The crystal structures of human calpains 1 and 9 imply diverse mechanisms of action and auto-inhibition.
Calpains are calcium activated cysteine proteases found throughout the animal, plant, and fungi kingdoms; 14 isoforms have been described in the human genome. Calpains have been implicated in multiple models of human disease; for instance, calpain 1 is activated in the brains of individuals with Alzheimer's disease, and the digestive tract specific calpain 9 is down-regulated in gastric cancer cell lines. We have solved the structures of human calpain 1 and calpain 9 protease cores using crystallographic methods; both structures have clear implications for the function of non-catalytic domains of full-length calpains in the calcium-mediated activation of the enzyme. The structure of minicalpain 1 is similar to previously solved structures of the protease core. Auto-inhibition in this system is most likely through rearrangements of a central helical/loop region near the active site cysteine, which occlude the substrate binding site. However, the structure of minicalpain 9 indicates that auto-inhibition in this enzyme is mediated through large intra-domain movements that misalign the catalytic triad. This disruption is reminiscent of the full-length inactive calpain conformation. The structures of the highly conserved, ubiquitously expressed human calpain 1 and the more tissue specific human calpain 9 indicate that although there are high levels of sequence conservation throughout the calpain family, isolated structures of family members are insufficient to explain the molecular mechanism of activation for this group of proteins.
Davis
Tara L
TL
Structural Genomics Consortium and the Department of Physiology, University of Toronto, 100 College Street, Toronto, Ontario, Canada M5G 1L5.
Walker
John R
JR
Finerty
Patrick J
PJ
Mackenzie
Farrell
F
Newman
Elena M
EM
Dhe-Paganon
Sirano
S
D001665:Binding Sites; D002154:Calpain / Q000737:chemistry*; D018360:Crystallography, X-Ray; D015195:Drug Design; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013329:Structure-Activity Relationship
C518121:CAPN9 protein, human; D002154:Calpain; C484806:CAPN1 protein, human