A plant alternative to animal caspases: subtilisin-like proteases

Abstract

Activities displaying caspase cleavage specificity have been well documented in various plant programmed cell death (PCD) models. However, plant genome analyses have not revealed clear orthologues of caspase genes, indicating that enzyme(s) structurally unrelated yet possessing caspase specificity have functions in plant PCD. Here, we review recent data showing that some caspase-like activities are attributable to the plant subtilisin-like proteases, saspases and phytaspases. These proteases hydrolyze a range of tetrapeptide caspase substrates following the aspartate residue. Data obtained with saspases implicate them in the proteolytic degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) during biotic and abiotic PCD, whereas phytaspase overproducing and silenced transgenics provide evidence that phytaspase regulates PCD during both abiotic (oxidative and osmotic stresses) and biotic (virus infection) insults. Like caspases, phytaspases and saspases are synthesized as proenzymes, which are autocatalytically processed to generate a mature enzyme. However, unlike caspases, phytaspases and saspases appear to be constitutively processed and secreted from healthy plant cells into the intercellular space. Apoplastic localization presumably prevents enzyme-mediated protein fragmentation in the absence of PCD. In response to death-inducing stimuli, phytaspase has been shown to re-localize to the cell interior. Thus, plant PCD-related proteases display both common (D-specific protein fragmentation during PCD) and distinct (enzyme structure and activity regulation) features with animal PCD-related proteases. Plants, like animals, employ programmed cell death (PCD) for a variety of purposes including development (e.g., xylem formation, seed germination and senescence), stress responses (e.g., salt, cold, heat) and defence (e.g., hypersensitive response (HR)). Although it is becoming increasingly evident that multiple forms of PCD exist in both plants and animals, 1,2 certain forms of PCD in both kingdoms share a number of hallmarks, such as DNA fragmentation, cytochrome c release from mitochondria, cell shrinkage, and so on.3 However, despite extensive similarities, the equivalence of the molecular mechanisms regulating plant PCD is less clearly understood. Among the more surprising findings is an absence of caspase orthologues in plants. Because caspases, a family of cysteine-dependent aspartate-specific proteases, are known to have important functions in the initiation and execution of PCD in animals through cleavage of a number of protein targets, their apparent absence in plants poses a dilemma. Either the principle of specific protein fragmentation at aspartate residues does not operate during PCD in plants or the function of caspases is assumed by other protease(s) that are structurally unrelated to caspases.Evidence accumulated during the past decade indicates that, in at least some cases, the second scenario is correct and that caspase-like (in a functional rather than a structural sense) prote...

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