Proteolytic activation of caspases is a key step in the process of apoptosis. According to their primary structure, caspases can be divided into a group with a long prodomain and a group with a short prodomain. Whereas long prodomains play a role in autocatalytic processing, little is known about the function of the short prodomain, for example the prodomain of caspase-3. We constructed caspase-3 variants lacking the prodomain and overexpressed these in HeLa and yeast cells. We found that removal of the caspase-3 prodomain resulted in spontaneous proteolytic activation of the protein when expressed in HeLa cells. This processing was only partially autocatalytic, as demonstrated by a catalytically inactive caspase-3 mutant. Co-expression of the anti-apoptotic protein XIAP (X-chromosome-linked inhibitor of apoptosis protein) completely blocked the observed spontaneous activation, which excluded a direct involvement of caspase-8. Our findings indicate that the short prodomain of caspase-3 serves as a silencing component in mammalian cells by retaining this executioner caspase in an inactive state.

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