FKBPL

FKBPL (WISp39/DIR1) is a TPR-domain Hsp90 co-chaperone that operates in two distinct compartments: intracellularly it controls protein stability and cytoskeletal dynamics, and extracellularly it acts as a secreted antiangiogenic ligand. As a co-chaperone it stabilizes newly synthesized p21(WAF1/CIP1) by forming a trimeric complex with Hsp90, an interaction that requires the C-terminal TPR domain; loss of FKBPL prevents p21 accumulation and the G2/M cell cycle arrest after ionizing radiation (PMID:15664193). FKBPL also engages estrogen receptor alpha within Hsp90 complexes, modulating ERα phosphorylation, p21 levels, and endocrine therapy sensitivity, and is itself subject to RBCK1-mediated ubiquitination that controls its stability and its co-occupancy with ERα at estrogen-responsive promoters (PMID:20103631, PMID:23912458). Through Hsp90, FKBPL further scaffolds directed cell migration by binding phospho-Coronin 1B to position Slingshot phosphatase, activating Cofilin and localizing the Arp2/3 complex at the leading edge (PMID:25800056). Secreted FKBPL and its peptide derivatives (AD-01, ALM201) bind the cell-surface receptor CD44 to drive an antimigratory, antiangiogenic phenotype involving Rac-1 inhibition, RhoA upregulation, and cortical actin reorganization (PMID:21364036, PMID:23457460), and this CD44 axis is required for FKBPL's essential role in developmental angiogenesis, where knockout is embryonic lethal and zebrafish vascular defects are rescued by human FKBPL in a CD44-dependent manner (PMID:25767277). The same secreted activity suppresses cancer stem cells and metastasis through CD44/STAT3 and DLL4/Notch4 signaling (PMID:23741069, PMID:30975104, PMID:31772325). Most recently, FKBPL was defined as a cytosolic ER-phagy regulator that scaffolds the ER-resident protein CKAP4 to LC3/GABARAP autophagy adaptors, driving ER fragmentation and autophagic turnover, with its deficiency redirecting cargo toward microvesicle-mediated secretion (PMID:39251576).

1. 2005 High

   Established FKBPL's first molecular function: how the cell stabilizes p21 to enforce a DNA-damage cell cycle checkpoint, answering whether a TPR co-chaperone gates p21 turnover.

   Evidence Co-IP of a trimeric WISp39-p21-Hsp90 complex, TPR-domain point mutagenesis, siRNA knockdown, and cell cycle analysis after ionizing radiation

   PMID:15664193 PMID:15846062

   Open questions at the time

   • Does not define how FKBPL physically shields p21 from the proteasome
   • Substrate range of the Hsp90-FKBPL module beyond p21 not addressed
2. 2000 Medium

   Linked FKBPL/DIR1 to acquired radioresistance, raising the question of whether it influences DNA single-strand break repair.

   Evidence Antisense oligonucleotide knockdown with alkaline comet assay and clonogenic survival across radioresistant and radiosensitive cell lines

   PMID:10866283

   Open questions at the time

   • No molecular mechanism connecting FKBPL to SSB repair machinery
   • Predates the p21/Hsp90 model and not mechanistically reconciled with it
3. 2010 Medium

   Extended FKBPL's chaperone role to hormone receptor signaling, testing whether it regulates ERα levels and endocrine therapy response.

   Evidence Reciprocal overexpression/knockdown in breast cancer lines with western blotting for ERα Ser118 phosphorylation, p21, and drug sensitivity assays

   PMID:20103631

   Open questions at the time

   • Direct ERα binding versus Hsp90-mediated association not resolved here
   • AR reporter activity (idx 12) is a separate low-confidence claim not integrated
4. 2013 High

   Resolved how FKBPL itself is regulated and how it acts at estrogen target genes, identifying RBCK1 as the post-translational controller.

   Evidence Co-IP, ubiquitination assays, and ChIP at the pS2 promoter with gain/loss-of-function in breast cancer cells

   PMID:23912458

   Open questions at the time

   • Ubiquitin chain type and degradative versus non-degradative fate of FKBPL not defined
   • Stoichiometry within Hsp90-ERα complexes unknown
5. 2013 High

   Defined FKBPL's extracellular mechanism: direct CD44 binding driving cytoskeletal reorganization to block migration, angiogenesis, and cancer stem cells.

   Evidence Recombinant FKBPL/AD-01 with CD44 siRNA, blocking antibody, Rac-1 activity and actin imaging, mammosphere and xenograft assays

   PMID:21364036 PMID:23457460 PMID:23741069

   Open questions at the time

   • How a TPR co-chaperone is secreted and presented to CD44 not established
   • Structural basis of FKBPL-CD44 binding unknown
6. 2015 High

   Demonstrated FKBPL is essential for developmental angiogenesis in vivo and that this function is CD44-dependent.

   Evidence Fkbpl knockout/heterozygote mice, zebrafish morpholino knockdown with human FKBPL rescue blocked by cd44 co-knockdown, and ex vivo/in vivo angiogenesis models

   PMID:25767277

   Open questions at the time

   • Cause of pre-E8.5 embryonic lethality not pinpointed
   • Connection between intracellular chaperone roles and secreted angiostatic activity in development unresolved
7. 2015 High

   Identified FKBPL's intracellular cytoskeletal mechanism, showing it scaffolds a phosphatase-cofilin module for directed motility.

   Evidence Phospho-specific reciprocal Co-IP of WISp39-Coronin 1B, SSH/Cofilin pathway analysis, Arp2/3 localization imaging, and rescue with Coronin 1B plus constitutively active Cofilin

   PMID:25800056

   Open questions at the time

   • Relationship between this promigratory intracellular role and the antimigratory extracellular CD44 role not reconciled
   • Hsp90-dependence mechanism in this context not detailed
8. 2019 Medium

   Broadened the anti-stem-cell/antimetastatic mechanism beyond CD44 to DLL4/Notch4 and CD44/STAT3 signaling.

   Evidence Overexpression and peptide treatment (AD-01, ALM201) with migration/invasion, metastasis, limiting dilution, and CD44/STAT3 and DLL4/Notch4 readouts in breast and ovarian models

   PMID:30975104 PMID:31772325

   Open questions at the time

   • Whether DLL4/Notch4 and STAT3 effects are downstream of CD44 or parallel not fully separated
   • Single-lab findings
9. 2024 High

   Revealed a distinct cytosolic function: FKBPL as an ER-phagy scaffold linking CKAP4 to autophagy adaptors and controlling secretory fate.

   Evidence Gain-of-function screen, Co-IP of FKBPL-CKAP4 and FKBPL-LC3/GABARAP, double knockdown/knockout, ER-phagy flux and organelle imaging, and microvesicle secretion analysis

   PMID:39251576

   Open questions at the time

   • Trigger for FKBPL oligomerization at the ER not defined
   • Relationship between ER-phagy role and Hsp90/CD44 functions unknown
10. 2025 Medium

    Positioned FKBPL within ischemic and diabetic vascular contexts, testing how hypoxia and metabolic stress modulate its angiostatic activity.

    Evidence Hindlimb ischemia and streptozotocin diabetic mouse models, 3D microfluidics with siRNA/overexpression, LC-MS/MS proteomics, and miRNA quantification

    PMID:40069829 PMID:41674464

    Open questions at the time

    • Causal hierarchy among CD44, VE-cadherin, FGF/PDGF/TGF-β pathways, and miRNAs not established
    • Single-lab pathway-level associations

• It remains unresolved how FKBPL's distinct compartmentalized activities — Hsp90/p21 chaperoning, secreted CD44 antiangiogenesis, leading-edge cytoskeletal scaffolding, and cytosolic ER-phagy — are mechanistically coordinated and switched within a single protein.
• No unifying structural or regulatory model linking the chaperone, secreted, and ER-phagy functions
• Mechanism of FKBPL secretion is undefined
• How FKBPL toggles between promigratory (Coronin 1B) and antimigratory (CD44) outcomes is unknown