Involvement of calpain-7 in epidermal growth factor receptor degradation via the endosomal sorting pathway.

Calpain-7 (CAPN7) is a unique intracellular cysteine protease that has a tandem repeat of microtubule interacting and trafficking (MIT) domains and lacks a penta-EF-hand domain. Although the MIT domains of CAPN7 were previously shown to interact with a subset of endosomal sorting complex required for transport (ESCRT)-III and ESCRT-III-related proteins, including charged multivesicular body protein 1 and increased sodium tolerance (IST)1, knowledge of the involvement of the protease in membrane trafficking has been limited. In the present study, compared with control cells, we found that epidermal growth factor receptor (EGFR) degradation was mildly delayed in CAPN7-knockdown HeLa cells and mouse embryonic fibroblast cells established from CAPN7 knockout (Capn7(-/-) ) mice. Re-expression of wild-type CAPN7 but not a protease-inactive mutant of CAPN7 (CAPN7(C290S) ) resulted in a recovery of the rate of EGFR degradation. We found, by immunofluorescence microscopic analysis, that monomeric GFP fused with the protease-inactive mutant of CAPN7 [monomeric green fluorescent protein (mGFP)-CAPN7(C290S) ] was mobilized to EGFR-positive endosomes upon epidermal growth factor stimulation in HeLa cells. Although mGFP-CAPN7(C290S) exhibited dominant-negative effects on EGFR degradation, a deletion mutant of MIT domains in mGFP-CAPN7(C290S) did not have such properties, suggesting that the interaction between the MIT domains and ESCRT proteins is important for the function of CAPN7. Moreover, we found that epidermal growth factor stimulation induces translocation of IST1 from the cytosol to endosomes positive in both EGFR and mGFP-CAPN7(C290S) . When IST1 was knocked down, mGFP-CAPN7(C290S) lost its co-localization with EGFR. These results demonstrate for the first time that the proteolytic activity of CAPN7 is important for the acceleration of EGFR degradation via the endosomal sorting pathway utilizing a part of the ESCRT system. STRUCTURED DIGITAL ABSTRACT: EGFR and CAPN7 colocalize by fluorescence microscopy (View interaction) EGFR, CAPN7 and IST1 colocalize by fluorescence microscopy (View interaction) EEA1 and CAPN7 colocalize by fluorescence microscopy (View interaction) CAPN7 and LAMP1 colocalize by fluorescence microscopy (View interaction).