Caspases play an important role in the ability of animal cells to kill themselves by apoptosis.
Caspase activity is regulated in vivo by members of three distinct
protease inhibitor families, two of which, baculovirus p35 and members of the inhibitor of apoptosis (IAP) family, are thought to be
caspase specific. However,
caspases are members of the clan of
cysteine proteases designated CD, which also includes animal and plant legumains, and the bacterial
proteases clostripain,
gingipain-R and
gingipain-K. Since these
proteases have been proposed to have a common mechanism and evolutionary origin, we hypothesized that the
caspase inhibitors may also regulate these other
proteases. We tested this hypothesis by examining the effect of the natural
caspase inhibitors on other members of
protease clan CD. The IAP family
proteins were found to have only a slight inhibitory effect on
gingipain-R. The
cowpox viral
cytokine-response modifier A (CrmA)
serpin had no effect on any of the
proteases tested but a single point mutation of CrmA (
Asp-->Lys) resulted in strong inhibition of
gingipain-K. More substantial, with respect to the hypothesis, was the strong inhibition of
gingipain-K by wild-type p35. The site in p35, required for inhibition of
gingipain-K, was mapped to Lys94, seven residues C-terminal to the
caspase inhibitory site. Our data indicate that the virally encoded
caspase inhibitors have adopted a mechanism that allows them to regulate disparate members of clan CD
proteases.