Purification and characterization of cathepsin B-like cysteine protease from cotyledons of daikon radish, Raphanus sativus

PDX proteins from Arabidopsis thaliana as novel substrates of cathepsin B: implications for vitamin B6 biosynthesis regulation

Vitamin B6 is a critical molecule for metabolism, development, and stress sensitivity in plants. It is a cofactor for numerous biochemical reactions, can serve as an antioxidant, and has the potential to increase tolerance against both biotic and abiotic stressors. Due to the importance of vitamin B6, its biosynthesis is likely tightly regulated. Plants can synthesize vitamin B6 de novo via the concerted activity of Pyridoxine Biosynthesis Protein 1 (PDX1) and PDX2. Previously, PDX proteins have been identified as targets for ubiquitination, indicating they could be marked for degradation by two highly conserved pathways: the Ubiquitin Proteasome Pathway (UPP) and the autophagy pathway. Initial experiments show that PDXs are in fact degraded, but surprisingly, in a ubiquitin-independent manner. Inhibitor studies pointed toward cathepsin B, a conserved lysosomal cysteine protease, which is implicated in both programed cell death and autophagy in humans and plants. In plants, cathepsin Bs are poorly described, and no confirmed substrates have been identified. Here, we present PDX proteins from Arabidopsis thaliana as interactors and substrates of a plant Cathepsin B. These findings not only describe a novel cathepsin B substrate in plants, but also provide new insights into how plants regulate de novo biosynthesis of vitamin B6.

Plant cathepsin B, a versatile protease

Plant proteases are essential enzymes that play key roles during crucial phases of plant life. Some proteases are mainly involved in general protein turnover and recycle amino acids for protein synthesis. Other proteases are involved in cell signalling, cleave specific substrates and are key players during important genetically controlled molecular processes. Cathepsin B is a cysteine protease that can do both because of its exopeptidase and endopeptidase activities. Animal cathepsin B has been investigated for many years, and much is known about its mode of action and substrate preferences, but much remains to be discovered about this potent protease in plants. Cathepsin B is involved in plant development, germination, senescence, microspore embryogenesis, pathogen defence and responses to abiotic stress, including programmed cell death. This review discusses the structural features, the activity of the enzyme and the differences between the plant and animal forms. We discuss its maturation and subcellular localisation and provide a detailed overview of the involvement of cathepsin B in important plant life processes. A greater understanding of the cell signalling processes involving cathepsin B is needed for applied discoveries in plant biotechnology.

Genome-wide identification and in-silico analysis of papain-family cysteine protease encoding genes in Tetrahymena thermophila

Tetrahymena thermophila is a promising host for recombinant protein production, but its utilization in biotechnology is mostly limited due to the presence of intracellular and extracellular papain-family cysteine proteases (PFCPs). In this study, we employed bioinformatics approaches to investigate the T. thermophila PFCP genes and their encoded proteases (TtPFCPs), the most prominent protease family in the genome. Results from the multiple sequence alignment, protein modeling, and conserved motif analyses revealed that all TtPFCPs showed considerably high homology with mammalian cysteine cathepsins and contained conserved amino acid motifs. The total of 121 TtPFCP-encoding genes, 14 of which were classified as non-peptidase homologs, were found. Remaining 107 true TtPFCPs were divided into four distinct subgroups depending on their homology with mammalian lysosomal cathepsins: cathepsin L-like (TtCATLs), cathepsin B-like (TtCATBs), cathepsin C-like (TtCATCs), and cathepsin X-like (TtCATXs) PFCPs. The majority of true TtPFCPs (96 out of the total) were in TtCATL-like peptidase subgroup. Both phylogenetic and chromosomal localization analyses of TtPFCPs supported the hypothesis that TtPFCPs likely evolved through tandem gene duplication events and predominantly accumulated on micronuclear chromosome 5. Additionally, more than half of the identified TtPFCP genes are expressed in considerably low quantities compared to the rest of the TtPFCP genes, which are expressed at a higher level. However, their expression patterns fluctuate based on the stage of the life cycle. In conclusion, this study provides the first comprehensive in-silico analysis of TtPFCP genes and encoded proteases. The results would help designing an effective strategy for protease knockout mutant cell lines to discover biological function and to improve the recombinant protein production in T. thermophila.

Activity-based proteomics reveals nine target proteases for the recombinant protein-stabilizing inhibitor SlCYS8 in Nicotiana benthamiana

Co-expression of protease inhibitors like the tomato cystatin SlCYS8 is useful to increase recombinant protein production in plants, but key proteases involved in protein proteolysis are still unknown. Here, we performed activity-based protein profiling to identify proteases that are inhibited by SlCYS8 in agroinfiltrated Nicotiana benthamiana. We discovered that SlCYS8 selectively suppresses papain-like cysteine protease (PLCP) activity in both apoplastic fluids and total leaf extracts, while not affecting vacuolar-processing enzyme and serine hydrolase activity. A robust concentration-dependent inhibition of PLCPs occurred in vitro when purified SlCYS8 was added to leaf extracts, indicating direct cystatin-PLCP interactions. Activity-based proteomics revealed that nine different Cathepsin-L/-F-like PLCPs are strongly inhibited by SlCYS8 in leaves. By contrast, the activity of five other Cathepsin-B/-H-like PLCPs, as well as 87 Ser hydrolases, was unaffected by SlCYS8. SlCYS8 expression prevented protein degradation by inhibiting intermediate and mature isoforms of granulin-containing proteases from the Resistant-to-Desiccation-21 (RD21) PLCP subfamily. Our data underline the key role of endogenous PLCPs on recombinant protein degradation and reveal candidate proteases for depletion strategies.

The two cathepsin B-like proteases of Arabidopsis thaliana are closely related enzymes with discrete endopeptidase and carboxydipeptidase activities

The genome of the model plant Arabidopsis thaliana encodes three paralogues of the papain-like cysteine proteinase cathepsin B (AtCathB1, AtCathB2 and AtCathB3), whose individual functions are still largely unknown. Here we show that a mutated splice site causes severe truncations of the AtCathB1 polypeptide, rendering it catalytically incompetent. By contrast, AtCathB2 and AtCathB3 are effective proteases which display comparable hydrolytic properties and share most of their substrate specificities. Site-directed mutagenesis experiments demonstrated that a single amino acid substitution (Gly336→Glu) is sufficient to confer AtCathB2 with the capacity to tolerate arginine in its specificity-determining S2 subsite, which is otherwise a hallmark of AtCathB3-mediated cleavages. A degradomics approach utilizing proteome-derived peptide libraries revealed that both enzymes are capable of acting as endopeptidases and exopeptidases, releasing dipeptides from the C-termini of substrates. Mutation of the carboxydipeptidase determinant His207 also affected the activity of AtCathB2 towards non-exopeptidase substrates, highlighting mechanistic differences between plant and human cathepsin B. This was also noted in molecular modeling studies which indicate that the occluding loop defining the dual enzymatic character of cathepsin B does not obstruct the active-site cleft of AtCathB2 to the same extent as in its mammalian orthologues.

The papain-like cysteine proteinases NbCysP6 and NbCysP7 are highly processive enzymes with substrate specificities complementary to Nicotiana benthamiana cathepsin B

The tobacco-related plant Nicotiana benthamiana is gaining interest as a versatile host for the production of monoclonal antibodies and other protein therapeutics. However, the susceptibility of plant-derived recombinant proteins to endogenous proteolytic enzymes limits their use as biopharmaceuticals. We have now identified two previously uncharacterized N. benthamiana proteases with high antibody-degrading activity, the papain-like cysteine proteinases NbCysP6 and NbCysP7. Both enzymes are capable of hydrolysing a wide range of synthetic substrates, although only NbCysP6 tolerates basic amino acids in its specificity-determining S2 subsite. The overlapping substrate specificities of NbCysP6 and NbCysP7 are also documented by the closely related properties of their other subsites as deduced from the action of the enzymes on proteome-derived peptide libraries. Notable differences were observed to the substrate preferences of N. benthamiana cathepsin B, another antibody-degrading papain-like cysteine proteinase. The complementary activities of NbCysP6, NbCysP7 and N. benthamiana cathepsin B indicate synergistic roles of these proteases in the turnover of recombinant and endogenous proteins in planta, thus representing a paradigm for the shaping of plant proteomes by the combined action of papain-like cysteine proteinases.

Inhibition of cathepsin B by caspase-3 inhibitors blocks programmed cell death in Arabidopsis

Programmed cell death (PCD) is used by plants for development and survival to biotic and abiotic stresses. The role of caspases in PCD is well established in animal cells. Over the past 15 years, the importance of caspase-3-like enzymatic activity for plant PCD completion has been widely documented despite the absence of caspase orthologues. In particular, caspase-3 inhibitors blocked nearly all plant PCD tested. Here, we affinity-purified a plant caspase-3-like activity using a biotin-labelled caspase-3 inhibitor and identified Arabidopsis thaliana cathepsin B3 (AtCathB3) by liquid chromatography with tandem mass spectrometry (LC-MS/MS). Consistent with this, recombinant AtCathB3 was found to have caspase-3-like activity and to be inhibited by caspase-3 inhibitors. AtCathepsin B triple-mutant lines showed reduced caspase-3-like enzymatic activity and reduced labelling with activity-based caspase-3 probes. Importantly, AtCathepsin B triple mutants showed a strong reduction in the PCD induced by ultraviolet (UV), oxidative stress (H₂O₂, methyl viologen) or endoplasmic reticulum stress. Our observations contribute to explain why caspase-3 inhibitors inhibit plant PCD and provide new tools to further plant PCD research. The fact that cathepsin B does regulate PCD in both animal and plant cells suggests that this protease may be part of an ancestral PCD pathway pre-existing the plant/animal divergence that needs further characterisation.

Nicotiana benthamiana cathepsin B displays distinct enzymatic features which differ from its human relative and aleurain-like protease

The tobacco-related plant species Nicotiana benthamiana has recently emerged as a versatile expression platform for the rapid generation of recombinant biopharmaceuticals, but product yield and quality frequently suffer from unintended proteolysis. Previous studies have highlighted that recombinant protein fragmentation in plants involves papain-like cysteine proteinases (PLCPs). For this reason, we have now characterized two major N. benthamiana PLCPs in detail: aleurain-like protease (NbALP) and cathepsin B (NbCathB). As typical for PLCPs, the precursor of NbCathB readily undergoes autocatalytic activation when incubated at low pH. On the contrary, maturation of NbALP requires the presence of a cathepsin L-like PLCP as processing enzyme. While the catalytic features of NbALP closely resemble those of its mammalian homologue cathepsin H, NbCathB displays remarkable differences to human cathepsin B. In particular, NbCathB appears to be a far less efficient peptidyldipeptidase (removing C-terminal dipeptides) than its human counterpart, suggesting that it functions primarily as an endopeptidase. Importantly, NbCathB was far more efficient than NbALP in processing the human anti-HIV-1 antibody 2F5 into fragments observed during its production in N. benthamiana. This suggests that targeted down-regulation of NbCathB could improve the performance of this plant-based expression platform.

Enzymatic characterization of germination-specific cysteine protease-1 expressed transiently in cotyledons during the early phase of germination

Papain-like cysteine protease activity that shows a unique transient expression profile in cotyledons of daikon radish during germination was detected. The enzyme showed a distinct elution pattern on DEAE-cellulose compared with cathepsin B-like and Responsive to dessication-21 cysteine protease. Although this activity was not detected in seed prior to imbibition, the activity increased markedly and reached a maximum at 2 days after imbibition and then decreased rapidly and completely disappeared after 5 days. Using cystatin-Sepharose, the 26 kDa cysteine protease (DRCP26) was isolated from cotyledons at 2 days after imbibition. The deduced amino acid sequence from the cDNA nucleotide sequence indicated that DRCP26 is an orthologue of Arabidopsis unidentified protein, germination-specific cysteine protease-1, belonging to the C1 family of cysteine protease predicted from genetic information. In an effort to characterize the enzymatic properties of DRCP26, the enzyme was purified to homogeneity from cotyledons at 48 h after imbibition. The best synthetic substrate for the enzyme was carbobenzoxy-Phe-Arg-4-methylcoumaryl-7-amide. All model peptides were digested to small peptides by the enzyme, suggesting that DRCP26 possesses broad cleavage specificity. These results indicated that DRCP26 plays a role in the mobilization of storage proteins in the early phase of seed germination.

Insecticide resistance mechanisms in the green peach aphid Myzus persicae (Hemiptera: Aphididae) II: Costs and benefits

BACKGROUND

Among herbivorous insects that have exploited agro-ecosystems, the peach-potato aphid, Myzus persicae, is recognized as one of the most important agricultural pests worldwide. Uses over 400 plant species and has evolved different insecticides resistance mechanisms. As M. persicae feeds upon a huge diversity of hosts, it has been exposed to a wide variety of plant allelochemicals, which probably have promoted a wide range of detoxification systems.

METHODOLOGY/PRINCIPAL FINDINGS

In this work we (i) evaluated whether insecticide resistance mutations (IRM) in M. persicae can give an advantage in terms of reproductive fitness when aphids face two hosts, pepper (Capsicum annuum) a suitable host and radish (Raphanus sativus) the unfavorable host and (ii) examined the transcriptional expression of six genes that are known to be up-regulated in response to insecticides. Our results show a significant interaction between host and IRM on the intrinsic rate of increase (r(m)). Susceptible genotypes (not carrying insensitivity mutations) had a higher r(m) on pepper, and the transcriptional levels of five genes increased on radish. The r(m) relationship was reversed on the unfavorable host; genotypes with multiple IRM exhibited higher r(m), without altering the transcriptional levels of the studied genes. Genotypes with one IRM kept a similar r(m) on both hosts, but they increased the transcriptional levels of two genes.

CONCLUSIONS/SIGNIFICANCE

Although we have studied only nine genotypes, overall our results are in agreement with the general idea that allelochemical detoxification systems could constitute a pre-adaptation for the development of insecticide resistance. Genotypes carrying IRM exhibited a higher r(m) than susceptible genotypes on radish, the more unfavorable host. Susceptible genotypes should be able to tolerate the defended host by up-regulating some metabolic genes that are also responding to insecticides. Hence, our results suggest that the trade-off among resistance mechanisms might be quite complex, with a multiplicity of costs and benefits depending on the environment.

Structural basis for specificity of propeptide-enzyme interaction in barley C1A cysteine peptidases

C1A cysteine peptidases are synthesized as inactive proenzymes. Activation takes place by proteolysis cleaving off the inhibitory propeptide. The inhibitory capacity of propeptides from barley cathepsin L and B-like peptidases towards commercial and barley cathepsins has been characterized. Differences in selectivity have been found for propeptides from L-cathepsins against their cognate and non cognate enzymes. Besides, the propeptide from barley cathepsin B was not able to inhibit bovine cathepsin B. Modelling of their three-dimensional structures suggests that most propeptide inhibitory properties can be explained from the interaction between the propeptide and the mature cathepsin structures. Their potential use as biotechnological tools is discussed.