LMF1

LMF1 (Lasso Maintenance Factor 1) is an outer mitochondrial membrane protein in Toxoplasma gondii that tethers the mitochondrion to the parasite pellicle to maintain its characteristic lasso-shaped peripheral morphology and ensure faithful organellar inheritance during intracellular division (PMID:32047127). LMF1, originally identified as a Fis1 interactor, is required for the mitochondrion to remain apposed to the pellicle; its loss causes the organelle to retract and collapse and impairs parasite fitness and mitochondrial segregation (PMID:32047127). LMF1 bridges the outer membrane to the inner membrane complex (IMC) by binding the IMC protein IMC10, and conditional knockdown of IMC10 phenocopies LMF1 disruption, establishing this interaction as the physical tether underlying lasso morphology and daughter-cell distribution (PMID:36314270). LMF1 further promotes the association of the mitochondrion with the actin cytoskeleton—mitochondrial vesicles from LMF1-null parasites bind less actin—and the putative LMF1 interactor Myosin A accumulates around the dividing mitochondrion and is itself required for mitochondrial morphology and inheritance (PMID:39374269).

1. 2020 High

   Establishing how the Toxoplasma mitochondrion maintains its peripheral lasso morphology, LMF1 was identified as an outer membrane factor whose loss causes the organelle to retract from the pellicle and collapse, linking morphology to segregation and fitness.

   Evidence Protein interaction trap screen (Fis1 interactor), conditional loss-of-function with mitochondrial morphology/division readouts, and subcellular fractionation in T. gondii

   PMID:32047127

   Open questions at the time

   • Did not identify the pellicle-side partner mediating tethering
   • Molecular basis of the actin/cytoskeletal contribution not addressed
2. 2022 High

   Defining the physical bridge between mitochondrion and pellicle, LMF1 was shown to bind the IMC protein IMC10, and IMC10 knockdown phenocopied LMF1 loss, establishing an outer-membrane-to-IMC tether responsible for lasso morphology and daughter-cell distribution.

   Evidence Yeast two-hybrid screen and protein-protein interaction assays confirming LMF1–IMC10 binding, plus conditional IMC10 knockdown with morphology/division phenotypes in T. gondii

   PMID:36314270

   Open questions at the time

   • Structural basis and stoichiometry of the LMF1–IMC10 interface unresolved
   • Whether additional pellicle proteins participate in the tether not determined
3. 2024 Medium

   Connecting the tether to the cytoskeleton, LMF1 was shown to promote mitochondrion–actin interaction and to associate with Myosin A, which localizes around the dividing mitochondrion and is required for its morphology and inheritance, implicating an actomyosin-based mechanism in organellar segregation.

   Evidence Mitochondrion-vesicle actin pulldowns from LMF1-null vs. wild-type parasites, U-ExM MyoA localization, MyoA conditional loss-of-function, and actin network disruption in T. gondii

   PMID:39374269

   Open questions at the time

   • Direct binding between LMF1 and MyoA (yeast two-hybrid candidate) not biochemically confirmed
   • Whether the actin interaction is direct or bridged by intermediate factors is unresolved
   • Mechanistic order relative to the IMC10 tether during division not established

• How LMF1 coordinates the static IMC10 tether with dynamic actomyosin-driven segregation, and the molecular architecture of the full tethering complex, remain open.
• No structural model of the LMF1 tethering complex
• Direct versus indirect nature of LMF1–actin and LMF1–MyoA interactions unresolved
• Regulation of tether assembly/disassembly across the division cycle uncharacterized