FCHO2

FCHO2 is an F-BAR/EFC domain protein that initiates clathrin-mediated endocytosis by sensing and generating membrane curvature at clathrin-coated pits (PMID:17540576, PMID:21762413). Its F-BAR domain forms an intrinsically curved antiparallel dimer that binds phosphatidylserine and phosphoinositides through its concave face and deforms membranes into narrow tubules, with a conformationally switching N-terminal helix phenylalanine governing curvature sensitivity (PMID:17540576, PMID:21762413). At the plasma membrane FCHO2 self-assembles into ring-like patches on PI(4,5)P2-enriched domains, clusters PI(4,5)P2 at cargo-receptor boundaries to enhance clathrin assembly, recruits the endocytic adaptor Eps15, and is required for transferrin uptake (PMID:21762413, PMID:35044298). The curvature it generates is itself a functional signal: the C2 domain of the ubiquitin ligase Nedd4L senses the specific degree of FCHO2-induced curvature, relieving Nedd4L autoinhibition to drive ubiquitination and endocytosis of the epithelial sodium channel ENaC without a direct FCHO2–Nedd4L interaction (PMID:39402328). Beyond endocytosis, FCHO2 binds directly to the HDRRE motif in the integrin β5 cytoplasmic tail to mediate talin-independent inside-out activation of αvβ5 in curved adhesions, a process dependent on β5 residue Y766 (PMID:41651837, PMID:41383757). FCHO2 additionally bundles septin filaments through its intrinsically disordered region, a function conserved from the yeast ortholog Syp1 (PMID:36476870), and interacts with Mon1a to maintain Golgi ribbon architecture and the uniform distribution of Golgi-resident enzymes (PMID:37461455).

1. 2007 High

   Established the structural basis by which FCHO2 acts on membranes: whether the F-BAR domain could intrinsically shape lipid bilayers was unknown until its curved dimeric architecture and tubulation activity were defined.

   Evidence Crystal structure, pulse EPR, liposome tubulation, and site-directed mutagenesis

   PMID:17540576

   Open questions at the time

   • Did not establish the cellular pathway in which curvature generation operates
   • Curvature-sensing role of the N-terminal helix phenylalanine defined only in vitro
2. 2011 Medium

   Placed FCHO2 in clathrin-mediated endocytosis by linking its lipid-deforming activity to coated-pit localization, an adaptor partner, and a functional uptake phenotype.

   Evidence Lipid-binding and tubulation assays, colocalization, FCHO2–Eps15 Co-IP, siRNA knockdown with transferrin readout, gel filtration

   PMID:21762413

   Open questions at the time

   • Mechanism of Eps15 recruitment not resolved at structural level
   • Functional role of FCHO2 monoubiquitination not determined
3. 2022 High

   Showed how FCHO2 selects sites and primes clathrin assembly: its self-assembly into ring patches on PI(4,5)P2 domains and reciprocal PI(4,5)P2 clustering link curvature generation to cargo organization.

   Evidence In vitro reconstitution on supported lipid bilayers plus live-cell imaging

   PMID:35044298

   Open questions at the time

   • Does not define how cargo-receptor boundaries are recognized
   • Stoichiometry of FCHo2 rings with clathrin not resolved
4. 2022 Medium

   Revealed a non-endocytic activity of FCHO2: its intrinsically disordered region bundles septin filaments, a function conserved from yeast Syp1.

   Evidence In vitro reconstitution with purified septins and FCHo2/Syp1 plus advanced microscopy

   PMID:36476870

   Open questions at the time

   • Cellular consequence of septin bundling by mammalian FCHO2 not established
   • Relationship between septin bundling and endocytic role unknown
5. 2023 Medium

   Implicated FCHO2 in Golgi organization through a Mon1a interaction, expanding its role beyond the plasma membrane.

   Evidence Yeast two-hybrid, Co-IP, siRNA knockdown, fluorescence microscopy, FRAP, live-cell imaging (preprint)

   PMID:37461455

   Open questions at the time

   • Preprint, single lab, not peer-reviewed
   • Molecular basis of how FCHO2 maintains Golgi ribbon architecture not defined
   • Whether the Mon1a interaction is direct vs. complex-mediated unresolved
6. 2024 High

   Demonstrated that FCHO2-generated curvature is a functional signal that activates an enzyme: the Nedd4L C2 domain reads the specific curvature to relieve autoinhibition and ubiquitinate ENaC, defining a curvature-coupled regulatory mechanism rather than a protein-protein bridge.

   Evidence In vitro reconstitution of Nedd4L recruitment/activation, cellular colocalization, siRNA knockdown, ubiquitination and ENaC endocytosis assays

   PMID:39402328

   Open questions at the time

   • Whether other C2-domain ligases are similarly activated by FCHO2 curvature not addressed
   • Physiological context of ENaC regulation in vivo not established
7. 2025 Medium

   Identified a talin-independent adhesion function: direct FCHO2 binding to the integrin β5 HDRRE motif drives inside-out αvβ5 activation in curved adhesions, with β5 Y766 determining adhesion type.

   Evidence Co-IP/binding assays, Y766W mutagenesis, live-cell imaging, integrin activation assays, knockout/rescue

   PMID:41383757 PMID:41651837

   Open questions at the time

   • Structural basis of the FCHO2–β5 tail interaction not resolved
   • How phosphorylation of Y766 switches adhesion type mechanistically not defined
   • Relationship between curved adhesions and clathrin-coated pits unclear

• How FCHO2's distinct activities — endocytic curvature generation, septin bundling, Golgi maintenance, and integrin activation — are coordinated within a cell and whether they share common regulatory inputs remains unresolved.
• No unifying model integrating membrane, cytoskeletal, and adhesion roles
• Post-translational regulation (e.g. monoubiquitination) of FCHO2 function uncharacterized