CAPN7

CAPN7 is a cysteine protease that functions at the interface of the ESCRT membrane-remodeling machinery and substrate-specific proteolysis (PMID:18316332, PMID:24953135, PMID:36107470). Its tandem N-terminal MIT domains bind ESCRT-III/CHMP subunits, an interaction that is both necessary and sufficient for CHMP binding and that targets a membrane-associated pool of CAPN7 to the endosomal pathway, where it co-localizes with endocytosed EGF (PMID:18316332). The MIT domains engage the ESCRT-III subunit IST1 specifically, with the two domains binding simultaneously to distinct MIM1 and MIM2 motifs within IST1; this interaction recruits CAPN7 to EGFR-positive endosomes and to the cytokinetic midbody (PMID:24953135, PMID:36107470, PMID:37772788). At endosomes, CAPN7 proteolytic activity accelerates EGFR degradation, with both the catalytic cysteine (C290) and MIT/IST1 engagement required (PMID:24953135). At the midbody, both IST1 binding and protease activity are independently required for efficient abscission and for maintenance of the NoCut abscission checkpoint (PMID:36107470, PMID:37772788). In other contexts CAPN7 cleaves substrates bearing PEST sequences: it degrades HOXA10 in a Ca2+-dependent, inhibitor-sensitive manner to reduce β3-integrin expression and impair embryo implantation (PMID:29459744), and it acts on AKT1 to enhance AKT1 signaling, drive FoxO1 phosphorylation and nuclear exclusion, and restrain endometrial stromal cell decidualization (PMID:35191464). CAPN7 also contributes to proteolytic turnover of SMN protein (PMID:30910806).

1. 2008 High

   Established that CAPN7 physically couples to the ESCRT-III machinery and accesses the endosomal pathway, identifying the structural basis for its membrane recruitment.

   Evidence Recombinant and Strep-tagged protein pulldowns mapping the interaction to the tandem MIT domains, subcellular fractionation, and co-localization with endocytosed EGF in HEK293T/microscopy

   PMID:18316332

   Open questions at the time

   • Did not demonstrate a proteolytic function at endosomes
   • Did not identify which specific CHMP/ESCRT-III subunit drives physiological recruitment
2. 2014 High

   Showed that CAPN7 is not merely an ESCRT passenger but a functional protease in endosomal sorting, accelerating EGFR degradation through combined catalytic and adaptor activities.

   Evidence Knockdown in HeLa and Capn7−/− MEFs with EGFR degradation assays, rescue with wild-type vs C290S catalytic mutant, MIT-deletion analysis, and IST1 knockdown epistasis

   PMID:24953135

   Open questions at the time

   • Did not identify the direct proteolytic substrate at the endosome
   • Mechanism by which proteolysis accelerates sorting not resolved
3. 2018 Medium

   Identified the first defined substrate motif, demonstrating CAPN7 degrades PEST-containing HOXA10 to control a transcriptional program governing embryo implantation.

   Evidence CAPN7 overexpression in Ishikawa cells and mouse uterus, co-IP, ALLN inhibitor rescue, HOXA10 PEST truncation, luciferase and implantation assays

   PMID:29459744

   Open questions at the time

   • Single lab
   • Ca2+-dependence of CAPN7 catalysis not biochemically reconstituted
   • Direct cleavage site not mapped
4. 2019 Medium

   Implicated CAPN7 among redundant cysteine proteases that turn over SMN protein, expanding its substrate range.

   Evidence SMN2-GFP reporter drug screen, cysteine protease inhibitor treatment, and knockdown of individual proteases with SMN Western blot

   PMID:30910806

   Open questions at the time

   • CAPN7-specific contribution confounded by parallel proteases (CAPN1, CTSB, CTSL)
   • No direct biochemical demonstration of CAPN7 cleaving SMN
   • No cleavage site or motif defined
5. 2022 High

   Extended CAPN7's ESCRT role to cell division, placing it at the midbody where IST1-specific recruitment supports abscission and the NoCut checkpoint.

   Evidence Quantitative MIT-domain/ESCRT-III interaction mapping, related MIT-IST1 crystal structure, midbody live imaging, abscission timing, and NoCut checkpoint assays (Sundquist/Ullman groups)

   PMID:36107470

   Open questions at the time

   • Did not resolve whether protease activity acts on a midbody substrate
   • Identity of any cleaved checkpoint regulator unknown
6. 2022 Medium

   Defined a second PEST substrate, AKT1, linking CAPN7 proteolysis to phospho-signaling output and FoxO1-dependent transcription in decidualization.

   Evidence CAPN7 knockdown/overexpression in HESCs, co-IP, phospho-Western for AKT1/FoxO1, nuclear-cytoplasmic fractionation, FoxO1 reporter, and in vivo mouse model

   PMID:35191464

   Open questions at the time

   • Single lab
   • How hydrolysis of AKT1 enhances its phosphorylation mechanistically unclear
   • Direct cleavage site not mapped
7. 2023 High

   Provided the atomic basis for CAPN7 recruitment, showing the two MIT domains clamp distinct MIM1/MIM2 motifs on IST1 and dissecting IST1-binding from catalytic requirements at the midbody.

   Evidence Crystallography of CAPN7 tandem MIT domains bound to IST1 MIM peptides, ITC binding, structure-guided mutagenesis, and depletion/rescue abscission and NoCut checkpoint assays

   PMID:37772788

   Open questions at the time

   • Did not identify the midbody proteolytic substrate
   • Did not resolve the full-length CAPN7 structure or catalytic-domain regulation

• The direct proteolytic substrates of CAPN7 at endosomes and midbodies, and the regulation of its catalytic activity, remain unresolved.
• No endosomal or midbody cleavage substrate identified
• Ca2+- and context-dependent regulation of catalysis not reconstituted
• No full-length structure linking MIT recruitment to protease domain