PPID

PPID (CyP-40/cyclophilin D) is a TPR-domain immunophilin that operates as an hsp90 co-chaperone in steroid receptor maturation and as a mitochondrial mediator of regulated cell death (PMID:7657624, PMID:41397247). It is a native component of glucocorticoid receptor–hsp90 heterocomplexes, binding hsp90 directly through a shared immunophilin site for which it competes with FKBP52, so the two immunophilins occupy mutually exclusive hsp90 complexes (PMID:7657624, PMID:8621687). Stable hsp90 association requires not only the TPR domain but also flanking acidic and basic α-helical microdomains, while the peptidylprolyl isomerase catalytic activity is dispensable for heterocomplex assembly and, in the yeast homolog Cpr7, for essential hsp90-dependent functions (PMID:8621687, PMID:9556552). Beyond chaperone scaffolding, PPID can act independently as a molecular chaperone in vitro, holding unfolded substrates in folding-competent intermediates (PMID:8939864), and the Cpr7/Hsp90 module negatively regulates the heat shock response by repressing HSF-dependent transcription (PMID:9668076). In a distinct mitochondrial role, PPID functions as cyclophilin D in overnutrition-induced β-cell death by promoting opening of the mitochondrial permeability transition pore downstream of mitochondrial oxidative stress and calcium, in a cyclosporin A–sensitive manner (PMID:41397247).

1. 1995 High

   Established that PPID is a bona fide constituent of receptor chaperone machinery by showing it binds hsp90 directly within glucocorticoid receptor heterocomplexes and shares a single immunophilin site with FKBP52.

   Evidence Co-immunoadsorption, direct binding of purified proteins, and competitive binding/reconstitution assays

   PMID:7657624

   Open questions at the time

   • Structural basis of the shared hsp90 site not resolved
   • Functional consequence of CyP-40 vs FKBP52 occupancy for receptor activity not defined
2. 1996 Medium

   Defined the molecular determinants of hsp90 binding, showing the TPR domain is necessary but requires flanking charged α-helical microdomains, and confirmed FKBP52 competition.

   Evidence GST-fusion deletion-mutant pulldown/hsp90 retention plus competition assay in myometrial cytosol

   PMID:8621687

   Open questions at the time

   • Precise residues mediating contact not mapped
   • Quantitative affinities not determined
3. 1996 Medium

   Demonstrated an hsp90-independent activity: PPID can itself act as a molecular chaperone holding substrates in non-native folding-competent states.

   Evidence In vitro folding assay with unfolded substrate proteins

   PMID:8939864

   Open questions at the time

   • Physiological substrates not identified
   • Single-lab in vitro assay without orthogonal validation
   • Cannot complete refolding alone
4. 1996 Medium

   Provided the genomic architecture of human PPID, indicating housekeeping-style regulation.

   Evidence Genomic sequencing, somatic cell hybrid PCR mapping, anchor-ligation PCR for transcription start site

   PMID:8812478

   Open questions at the time

   • No functional dissection of the promoter
   • Tissue-specific expression control not addressed
5. 1998 Medium

   Showed that PPIase catalytic activity is dispensable for the protein's essential hsp90-related functions, localizing function to the TPR-containing region.

   Evidence Catalytic-residue mutagenesis and PPIase domain deletion with growth and glucocorticoid receptor activity readouts in S. cerevisiae

   PMID:9556552

   Open questions at the time

   • Whether PPIase activity has any context-specific role unaddressed
   • Reliance on yeast homolog Cpr7
6. 1998 Medium

   Placed the CyP-40/Hsp90 module in negative regulation of the heat shock response via repression of HSF-dependent transcription.

   Evidence Genetic epistasis with HSF reporter and thermotolerance assays in S. cerevisiae

   PMID:9668076

   Open questions at the time

   • Mechanism of HSF repression unresolved
   • Conservation of this role to human PPID not tested in this study
7. 2025 High

   Identified a mitochondrial role for PPID as cyclophilin D mediating overnutrition-induced β-cell death through mPTP opening, downstream of mitochondrial ROS and calcium.

   Evidence Zebrafish global ppid knockout, β-cell-specific re-expression, and pharmacological inhibition (cyclosporin A, mito-TEMPO, Ru360) with β-cell mass quantification

   PMID:41397247

   Open questions at the time

   • Molecular composition/structure of the PPID-dependent mPTP not defined
   • Direct PPID interactors at the pore not identified
   • Conservation to mammalian β cells not shown here

• How PPID's cytosolic hsp90 co-chaperone activity and its mitochondrial mPTP function are coordinated, and what dictates its subcellular partitioning, remains unresolved.
• No study links the chaperone and mitochondrial roles
• Subcellular targeting determinants uncharacterized