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Crystal structure of calpain reveals the structural basis for Ca 2+ ‐dependent protease activity and a novel mode of enzyme activation
Author(s) -
Hosfield Christopher M.,
Elce John S.,
Davies Peter L.,
Jia Zongchao
Publication year - 1999
Publication title -
the embo journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.484
H-Index - 392
eISSN - 1460-2075
pISSN - 0261-4189
DOI - 10.1093/emboj/18.24.6880
Subject(s) - calpain , protease , proteases , biology , active site , cysteine protease , catalytic triad , protein subunit , enzyme activator , enzyme , biochemistry , protein structure , calmodulin , biophysics , gene
The combination of thiol protease activity and calmodulin‐like EF‐hands is a feature unique to the calpains. The regulatory mechanisms governing calpain activity are complex, and the nature of the Ca 2+ ‐induced switch between inactive and active forms has remained elusive in the absence of structural information. We describe here the 2.6 Å crystal structure of m‐calpain in the Ca 2+ ‐free form, which illustrates the structural basis for the inactivity of calpain in the absence of Ca 2+ . It also reveals an unusual thiol protease fold, which is associated with Ca 2+ ‐binding domains through heterodimerization and a C 2 ‐like β‐sandwich domain. Strikingly, the structure shows that the catalytic triad is not assembled, indicating that Ca 2+ ‐binding must induce conformational changes that re‐orient the protease domains to form a functional active site. The α‐helical N‐terminal anchor of the catalytic subunit does not occupy the active site but inhibits its assembly and regulates Ca 2+ ‐sensitivity through association with the regulatory subunit. This Ca 2+ ‐dependent activation mechanism is clearly distinct from those of classical proteases.