Caspases are cysteine proteases that cleave their substrates after an aspartate residue. Besides their multiple vital roles ranging from immune regulation to spermatogenesis, they are crucial in most cell death pathways, representing a sort of executing sword in the hands of apoptosis. In 2000, a psi-Blast in-silico approach led Uren et al. to identify two novel caspase-related families: metacaspases and paracaspases. Paracaspases are involved in the development of MALT lymphoma, but not in cell death execution, and are found both in eukaryotes owning caspases (animals), as well as in organisms lacking caspases. Metacaspases, on the other hand, are found only in eukaryotes that are devoid of caspases, for example, plants, protists and fungi. Similar to caspases, they contain a caspase-specific catalytic diad of histidine and cysteine, as well as a caspase-like secondary structure.
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