Arabidopsis thaliana phytaspase: identification and peculiar properties

Characterization of Phytaspase Proteolytic Activity Using Fluorogenic Peptide Substrates

Within the subtilase family of plant proteolytic enzymes, phytaspases are distinguished by their strict substrate cleavage specificity after an aspartate residue preceded by a characteristic tripeptide amino acid motif. This type of recognition resembles that of animal apoptotic proteases, caspases. Phytaspases attract attention not only because they are critically important for the accomplishment of stress-induced death of plant cells, but also due to their ability to specifically process precursor proteins, thus generating bioactive plant peptide hormones, systemin and phytosulfokine. As the activity of phytaspases appears to be essential for life and death decisions made by the plant cell, elaboration of an approach to characterize and quantitate phytaspase proteolytic activity is of importance. Here we provide a protocol for phytaspase activity determination and characterization using fluorogenic peptide substrates. This approach works well, both with purified phytaspase samples, and with crude extracts from plant tissues. We also discuss advantages of the assay, factors that may influence its sensitivity and specificity, as well as possible pitfalls.

Purification of Phytaspases Using a Biotinylated Peptide Inhibitor

Phytaspases are plant subtilisin-like proteases (subtilases) that possess a rarely occurring substrate cleavage specificity. These proteolytic enzymes hydrolyze their substrates strictly after an aspartate residue preceded by a characteristic, though degenerate, tripeptide amino acid motif. Having been initially discovered as proteases involved in the accomplishment of programmed cell death in plants, phytaspases were also demonstrated to be instrumental in specific processing of precursor proteins of several plant peptide hormones, thus generating biologically active peptides. Here we provide a protocol for isolation of active phytaspases from leaves, which was shown to be efficient for a wide range of plant species. The key element of the proposed scheme is the use of a specific and reversible biotinylated peptide aldehyde inhibitor of phytaspases for purification of the enzymes by means of affinity chromatography. We also discuss nuances, pitfalls, and possible alternatives for successful isolation of proteolytically active phytaspases.

Determination of Caspase-Like Activities in Roots by the Use of Fluorogenic Substrates

Activity of proteases in tissues can be influenced by various intrinsic and extrinsic factors. One of the activities that is regularly monitored in organisms ranging from prokaryotes to metazoans is the -aspase-like activity: activity of proteases, which cleave their substrates after the negatively charged amino acid residues, especially the aspartic acid. This activity is also known as the caspase-like activity, since the caspases, metazoan cysteine proteases, are one of the best characterized proteases with Asp-directed activities. Plants do not contain caspases; however, various plant proteases have been shown to exhibit caspase-like activity including saspases, phytaspases, and legumains (VPEs). The activity of these proteases can change in plants in response to stress. Here we present a simple method for monitoring of the caspase-like protease activity in roots, which have been treated with allelopathic extracts, using a set of commercially available caspase substrates. We show that activity towards some, but not all, caspase substrates is upregulated in treated but not control samples. The protocol can be used also for other plant tissues as well as for other stressors.

Identification of Phytaspase Interactors via the Proximity-Dependent Biotin-Based Identification Approach

Proteolytic enzymes are instrumental in various aspects of plant development, including senescence. This may be due not only to their digestive activity, which enables protein utilization, but also to fulfilling regulatory functions. Indeed, for the largest family of plant serine proteases, subtilisin-like proteases (subtilases), several members of which have been implicated in leaf and plant senescence, both non-specific proteolysis and regulatory protein processing have been documented. Here, we strived to identify the protein partners of phytaspase, a plant subtilase involved in stress-induced programmed cell death that possesses a characteristic aspartate-specific hydrolytic activity and unusual localization dynamics. A proximity-dependent biotin identification approach in Nicotiana benthamiana leaves producing phytaspase fused to a non-specific biotin ligase TurboID was employed. Although the TurboID moiety appeared to be unstable in the apoplast environment, several intracellular candidate protein interactors of phytaspase were identified. These were mainly, though not exclusively, represented by soluble residents of the endoplasmic reticulum, namely endoplasmin, BiP, and calreticulin-3. For calreticultin-3, whose gene is characterized by an enhanced expression in senescing leaves, direct interaction with phytaspase was confirmed in an in vitro binding assay using purified proteins. In addition, an apparent alteration of post-translational modification of calreticultin-3 in phytaspase-overproducing plant cells was observed.

Phytosulfokine (PSK) precursor processing by subtilase SBT3.8 and PSK signaling improve drought stress tolerance in Arabidopsis

Increasing drought stress poses a severe threat to agricultural productivity. Plants, however, have evolved numerous mechanisms to cope with such environmental stress. Here we report that the stress-induced production of a peptide signal contributes to stress tolerance. The expression of phytosulfokine (PSK) peptide precursor genes, and transcripts of three subtilisin-like serine proteases, SBT1.4, SBT3.7, and SBT3.8, were found to be up-regulated in response to osmotic stress. Stress symptoms were more pronounced in sbt3.8 loss-of-function mutants and could be alleviated by PSK treatment. Osmotic stress tolerance was improved in plants overexpressing the PSK1 precursor (proPSK1) or SBT3.8, resulting in higher fresh weight and improved lateral root development in transgenic plants compared with wild-type plants. We further showed that SBT3.8 is involved in the biogenesis of the bioactive PSK peptide. ProPSK1 was cleaved by SBT3.8 at the C-terminus of the PSK pentapeptide. Processing by SBT3.8 depended on the aspartic acid residue directly following the cleavage site. ProPSK1 processing was impaired in the sbt3.8 mutant. The data suggest that increased expression of proPSK1 in response to osmotic stress followed by the post-translational processing of proPSK1 by SBT3.8 leads to the production of PSK as a peptide signal for stress mitigation.

Cell Death in Cyanobacteria: Current Understanding and Recommendations for a Consensus on Its Nomenclature

Cyanobacteria are globally widespread photosynthetic prokaryotes and are major contributors to global biogeochemical cycles. One of the most critical processes determining cyanobacterial eco-physiology is cellular death. Evidence supports the existence of controlled cellular demise in cyanobacteria, and various forms of cell death have been described as a response to biotic and abiotic stresses. However, cell death research in this phylogenetic group is a relatively young field and understanding of the underlying mechanisms and molecular machinery underpinning this fundamental process remains largely elusive. Furthermore, no systematic classification of modes of cell death has yet been established for cyanobacteria. In this work, we analyzed the state of knowledge in the field of cyanobacterial cell death. Based on that, we propose unified criterion for the definition of accidental, regulated, and programmed forms of cell death in cyanobacteria based on molecular, biochemical, and morphologic aspects following the directions of the Nomenclature Committee on Cell Death (NCCD). With this, we aim to provide a guide to standardize the nomenclature related to this topic in a precise and consistent manner, which will facilitate further ecological, evolutionary, and applied research in the field of cyanobacterial cell death.

The PSY Peptide Family-Expression, Modification and Physiological Implications

Small post-translationally modified peptides are gaining increasing attention as important signaling molecules in plant development. In the family of plant peptides containing tyrosine sulfation (PSYs), only PSY1 has been characterized at the mature level as an 18-amino-acid peptide, carrying one sulfated tyrosine, and involved in cell elongation. This review presents seven additional homologs in Arabidopsis all sharing high conservation in the active peptide domain, and it shows that PSY peptides are found in all higher plants and mosses. It is proposed that all eight PSY homologs are post-translationally modified to carry a sulfated tyrosine and that subtilisin-like subtilases (SBTs) are involved in the processing of PSY propeptides. The PSY peptides show differential expression patterns indicating that they serve several distinct functions in plant development. PSY peptides seem to be at least partly regulated at the transcriptional level, as their expression is greatly influenced by developmental factors. Finally, a model including a receptor in addition to PSY1R is proposed.

The biogenesis of CLEL peptides involves several processing events in consecutive compartments of the secretory pathway

Post-translationally modified peptides are involved in many aspects of plant growth and development. The maturation of these peptides from their larger precursors is still poorly understood. We show here that the biogenesis of CLEL6 and CLEL9 peptides in Arabidopsis thaliana requires a series of processing events in consecutive compartments of the secretory pathway. Following cleavage of the signal peptide upon entry into the endoplasmic reticulum (ER), the peptide precursors are processed in the cis-Golgi by the subtilase SBT6.1. SBT6.1-mediated cleavage within the variable domain allows for continued passage of the partially processed precursors through the secretory pathway, and for subsequent post-translational modifications including tyrosine sulfation and proline hydroxylation within, and proteolytic maturation after exit from the Golgi. Activation by subtilases including SBT3.8 in post-Golgi compartments depends on the N-terminal aspartate of the mature peptides. Our work highlights the complexity of post-translational precursor maturation allowing for stringent control of peptide biogenesis.

Clathrin-Mediated Endocytosis Delivers Proteolytically Active Phytaspases Into Plant Cells

Phytaspases belong to the family of plant subtilisin-like proteases and are distinct from other family members, as they have strict and rarely occurring aspartate cleavage specificity and unusual localization dynamics. After being secreted into the apoplast of healthy plant tissues, phytaspases are able to return back into cells that have been committed to cell death due to a variety of biotic and abiotic stresses. It was recently discovered that retrograde transport of phytaspases involves clathrin-mediated endocytosis. Here, consequences of phytaspase internalization were studied. Proteolytic activity of phytaspases in the apoplast and intracellular protein fractions obtained from Nicotiana benthamiana leaves containing either endogenous phytaspase only or transiently producing Nicotiana tabacum phytaspase-EGFP protein (NtPhyt-EGFP) was determined. We demonstrated that triggering phytaspase internalization by antimycin A-induced oxidative stress is accompanied by re-distribution of phytaspase activity from the apoplast to the cell interior. Inhibition of clathrin-mediated endocytosis by co-production of the Hub protein prevented phytaspase internalization and phytaspase activity re-localization. Specificity of endocytic uptake of phytaspases was demonstrated by the co-production of an apoplast-targeted mRFP protein marker, which retained its apoplastic localization when phytaspase internalization was essentially complete. Overproduction of NtPhyt-EGFP, but not of the proteolytically inactive phytaspase mutant, per se caused moderate damage in young Nicotiana benthamiana seedlings, whereas antimycin A treatment induced a pronounced loss of cell viability independent of the NtPhyt-EGFP overproduction. Interestingly, inhibition of clathrin-mediated endocytosis abrogated cell death symptoms in both cases. In contrast to stress-induced internalization of tobacco phytaspase, Arabidopsis thaliana phytaspase-EGFP protein (AtPhyt-EGFP) was spontaneously internalized when transiently produced in N. benthamiana leaves. The AtPhyt-EGFP uptake was dependent on clathrin-mediated endocytosis as well, the internalized protein being initially visualized within the membranous vesicles. At later time points, the EGFP tag was cleaved off from AtPhyt, though the elevated level of intracellular AtPhyt proteolytic activity persisted. Our data, therefore, point to clathrin-mediated endocytosis as a means to deliver proteolytically active phytaspases into plant cells. It would be interesting to learn whether or not phytaspases are unique among the large family of plant subtilisin-like proteases in their ability to utilize retrograde trafficking.

The tomato subtilase family includes several cell death-related proteinases with caspase specificity

Phytaspases are Asp-specific subtilisin-like plant proteases that have been likened to animal caspases with respect to their regulatory function in programmed cell death (PCD). We identified twelve putative phytaspase genes in tomato that differed widely in expression level and tissue-specific expression patterns. Most phytaspase genes are tandemly arranged on tomato chromosomes one, four, and eight, and many belong to taxon-specific clades, e.g. the P69 clade in the nightshade family, suggesting that these genes evolved by gene duplication after speciation. Five tomato phytaspases (SlPhyts) were expressed in N. benthamiana and purified to homogeneity. Substrate specificity was analyzed in a proteomics assay and with a panel of fluorogenic peptide substrates. Similar to animal caspases, SlPhyts recognized an extended sequence motif including Asp at the cleavage site. Clear differences in cleavage site preference were observed implying different substrates in vivo and, consequently, different physiological functions. A caspase-like function in PCD was confirmed for five of the seven tested phytaspases. Cell death was triggered by ectopic expression of SlPhyts 2, 3, 4, 5, 6 in tomato leaves by agro-infiltration, as well as in stably transformed transgenic tomato plants. SlPhyts 3, 4, and 5 were found to contribute to cell death under oxidative stress conditions.

Phytaspase-mediated precursor processing and maturation of the wound hormone systemin

Peptide hormones are implicated in many important aspects of plant life and are usually synthesized as precursor proteins. In contrast to animals, data for plant peptide hormone maturation are scarce and the specificity of processing enzyme(s) is largely unknown. Here we tested a hypothesis that processing of prosystemin, a precursor of tomato (Solanum lycopersicum) wound hormone systemin, is performed by phytaspases, aspartate-specific proteases of the subtilase family. Following the purification of phytaspase from tomato leaves, two tomato phytaspase genes were identified, the cDNAs were cloned and the recombinant enzymes were obtained after transient expression in Nicotiana benthamiana. The newly identified tomato phytaspases hydrolyzed prosystemin at two aspartate residues flanking the systemin sequence. Site-directed mutagenesis of the phytaspase cleavage sites in prosystemin abrogated not only the phytaspase-mediated processing of the prohormone in vitro, but also the ability of prosystemin to trigger the systemic wound response in vivo. The data show that the prohormone prosystemin requires processing for signal biogenesis and biological activity. The identification of phytaspases as the proteases involved in prosystemin maturation provides insight into the mechanisms of wound signaling in tomato. Our data also suggest a novel role for cell death-related proteases in mediating defense signaling in plants.