Phytaspase, a relocalisable cell death promoting plant protease with caspase specificity

Abstract

Caspases are cysteine-dependent proteases and are important components of animal apoptosis. They introduce specific breaks after aspartate residues in a number of cellular proteins mediating programmed cell death (PCD). Plants encode only distant homologues of caspases, the metacaspases that are involved in PCD, but do not possess caspase-specific proteolytic activity. Nevertheless, plants do display caspase-like activities indicating that enzymes structurally distinct from classical caspases may operate as caspase-like proteases. Here, we report the identification and characterisation of a novel PCD-related subtilisinlike protease from tobacco and rice named phytaspase (plant aspartate-specific protease) that possesses caspase specificity distinct from that of other known caspase-like proteases. We provide evidence that phytaspase is synthesised as a proenzyme, which is autocatalytically processed to generate the mature enzyme. Overexpression and silencing of the phytaspase gene showed that phytaspase is essential for PCD-related responses to tobacco mosaic virus and abiotic stresses. Phytaspase is constitutively secreted into the apoplast before PCD, but unexpectedly is re-imported into the cell during PCD providing insights into how phytaspase operates.

NaStEP: A Proteinase Inhibitor Essential to Self-Incompatibility and a Positive Regulator of HT-B Stability inNicotiana alataPollen Tubes

“…However, the putative subtilisin-like component has yet to be identified, and a control mechanism for the NaStEP action remains unknown, since this protein is also present during SI pollination events, where rejection does take place. Besides their possible involvement in SI, plant proteases and protease inhibitors have been increasingly associated with several developmental processes such as plant-pathogen-insect interactions (Li et al, 2008;Hartl et al, 2010) and more recently in program cell death (Chichkova et al, 2010), which makes it a very intense field of study.…”

Morphological classification of plant cell deaths

“…7 Furthermore, the activation of plant proteases that possess caspase-like activity has not been shown to lead to apoptotic morphology. [10][11][12] Definition of 'Vacuolar' Plant Cell Death…”

Tobacco phytaspase: Successful expression in a heterologous system

“…[1][2][3][4] In spite of their structural dissimilarities, phytaspases are functionally homologous to animal caspases, as they both hydrolyze multiple synthetic caspase substrates. Phytaspases are highly specific in cleaving these substrates after aspartic acid residues.…”

“…Phytaspases are highly specific in cleaving these substrates after aspartic acid residues. 1,5 Subtilisin-like serine proteases (S8 family), commonly termed subtilases, are the second most ubiquitous serine proteases and can be found in bacteria, eukaryotes and even viruses. 6,7 Subtilases possess the same catalytic triad found in chymotrypsin, differing only in arrangement; Asp-His-Ser is found in subtilases, and His-Asp-Ser is found in chymotrypsin.…”