[Abstract unavailable

Vacuolar processing enzyme (VPE) is a cysteine proteinase originally identified as the proteinase responsible for the maturation and activation of vacuolar proteins in plants, and it is known to be an ortholog of animal asparaginyl endopeptidase (AEP/VPE/legumain). VPE has been shown to exhibit enzymatic properties similar to that of caspase 1, which is a cysteine protease that mediates the programmed cell death (PCD) pathway in animals. Although there is limited sequence identity between VPE and caspase 1, their predicted three-dimensional structures revealed that the essential amino-acid residues for these enzymes form similar pockets for the substrate peptide YVAD. In contrast to the cytosolic localization of caspases, VPE is localized in vacuoles. VPE provokes vacuolar rupture, initiating the proteolytic cascade leading to PCD in the plant immune response. It has become apparent that the VPE-dependent PCD pathway is involved not only in the immune response, but also in the responses to a variety of stress inducers and in the development of various tissues. This review summarizes the current knowledge on the contribution of VPE to plant PCD and its role in vacuole-mediated cell death, and it also compares VPE with the animal cell death executor caspase 1.

Balancing repair and degradation is essential for maintaining organellar and cellular homeostasis. Peroxisomes are ubiquitous organelles in eukaryotic cells that play pivotal roles in cell survival. However, the quality control mechanism used to maintain peroxisomes is unclear. Here, we demonstrate that LON protease 2 (LON2), which is encoded by ABERRANT PEROXISOME MORPHOLOGY 10 (APEM10), is responsible for the functional transition of peroxisomes with autophagy. The Arabidopsis apem10 mutant displayed accelerated peroxisome degradation and a dramatically reduced number of peroxisomes. LON2 deficiency caused enhanced peroxisome degradation by autophagy, and peroxisomal proteins accumulated in the cytosol due to a decrease in the number of peroxisomes. We also show the proteolytic consequence of LON2 for the degradation of peroxisomal proteins, and we demonstrated that unnecessary proteins are eliminated by LON2- and autophagy-dependent degradation pathways during the functional transition of peroxisomes. LON2 plays dual roles as an ATP-dependent protease and a chaperone. We show that the chaperone domain of LON2 is essential for the suppression of autophagy, whereas its peptidase domain interferes with this chaperone function, indicating that intramolecular modulation between the proteolysis and chaperone functions of LON2 regulates degradation of peroxisomes by autophagy.

Summary Vacuolar processing enzyme (VPE) is a cysteine‐type endopeptidase that has a substrate‐specificity for asparagine or aspartic acid residues and cleaves peptide bonds at their carboxyl‐terminal side. Various vacuolar proteins are synthesized as larger proprotein precursors, and VPE is an important initiator of maturation and activation of these proteins. It mediates programmed cell death (PCD) by provoking vacuolar rupture and initiating the proteolytic cascade leading to PCD. Vacuolar processing enzyme also possesses a peptide ligation activity, which is responsible for producing cyclic peptides in several plant species. These unique functions of VPE support developmental and environmental responses in plants. The number of VPE homologues is higher in angiosperm species, indicating that there has been differentiation and specialization of VPE function over the course of evolution. Angiosperm VPEs are separated into two major types: the γ‐type VPEs, which are expressed mainly in vegetative organs, and the β‐type VPEs, whose expression occurs mainly in storage organs; in eudicots, the δ‐type VPEs are further separated within γ‐type VPEs. This review also considers the importance of processing and peptide ligation by VPE in vacuolar protein maturation.

Microautophagy is a type of autophagy. It is characterized by direct enclosing with the vacuolar/lysosomal membrane, which completes the isolation and uptake of cell components in the vacuole. Several publications present evidence that plants exhibit microautophagy. Plant microautophagy is involved in anthocyanin accumulation in the vacuole, eliminating damaged chloroplasts and degrading cellular components during starvation. However, information on the molecular mechanism of microautophagy is less available than that on the general macroautophagy, because the research focusing on microautophagy has not been widely reported. In yeast and animals, it is suggested that microautophagy can be classified into several types depending on morphology and the requirements of autophagy-related (ATG) genes. This review summarizes the studies on plant microautophagy and discusses possible techniques for a future study in this field while taking into account the information on microautophagy obtained from yeast and animals.