Caspase-1 was cloned by virtue of its Interleukin-1b (IL-1b ) cleaving activity. IL-1b mediates a wide range of immune and inflammatory responses. The active cytokine is generated by proteolytic cleavage of an inactive precursor. Recombinant expression in COS-7 cells enabled the cells to process precursor IL-1b to the mature form. The gene encoding the protease was mapped to chromosomal band 11q23, a site frequently involved in rearrangement in human cancers. Caspase-1 is found predominantly in the cytoplasm of cells as the p45 pro-form (Ayala et al., 1994), although some is also localised to the external cell surface membrane, where it activates pro-IL-1b to its mature form during secretion (Singer et al., 1995).
Sequence analysis indicated that the enzyme itself may undergo (auto)proteolytic processing, since all four cleavage sites i.e. Asp-103, Asp-119, Asp-297 and Asp-316, arise at Asp-Xaa bonds. Caspase-1 is composed of two nonidentical subunits, p20 and p10, that are derived from a single proenzyme (p45). (Cerretti et al., 1992; Thornberry et al., 1992; Howard et al., 1991; Nett et al., 1992). Following the initial cleavage at Asp-297-Ser-298, autoproteolysis occurs in a series of steps, generating fragments of increasing activity resulting in the formation of active heterodimeric p20/p10 caspase-1 (Ramage et al., 1995; Yamin et al., 1996).
The crystal structure of ICE in complex with a tetrapeptide inhibitor has revealed that active ICE, lacking the prodomain and linker, is a tetramer of two p20 subunits surrounding two adjacent p10 subunits that form the dimer-dimer interface (Walker et al., 1994; Wilson et al., 1994). The active site of ICE is comprised of amino acid residues from both p20 and p10 with Cys285 and His237 forming a catalytic dyad in the active site (Wilson et al., 1994). The hall mark of all caspases namely the active-site pentapeptide, Gln-Ala,Cys-Xaa-Gly, is in the p20 subunit. However, amino acid residues involved in forming the Asp pocket include Arg-179, Gln-283, Arg-341 and Ser-347 with only the first two residues being contributed by the p20 subunit. The two arginine residues (Arg-179 and Arg-341) form hydrogen bonds with the P1 Asp residue of the substrate, and mutation of these residues results in the loss of catalytic activity. Side chains of residues of p10 from Val-338 to Pro-343 interact with P2-P4 sites of the inhibitor (Walker et al., 1994; Wilson et al., 1994).
Starting from the specific quaternary structure of ICE, two models for the maturation of the ICE pro-enzyme were proposed. The first model assumes an association of two p45 precursor proteins and subsequent processing. Hereby was stated that the p20 and p10 subunits forming one active site are derived from two distinct precursor proteins after autoprocessing of interdigitating precursors (Wilson et al., 1994). The second model supposes processing followed by association of mature proteins (Wilson et al., 1994). Using the yeast two-hybrid system it was demonstrated that the prodomain of caspase-1 is absolutely required for dimerisation and auto-activation of caspase-1 (Van Criekinge et al., 1996). Indeed time course experiments showed that the 10-kDa subunit was released from the 45-kDa precursor before the 20-kDa subunit, implying that the N-terminal portion of the precursor is released last and may play a regulatory role (Ramage et al., 1995). Interestingly by use of reverse transcriptase-PCR, four alternatively spliced isoforms of caspase-1 were identified that have different effects on apoptosis (Alnemri et al., 1995)