FRM1

TgFRM1 is an apical formin that serves as the primary actin nucleator driving Toxoplasma gondii gliding motility, host cell invasion, and egress (PMID:20949068, PMID:30753127). Its FH1-FH2 domains nucleate and polymerize TgACTI filaments and bundle the resulting filaments in vitro, doing so independently of profilin—indeed TgPRF sequesters actin monomers and opposes polymerization—establishing FRM1 as the dominant nucleator for parasite actin (PMID:22397711). FRM1 localizes to the conoid at the parasite apex, where it initiates an apical-to-posterior F-actin flux that depends on myosins A and H and is controlled by PKG- and CDPK1-mediated phosphorylation and AKMT-mediated methylation (PMID:30753127, PMID:41366525). Its apical positioning is achieved by two mechanisms: anchoring to the mature preconoidal ring scaffold via the CGP protein, and complex formation with the methyltransferase AKMT, which stabilizes FRM1 against degradation and positions it at the apex (PMID:41315181, PMID:41370370). FRM1 activity is gated by a conserved intramolecular autoinhibitory switch in which two regulatory elements upstream of the FH2 domain suppress polymerization until an intramolecular repositioning relieves autoinhibition, a mechanism conserved across apicomplexan parasites (PMID:41370370).

1. 2010 High

   Established that FRM1 is functionally required for the actin-dependent processes of parasite motility and invasion, moving it from a candidate to a validated nucleator in vivo.

   Evidence Conditional gene disruption (TetO/TetR) and dominant-negative FH2 domain mutants in T. gondii, scored by gliding and invasion assays

   PMID:20949068

   Open questions at the time

   • Did not demonstrate direct actin nucleation biochemically
   • Did not resolve subcellular site of FRM1 action
   • Individual vs joint roles of FRM1 and FRM2 not fully separated
2. 2012 High

   Reconstitution with purified proteins showed FRM1 directly nucleates and bundles TgACTI without profilin, resolving how parasite actin polymerizes and revealing that profilin acts as a sequestering inhibitor rather than a formin cofactor.

   Evidence In vitro actin polymerization and sequestration assays with purified TgFRM1 FH1-FH2 domains, TgACTI, and TgPRF

   PMID:22397711

   Open questions at the time

   • In vitro activity of isolated FH1-FH2 does not address in vivo regulation of full-length protein
   • Did not address how nucleation is spatially restricted
   • Physiological role of profilin sequestration unresolved
3. 2019 High

   Live F-actin imaging with a genetic epistasis panel placed FRM1 at the conoid as the initiator of a directional F-actin flux and identified myosins A/H, PKG, CDPK1, and AKMT as the effectors and regulators, defining the pathway context of FRM1 activity.

   Evidence F-actin chromobody reporters and live imaging across gene disruptions of FRM1/2/3, myosin A/H, PKG, CDPK1, AKMT, and GAC in T. gondii

   PMID:30753127

   Open questions at the time

   • Did not identify the physical anchor for FRM1 at the apex
   • Molecular mechanism of phosphorylation/methylation control on FRM1 not defined
   • Direction of causality between regulators and FRM1 not molecularly resolved
4. 2025 High

   Identified CGP as the preconoidal ring anchor for FRM1, explaining how the nucleator is retained at the apex in mature parasites versus during daughter assembly.

   Evidence Conditional CGP depletion with FRM1 immunofluorescence and cryo-electron tomography in T. gondii

   PMID:41315181

   Open questions at the time

   • Direct physical interaction between CGP and FRM1 not established
   • Why initial daughter-cell assembly is CGP-independent unresolved
   • Does not address activation state of anchored FRM1
5. 2025 High

   Defined a conserved intramolecular autoinhibitory switch upstream of the FH2 domain and showed AKMT complexes with FRM1 to stabilize and position it, explaining how nucleation is kept off until activation and how the protein is protected from degradation.

   Evidence Structural modeling, in vitro polymerization with regulatory-domain constructs, and AKMT co-IP/mass spectrometry in T. gondii, with conservation noted in Plasmodium

   PMID:41370370

   Open questions at the time

   • The in vivo trigger that flips the intramolecular switch is not identified
   • Link between AKMT methylation activity and switch relief not mechanistically resolved
   • Structural model not experimentally solved at high resolution
6. 2025 Medium

   Cryo-EM localization of FRM1 to the preconoidal ring pinpointed this site as the physical origin of actin polymerization and the starting point of the translocation flux.

   Evidence Cryo-electron microscopy of conoid complexes with conoid protein disruption studies in T. gondii

   PMID:41366525

   Open questions at the time

   • FRM1-specific mechanistic role inferred from structural context rather than direct FRM1 mutagenesis in this study
   • Does not resolve activation state of FRM1 within the ring
   • Does not connect structure to phosphoregulation

• The in vivo signal that relieves FRM1 autoinhibition and the mechanistic link between PKG/CDPK1 phosphorylation, AKMT methylation, CGP anchoring, and switch activation remain unresolved.
• No demonstrated molecular trigger for the intramolecular switch in vivo
• Phosphorylation and methylation sites on FRM1 and their functional consequences not mapped
• Direct FRM1-CGP and FRM1-AKMT interaction interfaces not structurally defined