CAPN7

CAPN7 is an atypical calpain family cysteine protease that functions at the interface of endosomal sorting, cytokinetic abscission, and transcription factor turnover. It lacks the classical penta-EF-hand domain but possesses tandem MIT domains that simultaneously engage two distinct MIMs on the ESCRT-III subunit IST1, as revealed by crystal structures and structure-guided mutagenesis; this interaction recruits CAPN7 to EGFR-positive endosomes and to the cytokinetic midbody, where its proteolytic activity is required for efficient EGFR degradation, timely abscission, and maintenance of the NoCut checkpoint (PMID:18316332, PMID:24953135, PMID:36107470, PMID:37772788). CAPN7 also cleaves PEST-sequence-containing substrates including the transcription factor HOXA10, whose Ca²⁺-dependent degradation by CAPN7 impairs embryo implantation, and AKT1, whose CAPN7-mediated processing enhances AKT1 phosphorylation and promotes FoxO1 nuclear exclusion to negatively regulate endometrial decidualization (PMID:29459744, PMID:35191464).

1. 2008 High

   Establishing that CAPN7 physically engages the ESCRT-III machinery resolved how an atypical calpain without a penta-EF-hand domain could be targeted to endosomal membranes.

   Evidence Recombinant pulldown confirmed direct CAPN7–CHMP1B interaction via tandem MIT domains; subcellular fractionation and fluorescence microscopy showed partial endosomal localization enhanced by dominant-negative VPS4B in HeLa cells.

   PMID:18316332

   Open questions at the time

   • Proteolytic activity was not tested; whether ESCRT-III binding serves a catalytic or scaffolding purpose remained open.
   • The preferred ESCRT-III partner(s) beyond CHMP1B were not ranked quantitatively.
   • Endogenous substrate(s) at endosomes were unknown.
2. 2014 High

   Demonstrating that CAPN7 protease activity is required for EGFR degradation linked the MIT-domain-mediated ESCRT-III recruitment to a specific endosomal sorting function.

   Evidence CAPN7 knockdown and Capn7−/− MEFs showed delayed EGF-stimulated EGFR degradation; rescue with catalytic-dead C290S mutant acted dominant-negatively, an effect abolished by MIT domain deletion; IST1 knockdown prevented CAPN7(C290S) co-localization with EGFR in HeLa cells.

   PMID:24953135

   Open questions at the time

   • The direct endosomal substrate(s) cleaved by CAPN7 to facilitate EGFR degradation were not identified.
   • Whether CAPN7 acts on EGFR itself or on ESCRT-associated factors was unresolved.
3. 2018 High

   Identifying HOXA10 as a direct PEST-dependent substrate expanded CAPN7 function beyond endosomal sorting to transcription factor turnover and reproductive biology.

   Evidence In vitro Ca²⁺-dependent degradation assay, PEST-motif truncation abolished cleavage, calpain inhibitor ALLN rescued HOXA10 stability; CAPN7 overexpression in mouse uterus inhibited embryo implantation.

   PMID:29459744

   Open questions at the time

   • The precise cleavage site(s) within HOXA10 were not mapped.
   • Whether CAPN7-mediated HOXA10 degradation operates via endosomal or cytosolic pools was unclear.
4. 2022 High

   Systematic MIT–ESCRT-III interaction mapping identified IST1 as the principal CAPN7 binding partner and showed that this interaction recruits CAPN7 to the cytokinetic midbody for abscission and NoCut checkpoint control, revealing a second major cellular function beyond endosomal sorting.

   Evidence Quantitative pairwise mapping of 228 MIT–ESCRT-III interactions, live-cell abscission timing assays, and NoCut checkpoint assays upon CAPN7 depletion/rescue.

   PMID:36107470

   Open questions at the time

   • The midbody substrate(s) of CAPN7 protease activity were not identified.
   • Whether the abscission and checkpoint functions require the same or different proteolytic events was unresolved.
5. 2022 Medium

   Demonstrating that CAPN7 cleaves the PEST-containing kinase AKT1 and modulates FoxO1 phosphorylation extended the substrate repertoire to a major signaling node controlling endometrial decidualization.

   Evidence CAPN7 overexpression/knockdown in human endometrial stromal cells, co-immunoprecipitation with AKT1, phospho-specific Western blotting for FoxO1, nuclear/cytoplasmic fractionation.

   PMID:35191464

   Open questions at the time

   • Reconstituted in vitro cleavage of AKT1 by purified CAPN7 was not fully demonstrated.
   • The precise AKT1 cleavage site and whether this generates a constitutively active fragment were not determined.
   • The relationship between AKT1 hydrolysis and enhanced AKT1 phosphorylation is mechanistically unclear.
6. 2023 High

   Crystal structures of the CAPN7 tandem-MIT–IST1 complex revealed a bipartite binding mode in which both MIT domains simultaneously engage two MIMs on IST1, providing the structural basis for CAPN7 recruitment to midbodies and its roles in abscission and the NoCut checkpoint.

   Evidence X-ray crystallography of tandem-MIT–IST1 complex, structure-guided point mutations disrupting IST1 binding in vitro and in cells, depletion/rescue abscission and checkpoint assays demonstrating that both MIT-mediated recruitment and catalytic activity are independently required.

   PMID:37772788

   Open questions at the time

   • The midbody substrate(s) whose cleavage mediates abscission timing and checkpoint signaling remain unidentified.
   • How catalytic activity and IST1-mediated recruitment are coordinated is not structurally resolved.
   • Whether additional ESCRT-III subunits contribute to CAPN7 regulation at the midbody in vivo is unknown.

• The direct proteolytic substrates of CAPN7 at endosomes and at the cytokinetic midbody remain unidentified, leaving the terminal mechanistic steps in EGFR degradation, abscission, and NoCut checkpoint maintenance unresolved.
• No endosomal or midbody substrate has been identified by cleavage-site mapping or proteomics.
• Whether CAPN7 acts catalytically on ESCRT-III subunits themselves or on other midbody factors is unknown.
• A full-length CAPN7 structure including the protease domain is lacking.