FCHSD2

FCHSD2 is an F-BAR/SH3 domain membrane-remodeling protein that couples membrane curvature sensing to Arp2/3-dependent branched actin assembly across multiple compartments (PMID:23437151, PMID:34520816). It directly binds WASP/N-WASP and stimulates F-actin polymerization through the Arp2/3 pathway in vitro (PMID:23437151). At the plasma membrane, FCHSD2 is activated by ERK1/2 phosphorylation to promote clathrin-coated pit initiation and clathrin-mediated endocytosis in cancer cells, and its loss raises cell-surface EGFR and enhances proliferation and migration (PMID:30249660). Beyond internalization, FCHSD2 governs the post-endocytic fate of EGFR and MET: it directs their recycling, and its depletion shunts these receptors into Rab7-dependent late endosomal/lysosomal degradation while driving nuclear ERK1/2 translocation and transcriptional upregulation of both receptors (PMID:32678845). At the endosome, FCHSD2 is directly recruited by the scaffold MICAL-L1 to generate Arp2/3-mediated branched actin required for endosome fission and receptor recycling, acting upstream of EHD1 in the fission pathway (PMID:39382837). FCHSD2 also builds cell-surface protrusions: apical microvilli/filopodia formation requires a trimeric complex with CDC42 and N-WASP, while lateral protrusions depend on its F-BAR domain independently of N-WASP, and both require its plasma-membrane binding (PMID:34520816). In vivo, FCHSD2 localizes along cochlear hair cell stereocilia and cooperates genetically with CDC42 to maintain them, with Fchsd2 knockout mice developing progressive hearing loss and stereocilia degeneration (PMID:23437151, PMID:35892293). Its C-terminal PDZ-binding motif directly engages the PDZ3 domain of the deafness-related protein PDZD7, resolved at atomic resolution (PMID:35695292). FCHSD2 is additionally required for glucose-stimulated insulin secretion in human pancreatic β-cells (PMID:33535042).

1. 2013 Medium

   Established FCHSD2 as an actin-nucleation effector by showing it binds the WASP/N-WASP machinery and stimulates Arp2/3-dependent F-actin assembly, defining its core biochemical activity.

   Evidence Co-IP/pulldown and in vitro F-actin polymerization assay; immunofluorescence in mouse cochlear hair cells

   PMID:23437151

   Open questions at the time

   • Did not define which membrane compartments host this activity in vivo
   • No domain mapping of the FCHSD2-N-WASP interaction
2. 2018 High

   Resolved how FCHSD2 activity is controlled and where it acts at the cell surface, identifying it as an ERK1/2 substrate whose phosphorylation gates clathrin-coated pit initiation and CME in cancer cells.

   Evidence Kinase inhibitor studies, siRNA/shRNA knockdown, live-cell CME imaging, ERK1/2 substrate identification in NSCLC cells

   PMID:30249660

   Open questions at the time

   • Phosphosite(s) and how phosphorylation changes FCHSD2 conformation/activity not defined
   • Cancer-cell specificity of the ERK1/2-FCHSD2 axis not mechanistically explained
3. 2020 High

   Extended FCHSD2 function beyond uptake to receptor recycling, showing it prevents EGFR/MET degradation and that its loss triggers a Rab7-dependent rerouting to lysosomes plus a nuclear ERK1/2 feedback loop upregulating the receptors.

   Evidence siRNA knockdown, live-cell endosomal trafficking imaging, subcellular fractionation, Rab7 co-depletion epistasis

   PMID:32678845

   Open questions at the time

   • Molecular link between FCHSD2 loss and ERK1/2 nuclear translocation not resolved
   • Did not identify the endosomal machinery FCHSD2 acts with
4. 2021 Medium

   Dissected FCHSD2's role in cell-surface morphogenesis, separating an N-WASP/CDC42-dependent apical protrusion activity from an F-BAR-domain-driven, N-WASP-independent lateral protrusion activity.

   Evidence Domain-deletion/overexpression constructs, reciprocal Co-IP of FCHSD2-CDC42-N-WASP complex, protrusion morphology assays

   PMID:34520816

   Open questions at the time

   • Performed largely by overexpression; endogenous contribution unquantified
   • Stoichiometry and assembly order of the trimeric complex unknown
5. 2021 Medium

   Identified a physiological role in metabolic secretion, showing FCHSD2 is required for glucose-stimulated insulin secretion in human β-cells.

   Evidence CRISPR-Cas9 knockout in EndoC-βH1 cells with glucose-stimulated insulin secretion assay

   PMID:33535042

   Open questions at the time

   • Single method; the membrane-trafficking step FCHSD2 controls in β-cells not defined
   • Not linked to its endocytic or actin functions mechanistically
6. 2022 High

   Provided in vivo proof that FCHSD2 maintains sensory hair cell structure, with knockout mice showing progressive hearing loss and stereocilia degeneration and a cooperative genetic interaction with CDC42.

   Evidence Fchsd2 knockout mice, ABR measurements, scanning EM of stereocilia, Fchsd2/Cdc42 double-knockout epistasis

   PMID:35892293

   Open questions at the time

   • Molecular mechanism by which FCHSD2 sustains stereocilia actin not defined
   • Whether the cochlear role uses the same N-WASP/Arp2/3 pathway as in cultured cells unclear
7. 2022 High

   Defined an atomic-resolution interaction linking FCHSD2 to deafness machinery, showing its C-terminal PDZ-binding motif threads through the PDZ3 groove of PDZD7.

   Evidence Yeast two-hybrid, Co-IP in COS-7 cells, 2.0 Å X-ray crystal structure

   PMID:35695292

   Open questions at the time

   • Functional consequence of the FCHSD2-PDZD7 interaction in vivo not tested
   • Whether PDZD7 localizes FCHSD2 to stereocilia not established
8. 2024 High

   Placed FCHSD2 within the endosomal fission pathway, showing MICAL-L1 directly recruits it to drive Arp2/3 branched actin for endosome fission and recycling, acting upstream of EHD1.

   Evidence Reciprocal Co-IP, live-cell imaging, siRNA knockdown, endosome fission and receptor recycling assays

   PMID:39382837

   Open questions at the time

   • How branched actin mechanically couples to EHD1-driven scission not resolved
   • Whether ERK1/2 phosphorylation regulates the endosomal pool of FCHSD2 unknown

• How a single F-BAR/SH3 protein is partitioned between plasma-membrane CME, protrusion formation, and endosomal fission, and whether ERK1/2 phosphorylation coordinates these pools, remains unresolved.
• No unified model of FCHSD2 compartment targeting
• Regulatory inputs beyond ERK1/2 unidentified
• Phosphosite-level control across compartments undefined