Towards decoding the regulation principles of vacuolar processing enzymes.

Plant legumains, also known as vacuolar processing enzymes (VPEs) or asparaginyl endopeptidases (AEPs), are cysteine proteases involved in protein maturation, seed development, and programmed cell death. In addition to their proteolytic function, plant legumains exhibit ligase and peptide cyclase activity, highlighting their catalytic versatility and biotechnological potential. This review summarizes current insights into the structural and regulatory mechanisms governing plant legumain activity. We discuss how domain architecture, autocatalytic activation, substrate-binding sites, pH dependence, oligomeric state, and endogenous inhibitors contribute to the regulation of protease and ligase activities in plant legumains. Despite substantial progress, important questions remain, particularly regarding the specific regulatory roles of individual structural elements, highlighting the need for further investigation.