Direct cleavage by the calcium-activated protease calpain can lead to inactivation of caspases.
Caspases, a unique family of cysteine proteases involved in cytokine activation and in the execution of apoptosis can be sub-grouped according to the length of their prodomain. Long prodomain caspases such as caspase-8 and caspase-9 are believed to act mainly as upstream caspases to cleave downstream short prodomain caspases such as caspases-3 and -7. We report here the identification of caspases as direct substrates of calcium-activated proteases, calpains. Calpains cleave caspase-7 at sites distinct from those of the upstream caspases, generating proteolytically inactive fragments. Caspase-8 and caspase-9 can also be directly cleaved by calpains. Two calpain cleavage sites in caspase-9 have been identified by N-terminal sequencing of the cleaved products. Cleavage of caspase-9 by calpain generates truncated caspase-9 that is unable to activate caspase-3 in cell lysates. Furthermore, direct cleavage of caspase-9 by calpain blocks dATP and cytochrome-c induced caspase-3 activation. Therefore our results suggest that calpains may act as negative regulators of caspase processing and apoptosis by effectively inactivating upstream caspases.
Chua
B T
BT
Institute of Molecular and Cell Biology, 30 Medical Drive, Singapore 117609, Singapore.
Guo
K
K
Li
P
P
D000818:Animals; D017209:Apoptosis; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002154:Calpain / Q000378:metabolism*; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism; D002460:Cell Line; D006801:Humans; D006868:Hydrolysis; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity
D061945:Caspase Inhibitors; D011994:Recombinant Proteins; D002154:Calpain; D020169:Caspases; D002118:Calcium