TPP2

TPP2 encodes a giant cytosolic serine peptidase that assembles into ~6 MDa oligomeric complexes (36-mers and 32-mers) and functions downstream of the 26S proteasome by removing tripeptides from proteasomal degradation products, spatially associating with proteasomes in situ and being recruited to aggresomes upon proteasome inhibition (PMID:28396430, PMID:30226264). In MHC class I antigen processing, TPP2 plays a predominantly destructive role: TPP2-deficient mice display increased surface MHC I–peptide complexes and delayed cytosolic epitope degradation (PMID:18056356). TPP2 deficiency activates cell type–specific death programs—proliferative apoptosis in T cells and premature senescence in fibroblasts—coinciding with p53 upregulation and NF-κB dysregulation, while TPP2 overexpression promotes mitotic exit and resistance to apoptosis (PMID:18362329, PMID:16762321). A homozygous loss-of-function missense mutation (p.Cys28Gly) in TPP2 causes sterile brain inflammation in humans (PMID:30533531).

1. 2004 High

   Establishing that TPP2 operates downstream of the proteasome using both endo- and exopeptidase activities to process proteasomal products resolved where TPP2 sits in the intracellular proteolytic cascade and linked it to MHC class I epitope generation and destruction.

   Evidence Biochemical analysis and integration of multiple functional studies across labs

   PMID:15224091

   Open questions at the time

   • Relative contribution of endopeptidase versus exopeptidase activity to epitope generation not delineated
   • No structural basis for dual catalytic modes
2. 2006 Medium

   Demonstrating that TPP2 overexpression shortens mitosis, enables evasion of spindle checkpoint arrest, and promotes polyploidy while knockdown causes mitotic delay established a non-proteolytic-pathway role for TPP2 in cell cycle progression and apoptosis resistance.

   Evidence Overexpression and shRNA knockdown in HEK293 cells with cell cycle analysis, spindle poison treatment, and IAP expression analysis

   PMID:16762321

   Open questions at the time

   • Mechanism by which TPP2 promotes mitotic exit is unknown
   • Relevance of IAP upregulation to TPP2 catalytic activity not tested
   • Single cell line studied
3. 2007 High

   Genetic knockout in mice showed TPP2 predominantly destroys rather than generates MHC class I epitopes, reversing earlier models that emphasized a generative role and clarifying TPP2's net contribution to antigen processing.

   Evidence TPP2 knockout mouse with surface MHC I quantification, cytosolic peptide degradation assays, and dendritic cell cross-presentation assays

   PMID:18056356

   Open questions at the time

   • Epitope-specific effects not fully catalogued
   • Whether TPP2 contributes to generation of specific epitopes in some contexts remains unresolved
4. 2008 High

   TPP2 deficiency was shown to trigger cell type–specific apoptosis and senescence programs linked to p53 and NF-κB dysregulation, establishing TPP2 as a pro-survival factor beyond its peptide-trimming role.

   Evidence TPP2 knockout mouse with flow cytometry, apoptosis/senescence assays, and p53/NF-κB expression analysis

   PMID:18362329

   Open questions at the time

   • Direct substrate(s) whose accumulation triggers p53 upregulation not identified
   • Whether the viability defect is catalytic-activity-dependent was not dissected
5. 2014 Medium

   Identification of MYBBP1A and CDK2 as physical interactors of TPP2—with the MYBBP1A interaction being activity-dependent—provided the first molecular links connecting TPP2's enzymatic function to tumor suppression and cell cycle regulation pathways.

   Evidence Co-immunoprecipitation and in situ proximity ligation assay in HEK293 cells with butabindide inhibitor

   PMID:25303791

   Open questions at the time

   • Functional consequences of MYBBP1A and CDK2 interactions on cell cycle not established
   • Whether MYBBP1A is a substrate or regulatory partner is unknown
   • Single lab without reciprocal genetic validation
6. 2015 Low

   Detection of physical complexes between TPP2, p53, and SIRT7 in both cytoplasm and nucleus suggested a direct mechanism by which TPP2 could regulate p53 stability or activity.

   Evidence Co-immunoprecipitation from HeLa lysates and mouse liver fractions, PLA in HEK293 cells

   PMID:26169984

   Open questions at the time

   • No functional follow-up demonstrating that the interaction modulates p53 activity or stability
   • Single lab without reciprocal IP or genetic perturbation
   • Stoichiometry and directness of the ternary complex unclear
7. 2017 High

   In situ cryo-ET visualization of TPP2 oligomeric states (36-mers and 32-mers) and their spatial proximity to 26S proteasomes in intact neurons provided the first native structural evidence for a postproteasomal processing relay.

   Evidence Cryo-electron tomography with Volta phase plate and template matching in rat hippocampal neurons

   PMID:28396430

   Open questions at the time

   • Whether spatial proximity reflects direct physical contact or functional coupling is unresolved
   • Regulation of interconversion between 36-mer and 32-mer states unknown
8. 2018 Medium

   A homozygous TPP2 missense mutation (p.Cys28Gly) confirmed as loss-of-function in patient cells established TPP2 deficiency as a cause of sterile brain inflammation, linking the enzyme to neuroinflammatory disease in humans.

   Evidence Next-generation sequencing with enzymatic activity assay and protein expression studies in patient blood cells

   PMID:30533531

   Open questions at the time

   • Mechanism by which TPP2 loss triggers neuroinflammation not determined
   • Single family—genetic confirmation in additional kindreds needed
9. 2018 Medium

   Demonstrating that TPP2 redistributes from diffuse cytoplasmic localization to a perinuclear aggresomal mantle upon proteasome inhibition revealed a dynamic spatial relationship between TPP2 and the proteasome under proteotoxic stress.

   Evidence Confocal microscopy with fluorescent proteasome inhibitor and co-immunostaining in C26 murine colon adenocarcinoma cells

   PMID:30226264

   Open questions at the time

   • Whether aggresomal recruitment is functionally protective or a consequence of substrate accumulation is unknown
   • Signals mediating TPP2 relocalization not identified

• The direct substrates whose accumulation upon TPP2 loss triggers p53 upregulation, apoptosis, senescence, and neuroinflammation remain unidentified, and the catalytic versus non-catalytic contributions of TPP2 to its pro-survival and cell cycle functions have not been dissected.
• No substrate identification linking TPP2 loss to p53/NF-κB dysregulation
• Catalytic versus scaffolding functions not separated genetically
• Structural basis for oligomeric state regulation and its functional consequences unknown