FIS1

In Toxoplasma gondii, TgFis1 is a mitochondrial outer membrane protein involved in mitochondrial morphology maintenance and membrane remodeling rather than in canonical organelle fission, which instead depends on the divergent dynamin-related protein TgDrpC (PMID:30947298). Mislocalization of endogenous Fis1 exerts a dominant-negative effect on mitochondrial structure, indicating that Fis1 acts through interactions with proteins critical for maintaining mitochondrial architecture (PMID:32047127). Fis1 is required for the reversible constriction of the mitochondrial outer membrane induced by the ionophore monensin, placing it in a membrane-remodeling pathway responsive to ionophore stress (PMID:32047127). Interaction screens using Fis1 as bait identified LMF1, an outer mitochondrial membrane protein essential for the peripheral, lasso-shaped positioning of the mitochondrion in intracellular parasites (PMID:32047127). Beyond these findings, the biochemical mechanism by which Fis1 contributes to membrane remodeling and positioning has not been characterized in the available corpus.

1. 2019 High

   Whether the canonical fission adaptor Fis1 drives mitochondrial fission in Toxoplasma was unknown; reciprocal genetic dissection established that TgFis1 is dispensable for fission and that the divergent dynamin-related protein TgDrpC carries this role.

   Evidence Genetic depletion and dominant-negative expression of TgDrpC alongside loss-of-function assessment of TgFis1, scored by organellar phenotype

   PMID:30947298

   Open questions at the time

   • Does not define what Fis1 does if not fission
   • Molecular mechanism of TgDrpC recruitment not addressed
2. 2020 Medium

   Having excluded a fission role, the question became whether Fis1 has any structural function; mislocalization of endogenous Fis1 produced a dominant-negative mitochondrial morphology defect, showing Fis1 engages partners important for organelle architecture.

   Evidence Endogenous protein mislocalization assay with mitochondrial morphology readout

   PMID:32047127

   Open questions at the time

   • Binding partners not directly identified in this experiment
   • Direct versus indirect effect on morphology not resolved
3. 2020 Medium

   To place Fis1 in a defined remodeling process, monensin-induced outer membrane constriction was tested and shown to require Fis1 function, embedding Fis1 in ionophore-responsive membrane remodeling.

   Evidence Monensin (ionophore) treatment combined with Fis1 functional perturbation and morphology readout

   PMID:32047127

   Open questions at the time

   • Single method with no molecular mechanism
   • How Fis1 senses or transduces the ionophore signal is unknown
4. 2020 Medium

   To identify the partners through which Fis1 acts, interaction trap screens using Fis1 as bait recovered LMF1, an outer membrane protein required for peripheral lasso-shaped positioning of the mitochondrion in intracellular parasites.

   Evidence Protein interaction trap/pulldown screens with Fis1 bait, followed by LMF1 loss-of-function morphology and localization assays

   PMID:32047127

   Open questions at the time

   • Direct Fis1-LMF1 binding interface not defined
   • Single lab; reciprocal validation of the interaction not described

• The biochemical activity of Fis1 and the mechanism by which it links to LMF1 to drive membrane remodeling and mitochondrial positioning remain undefined.
• No structural model of Fis1 or the Fis1-LMF1 complex
• Enzymatic or scaffolding activity of Fis1 undetermined
• Signal driving peripheral positioning not established