Plant phytaspases and animal caspases: structurally unrelated death proteases with a common role and specificity.

Proteases with an aspartate cleavage specificity are known to contribute to programmed cell death (PCD) in animals and plants. In animal cells this proteolytic activity belongs to caspases, a well-characterized family of cysteine-dependent death proteases. Plants, however, lack caspase homologs and thus the origin of this type of proteolytic activity in planta was poorly understood. Here, we review recent data demonstrating that a plant serine-dependent protease, phytaspase, shares cleavage specificity and a role in PCD analogous to that of caspases. However, unlike caspases, regulation of phytaspase-mediated cleavage of intracellular target proteins appears to be attained not at the level of proenzyme processing/activation, which occurs, in the case of phytaspase, autocatalytically and constitutively. Rather, the mature phytaspase is excluded from healthy cells into the apoplast and is allowed to re-enter cells upon the induction of PCD. Thus, PCD-related proteases in animals and plants display both common features and important distinctions.