Apoptosis is a controlled form of cell suicide that can be triggered by a variety of internal or external signals (for reviews see Thompson, 1995; White, 1996; Jacobson et al., 1997). Recent studies have demonstrated that specific protease activation is a critical element of the apoptotic process. In all systems described to date, the implicated proteases are members of a novel cysteine protease family which share an absolute requirement for aspartic acid in the P 1 position of proteolytic substrates (Martin and Green, 1995; Chinnaiyan and Dixit, 1996; Nicholson and Thornberry, 1997; Salvesen and Dixit, 1997). The 12 described members of the mammalian family of these proteases, which are all present as inactive precursors in resting cells, have been termed caspases (for cysteinyl aspartate-specific proteinase [Alnemri et al., 1996]). During apoptosis, these precursors are cleaved at Asp-X sites, generating a large and small subunit, which together constitute the active protease. Numerous observations, including identification of the individual substrate specificities of the caspases, suggest that this protease family forms a multitiered proteolytic system that collectively responds to apoptotic stimuli (activator caspases) and Address all correspondence to Antony Rosen, Johns Hopkins Universit
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