FILIP1

FILIP1 is a cytoskeletal adaptor protein that controls filamin turnover and actomyosin organization in both neurons and skeletal muscle. FILIP1 directly binds Filamin A and Filamin C and promotes their degradation; in developing neocortex this regulates the Filamin A pool to govern cortical neuron polarity and radial migration, while in skeletal muscle the FILIP1–FLNc interaction is gated by Akt/PKCα phosphorylation of FLNc Ser2234, which is reversed by PP1 under mechanical stress to permit FILIP1-dependent FLNc degradation (PMID:12055638, PMID:32444788, PMID:39521905). FILIP1 also binds the ATPase domain of non-muscle myosin heavy chain IIb in an Hsc70-dependent manner, regulating myosin IIb subcellular distribution and dendritic spine morphology (PMID:25220605, PMID:28234934). During myogenic differentiation, FILIP1 transitions from microtubule association (with EB3) to Z-disc localization with FLNc and is recruited to myofibrillar lesion sites upon mechanical stress, functioning as a cytolinker that bridges actin filaments and microtubules during myofibril assembly and repair (PMID:37178194, PMID:29618024).

1. 2002 High

   The question of how Filamin A abundance is controlled in migrating neurons was answered by the discovery that FILIP1 binds Filamin A and induces its degradation, establishing FILIP1 as a regulator of the actin cytoskeleton during neocortical migration.

   Evidence Co-immunoprecipitation in COS-7 cells and cortical explant migration assays

   PMID:12055638

   Open questions at the time

   • Proteolytic pathway mediating FILIP1-induced Filamin A degradation not identified
   • Whether FILIP1 acts on other filamins unknown
   • In vivo loss-of-function phenotype not yet established
2. 2004 High

   The cell-biological consequence of FILIP1-mediated Filamin A control was resolved: FILIP1 determines whether migrating cortical neurons adopt multipolar versus bipolar morphology by titrating Filamin A levels, establishing that FILIP1 is a polarity switch during cortical development.

   Evidence siRNA knockdown of FILIP1 in cortical neurons combined with dominant-negative Filamin A and in vivo migration assays

   PMID:15509752

   Open questions at the time

   • Upstream signals controlling FILIP1 expression in cortical neurons not identified
   • Whether FILIP1 function is required postnatally unknown
3. 2014 Medium

   Beyond filamins, FILIP1 was shown to bind non-muscle myosin heavy chain IIb near its ATPase domain and modulate its actin-binding activity, establishing a second cytoskeletal target and linking FILIP1 to dendritic spine morphology and neural excitation.

   Evidence Co-immunoprecipitation, actin-binding interference assay, and FILIP1-knockout mouse electrophysiology

   PMID:25220605

   Open questions at the time

   • Single lab; independent replication of myosin IIb interaction needed
   • Structural basis of FILIP1–myosin IIb binding unresolved
   • Whether FILIP1 degrades myosin IIb or only redistributes it not determined
4. 2017 Medium

   The mechanism by which FILIP1 controls myosin IIb distribution was extended by identifying Hsc70 as a required co-factor: Hsc70 binds FILIP1 via a domain distinct from the myosin IIb-binding region, and its ATPase activity is necessary for FILIP1-dependent regulation of spine morphology.

   Evidence Pull-down with mass spectrometry identification of Hsc70, ATPase inhibitor experiments in primary neurons

   PMID:28234934

   Open questions at the time

   • Direct structural or reconstituted biochemical evidence for a tripartite FILIP1–Hsc70–myosin IIb complex lacking
   • Whether Hsc70 is also involved in FILIP1-mediated filamin degradation untested
5. 2018 Medium

   FILIP1 was placed in a myogenesis regulatory pathway: its expression is controlled by the lncRNA Myolinc, and FILIP1 knockdown impairs myogenic differentiation, revealing a muscle-specific function beyond the neuronal context.

   Evidence siRNA knockdown of FILIP1 in C2C12 myoblasts with qRT-PCR and myotube formation assays

   PMID:29618024

   Open questions at the time

   • Whether the myogenesis role operates through filamin degradation or an independent mechanism not determined
   • In vivo muscle phenotype of FILIP1 loss not reported
6. 2020 High

   The question of how FILIP1-mediated filamin degradation is regulated was answered: dual phosphorylation of FLNc Ser2234 (by Akt) and a second site (by PKCα) shields FLNc from FILIP1 binding, establishing a phosphorylation-dependent gating mechanism for FILIP1 activity in muscle.

   Evidence BioID proximity proteomics, quantitative phosphoproteomics, kinase inhibitor experiments, and phosphosite mutagenesis

   PMID:32444788

   Open questions at the time

   • Identity of the E3 ligase mediating FILIP1-dependent FLNc proteasomal degradation unknown
   • Whether the same phospho-switch regulates FILIP1 binding to Filamin A untested
7. 2023 Medium

   FILIP1's dynamic localization during myogenesis was mapped — it transitions from microtubules (EB3-positive) to Z-discs with FLNc, and both proteins relocate to myofibrillar lesion sites under mechanical stress, establishing FILIP1 as a cytolinker involved in myofibril assembly and damage response.

   Evidence Immunofluorescence in differentiating myotubes with electrical pulse stimulation and nocodazole treatment

   PMID:37178194

   Open questions at the time

   • Direct microtubule-binding domain in FILIP1 not mapped
   • Whether FILIP1 promotes repair or degradation at lesion sites not distinguished
   • Single lab observation
8. 2024 High

   The upstream phosphatase completing the FLNc phospho-switch was identified: PP1 dephosphorylates FLNc Ser2234 under acute mechanical stress, promoting FILIP1 binding and thus coupling mechanical damage sensing to FILIP1-dependent filamin degradation.

   Evidence In vitro phosphatase assay, quantitative phosphoproteomics, and PP1 modulator treatments in skeletal myotubes

   PMID:39521905

   Open questions at the time

   • Mechanosensor upstream of PP1 activation not identified
   • Whether PP1-FILIP1 axis operates in non-muscle contexts unknown
   • In vivo validation of PP1-FLNc-FILIP1 axis in muscle injury models lacking

• The E3 ubiquitin ligase that mediates FILIP1-dependent filamin degradation has not been identified, and the structural basis of FILIP1 binding to its multiple cytoskeletal partners (Filamin A, Filamin C, myosin IIb, EB3/microtubules) remains unresolved.
• No E3 ligase identified for FILIP1-induced proteasomal degradation of filamins
• No structural model for any FILIP1 interaction
• In vivo muscle phenotype of FILIP1 loss-of-function not reported