Toxoplasma gondii centrosome that couples centrosome function to nuclear segregation during endodyogeny (PMID:25734885, PMID:36069445). A distant ortholog of the CEP250/C-Nap protein family, it localizes to the inner core of the centrosome, which is closely aligned with the spindle pole (centrocone) and duplicates sequentially after the outer core during S phase (PMID:25734885). Conditional degradation of TgCEP250L1 is lethal: daughter cells still form but nuclei fail to segregate and accumulate at the residual body, while the centriole-containing outer/middle centrosome disconnects from the nucleus (PMID:36069445). Loss of TgCEP250L1 abolishes proper localization of the microtubule-binding protein TgEB1 and prevents assembly or stabilization of the intranuclear mitotic spindle, linking its inner-core function directly to spindle nucleation (PMID:36069445). TgCEP250L1 repositions within the centrosome across the cell cycle, coincident with the transition to mitotic spindle formation (PMID:36069445).

1. 2015 Medium

   Establishing that the T. gondii centrosome is bipartite required identifying markers of its distinct cores; TgCEP250L1 defined the inner core aligned with the spindle pole, showing the two cores are spatially and functionally separable and duplicate sequentially.

   Evidence Fluorescence microscopy localization plus genetic epistasis in temperature-sensitive mutants

   PMID:25734885

   Open questions at the time

   • Functional consequence of inner-core loss not tested
   • Molecular partners of TgCEP250L1 within the inner core not identified
   • Mechanism of sequential core duplication unresolved
2. 2022 High

   Whether the inner core is functionally essential was unknown; conditional degradation showed TgCEP250L1 loss is lethal and specifically blocks nuclear segregation while daughter buds still form, establishing it as a nuclear-centrosome tether rather than a budding factor.

   Evidence Conditional protein degradation with fluorescence microscopy of nuclear segregation and centrosome positioning in T. gondii

   PMID:36069445

   Open questions at the time

   • Direct molecular tether to the nuclear envelope not identified
   • How TgCEP250L1 maintains outer-core to nucleus connection mechanistically unresolved
3. 2022 High

   The downstream consequence of inner-core loss for the spindle was untested; TgEB1 mislocalization and failed intranuclear spindle assembly upon TgCEP250L1 depletion tied inner-core function to mitotic spindle nucleation or stabilization.

   Evidence Conditional degradation plus TgEB1 localization by fluorescence and ultrastructure expansion microscopy

   PMID:36069445

   Open questions at the time

   • Whether TgCEP250L1 acts in spindle nucleation versus stabilization not distinguished
   • No direct biochemical link between TgCEP250L1 and TgEB1 or spindle microtubules
4. 2022 Medium

   Whether TgCEP250L1 is a static or dynamic centrosome component was open; U-ExM showed its position shifts relative to other markers coincident with spindle formation, indicating cell-cycle-regulated repositioning.

   Evidence Ultrastructure expansion microscopy with multi-marker co-localization across cell cycle stages

   PMID:36069445

   Open questions at the time

   • Signals driving repositioning unknown
   • Post-translational regulation of TgCEP250L1 not characterized

• The direct molecular partners and biochemical mechanism by which TgCEP250L1 tethers the centrosome to the nucleus and enables spindle assembly remain undefined.
• No identified binding partners at the nuclear envelope
• No structural model of the inner core
• Mechanism connecting TgCEP250L1 loss to TgEB1 mislocalization not established