FKBPL

FKBPL is a multifunctional immunophilin-like scaffold protein that operates at the intersection of protein quality control, cell cycle regulation, angiogenesis, and selective autophagy. Intracellularly, FKBPL recruits Hsp90 via its C-terminal TPR domain to form a trimeric complex with p21(WAF1/CIP1), stabilizing newly synthesized p21 against proteasomal degradation and thereby enforcing the p53-dependent G2/M checkpoint after DNA damage (PMID:15664193); it also scaffolds Hsp90-dependent complexes containing steroid receptors (ERα, AR) and the E3 ligase RBCK1, which ubiquitinates FKBPL to regulate its own stability (PMID:23912458, PMID:20103631, PMID:20210997). Extracellularly, FKBPL functions as an antiangiogenic factor by binding the CD44 receptor to suppress Rac1, activate RhoA, and induce cortical actin remodeling, thereby inhibiting endothelial and tumor cell migration, tubule formation, and cancer stem cell self-renewal through downstream modulation of DLL4/Notch4 signaling (PMID:21364036, PMID:23457460, PMID:23741069, PMID:25767277). FKBPL additionally serves as an ER-phagy scaffold that bridges ER-resident CKAP4 to LC3/GABARAPs to drive ER fragmentation and autophagy, with its deficiency leading to enhanced protein secretion via microvesicle shedding (PMID:39251576).

1. 2000 Medium

   The earliest functional evidence established that FKBPL participates in the DNA damage response, as its repression enhanced single-strand break repair and radioresistance, raising the question of how an immunophilin-like protein interfaces with genotoxic stress pathways.

   Evidence Antisense oligonucleotide knockdown in irradiated cell lines with comet assay and clonogenic survival readouts

   PMID:10866283

   Open questions at the time

   • Downstream mechanism linking FKBPL to SSB repair not identified
   • No direct target in the repair pathway established
2. 2005 High

   The molecular mechanism was resolved: FKBPL stabilizes newly synthesized p21 by forming a trimeric complex with Hsp90 via its TPR domain, preventing proteasomal degradation of p21 and thereby enabling the G2/M checkpoint after ionizing radiation — connecting the earlier radiobiology observation to a defined chaperone-client pathway.

   Evidence Reciprocal co-immunoprecipitation, TPR domain point mutagenesis, siRNA knockdown with cell cycle analysis after irradiation

   PMID:15664193

   Open questions at the time

   • Whether FKBPL stabilizes p21 independently of p53 not tested
   • Structural basis of trimeric complex unknown
3. 2010 Medium

   FKBPL's Hsp90-dependent co-chaperone role was extended to steroid hormone receptors: it interacts with ERα to modulate antiestrogen sensitivity and enhances AR transcriptional activity, broadening its function beyond cell cycle regulation to hormone signaling.

   Evidence Stable overexpression/knockdown with western blotting for ERα and p21 in breast cancer cells; AR reporter assay in vitro with patient mutation data

   PMID:20103631 PMID:20210997

   Open questions at the time

   • Direct FKBPL–ERα binding assay not shown
   • Mechanism by which FKBPL modulates ERα phosphorylation at Ser118 not resolved
   • AR co-chaperone mechanism inferred from reporter assay only
4. 2011 High

   A second major axis of FKBPL function was uncovered: extracellular FKBPL acts as an antiangiogenic factor by binding CD44, with CD44 knockdown abrogating its effects on endothelial migration and tubule formation, establishing CD44 as the obligate receptor.

   Evidence Recombinant FKBPL/AD-01 peptide in migration, tubule formation, aortic ring, and in vivo sponge assays with CD44 siRNA epistasis

   PMID:21364036

   Open questions at the time

   • Binding site on CD44 not mapped
   • How FKBPL reaches the extracellular space mechanistically was unclear at this stage
5. 2013 High

   The downstream signaling cascade of the FKBPL–CD44 axis was delineated: FKBPL suppresses Rac1 activity, upregulates RhoA, profilin, and vinculin, and induces cortical actin rearrangement, explaining its anti-migratory effects; simultaneously, FKBPL was shown to regulate cancer stem cell self-renewal and differentiation via CD44 and in conjunction with Notch signaling.

   Evidence CD44 siRNA epistasis, Rac1 activity assays, immunofluorescence for actin cytoskeleton, mammosphere assays with Notch inhibitor DAPT epistasis, in vivo tumor initiation assays

   PMID:23457460 PMID:23741069

   Open questions at the time

   • Whether Rac1 inhibition is direct or indirect not resolved
   • Precise molecular link between CD44 engagement and Notch pathway modulation unknown
6. 2013 High

   FKBPL's own post-translational regulation was elucidated: RBCK1 ubiquitinates FKBPL, and the two co-exist with ERα in Hsp90 complexes and co-associate at the pS2 promoter, revealing a feedback mechanism controlling FKBPL protein levels within steroid receptor complexes.

   Evidence Reciprocal co-immunoprecipitation, ubiquitination assay, chromatin immunoprecipitation at pS2 promoter

   PMID:23912458

   Open questions at the time

   • Ubiquitination site on FKBPL not mapped
   • Whether RBCK1-mediated degradation is the dominant turnover pathway in vivo unknown
7. 2015 High

   Genetic loss-of-function in two organisms confirmed FKBPL as essential for vascular development: Fkbpl knockout mice die before E8.5, zebrafish knockdown disrupts vasculature in a CD44-dependent manner, and FKBPL secretion by endothelial cells is specifically downregulated by hypoxia, linking microenvironmental oxygen sensing to FKBPL bioavailability.

   Evidence Knockout mouse, zebrafish morpholino with human FKBPL rescue and CD44 co-knockdown, ELISA of conditioned media under hypoxia

   PMID:25767277

   Open questions at the time

   • Mechanism of hypoxia-mediated secretion downregulation not resolved
   • Whether embryonic lethality is due to vascular or other defects not fully dissected
8. 2015 High

   An intracellular cytoskeletal regulatory circuit was mapped: FKBPL binds phosphorylated Coronin 1B and, through Hsp90, assembles a complex with Slingshot phosphatase to activate Cofilin and regulate Arp2/3 localization at the leading edge, providing a mechanism for its control of directional cell migration.

   Evidence Reciprocal co-immunoprecipitation for phospho-Coronin 1B, siRNA knockdown with rescue by constitutively active Cofilin, immunofluorescence for Arp2/3

   PMID:25800056

   Open questions at the time

   • Relationship between this intracellular pathway and the extracellular CD44-mediated anti-migratory function not integrated
   • Whether Coronin 1B-dependent mechanism operates in endothelial versus tumor cells not tested
9. 2019 Medium

   The cancer stem cell mechanism was refined: FKBPL and its peptides inhibit metastasis by downregulating DLL4 and Notch4 expression downstream of CD44, and the CD44/STAT3 pathway was identified as a mediator in ovarian cancer, extending the stem cell regulatory axis beyond breast cancer.

   Evidence qRT-PCR and western blotting for DLL4/Notch4, in vivo metastasis and limiting dilution assays; RNAseq pathway analysis in ovarian cancer xenografts

   PMID:30975104 PMID:31772325

   Open questions at the time

   • DLL4/Notch4 modulation shown at expression level only; direct signaling mechanism not established
   • Whether STAT3 activation is directly downstream of CD44 or involves intermediates is not resolved
10. 2024 High

    A novel function was discovered: cytosolic FKBPL acts as an ER-phagy scaffold by connecting ER-resident CKAP4 to LC3/GABARAPs, driving ER fragmentation; its deficiency leads to Golgi disassembly, lysosome impairment, and enhanced cytosolic protein secretion via microvesicle shedding, fundamentally expanding FKBPL's role to organelle homeostasis.

    Evidence Gain-of-function screen, reciprocal co-immunoprecipitation, live imaging, LC3/GABARAP interaction assays, ER fragmentation and secretory vesicle analysis

    PMID:39251576

    Open questions at the time

    • Whether ER-phagy and CD44-mediated extracellular functions are coordinated is unknown
    • Structural basis of FKBPL oligomerization at the ER not resolved
    • How FKBPL is partitioned between cytosolic ER-phagy and Hsp90 chaperone functions is unclear
11. 2025 Medium

    The vascular protective function of FKBPL was extended to diabetic and ischemic contexts: FKBPL modulates endothelial barrier integrity through VE-cadherin regulation, influences FGF/PDGF and proinflammatory pathways under high glucose, and its peptide AD-01 stabilizes HIF-1α signaling during hypoxia, identifying collagen alpha-1(XIX) and JCAD as downstream mediators.

    Evidence Transgenic mouse diabetes model, siRNA/overexpression in endothelial cells, transcriptomic/proteomic pathway analysis, 3D microfluidic hypoxia model, hindlimb ischemia mouse model

    PMID:34149611 PMID:40069829 PMID:41674464

    Open questions at the time

    • Mechanisms linking FKBPL to HIF-1α stabilization not directly established
    • Whether collagen alpha-1(XIX) and JCAD are direct or indirect targets unknown
    • Contribution of specific miRNAs (miR-29b-3p, miR-302b-5p) to FKBPL-mediated effects requires validation

• Key unresolved questions include: how FKBPL is partitioned among its diverse intracellular (ER-phagy, Hsp90 co-chaperoning) and extracellular (CD44-mediated antiangiogenic) functions; the structural basis of FKBPL interactions with Hsp90, CKAP4, and CD44; and whether its roles in ER-phagy, protein secretion, and vascular biology are mechanistically integrated.
• No structural model of FKBPL in any complex
• Mechanism of FKBPL secretion unresolved
• Integration of ER-phagy scaffold and CD44 antiangiogenic functions not explored