Programmed Cell Death Regulation by Plant Proteases with Caspase-Like Activity

Abstract

Caspases are key proteases for the initiation and completion of animal apoptosis, and caspase enzymatic activities can be detected in plant cell extracts. In addition, various caspase inhibitors block plant programmed cell death (PCD), suggesting that the proteases involved are required for completion of the process. Yet plants do not have caspase orthologous genes in their genomes, and the proteases phylogenetically closest to caspases, the metacaspases, do not have caspase activities. A major challenge has therefore been to identify the plant proteases that are responsible for caspase-like activities in order to understand how, or indeed if, they are related to animal caspases. We now know three proteases with caspase-like activity: vacuolar processing enzyme (VPE), a legumain that cleaves caspase-1 synthetic substrates; phytaspase, a subtilisin-like protease that cleaves caspase-6 synthetic substrates; and the proteasome subunit PBA1 that has caspase-3-like activity. Although these proteases participate in plant PCD, they are unrelated to animal caspases and do not directly form a protease cascade. Additional plant proteases with caspase-like enzymatic activity remain to be uncovered.