Inhibition of CED-3 zymogen activation and apoptosis in Caenorhabditis elegans by caspase homolog CSP-3

Abstract

Caspases are cysteine proteases that have a central role in triggering apoptosis; thus, it is essential to tightly control their activity. Now a caspase inhibitor has been identified in Caenorhabditis elegans: CSP-3 is a caspase homolog that associates with the CED-3 zymogen, inhibiting its autoactivation. Inhibitor of apoptosis (IAP) proteins have a crucial role in apoptosis, through negative regulation of caspases in species from fruitflies to mammals. In Caenorhabditis elegans, however, no IAP homolog or caspase inhibitor has been identified, calling into question how the cell-killing caspase CED-3 can be negatively regulated. Here we show that inactivation of the C. elegans csp-3 gene, which encodes a protein similar to the small subunit of the CED-3 caspase, causes cells that normally live to undergo apoptosis in a CED-3–dependent manner. Biochemical analysis reveals that CSP-3 associates with the large subunit of the CED-3 zymogen and inhibits zymogen autoactivation. However, CSP-3 does not block CED-3 activation induced by CED-4, nor does it inhibit the activity of the activated CED-3 protease. Therefore CSP-3 uses a previously unreported mechanism to protect cells from apoptosis.