Substrates of Caspases
During the execution phase of apoptosis, several proteins are cleaved
which were shown to be substrates of caspases
(see Table ). Since apoptosis
tends to be morphologically similar among different cells, it could be
expected that the most important substrates (involved in the characteristic
features of apoptosis) are ubiquitous and are evolutionary conserved,
at least at their cleavage sites. Indeed, a relatively limited number of
targets for proteolysis during apoptosis was identified and many of those
cleaved proteins probably directly or indirectly contribute to apoptotic events. The substrate specificities of caspases (as given in the Table ) were often determined in vitro, so it is still not clear which caspase(s) is/are responsible for cleavage under physiological conditions under varying substrate concentrations, pH or the tissue specific expression of specific regulatory factors. The caspases have overlapping substrate specificities that suggest at least partially overlapping functions. Poly(ADP-ribose)polymerase and its derived tetrapeptide DEVD, for example, can be cleaved by virtually all caspases, but Caspase-3 and Caspase-7 are by far most efficient in cleaving PARP. Caspase-3 and Caspase-7 are very similiar in their primary cleavage site specificities: both show a very strong preference for Asp in the P4 position of the cleavage site and both are efficiently inhibited by the AC-DEVD-CHO tetrapeptide. So it is likely that many substrates described as being cleaved by Caspase-3 (see Table ) will also be found to be cleaved by Caspase-7.
In contrast, caspase-6 ist the only caspase known to cleave lamin A
with the recognition sequence VEID-N, what reflects the preference of
Caspase-6 for a Val (branched amino acid) in P4 position. Caspase-1
cleaves the cytokine pro-IL-1beta. Since Caspase-4 (and maybe Caspase-5)
displays similar specificities in the cleavage site of testpeptides it
maybe able to substitute for some Caspase-1 functions.
In general, the most significant differences in caspase specificities are
found in substrate position P4. In contrast, P3 specificities a are similiar
between the caspases, and in P2 a wide range of amino acids is tolerated.
This is also consistent with the structures of Caspase-1 and Caspase-3 which
show that the P3 and P2 side chains of peptidic inhibitors are relatively
solvent-exposed, and that their P4 side chains occupy defined pockets that
vary significantly between those enzymes. The P4 preferences can be
categorized as hydrophobic (Caspase-1, -4, and -6) or Asp (caspase-2, -3,
References: Talaniant et al., 1997, J. Biol. Chem., vol. 272, no.15: 9677-82,
Thornberry et al., 1997, J. Biol. Chem., vol. 272, no.29: 17907-17911, and
Martin and Green, 1995, Cell, vol.82: 349-352.