C-terminal KDEL sequence of a KDEL-tailed cysteine proteinase (sulfhydryl-endopeptidase) is involved in formation of KDEL vesicle and in efficient vacuolar transport of sulfhydryl-endopeptidase

A View into Seed Autophagy: From Development to Environmental Responses

Autophagy is a conserved cellular mechanism involved in the degradation and subsequent recycling of cytoplasmic components. It is also described as a catabolic process implicated in the specific degradation of proteins in response to several stimuli. In eukaryotes, the endoplasmic reticulum accumulates an excess of proteins in response to environmental changes, and is the major cellular organelle at the crossroads of stress responses. Return to proteostasis involves the activation of the Unfolded Protein Response (UPR) and eventually autophagy as a feedback mechanism to relieve protein overaccumulation. Recent publications have focused on the relevance of autophagy in two central processes of seed biology: (i) seed storage protein accumulation upon seed maturation and (ii) reserve mobilization during seed imbibition. Although ER-protein accumulation and the subsequent activation of autophagy resemble the Seed Storage Protein (SSP) deposition during seed maturation, the molecular connection between seed development, autophagy, and seed response to abiotic stresses is still an underexplored field. This mini-review presents current advances in autophagy in seeds, highlighting its participation in the normal course of seed development from embryogenesis to germination. Finally, the function of autophagy in response to the seed environment is also considered, as is its involvement in controlling seed dormancy and germination.

Specialized endoplasmic reticulum-derived vesicles in plants: Functional diversity, evolution, and biotechnological exploitation

A central role of the endoplasmic reticulum (ER) is the synthesis, folding and quality control of secretory proteins. Secretory proteins usually exit the ER to enter the Golgi apparatus in coat protein complex II (COPII)-coated vesicles before transport to different subcellular destinations. However, in plants there are specialized ER-derived vesicles (ERDVs) that carry specific proteins but, unlike COPII vesicles, can exist as independent organelles or travel to the vacuole in a Golgi-independent manner. These specialized ERDVs include protein bodies and precursor-accumulating vesicles that accumulate storage proteins in the endosperm during seed development. Specialized ERDVs also include precursor protease vesicles that accumulate amino acid sequence KDEL-tailed cysteine proteases and ER bodies in Brassicales plants that accumulate myrosinases that hydrolyzes glucosinolates. These functionally specialized ERDVs act not only as storage organelles but also as platforms for signal-triggered processing, activation and deployment of specific proteins with important roles in plant growth, development and adaptive responses. Some specialized ERDVs have also been exploited to increase production of recombinant proteins and metabolites. Here we discuss our current understanding of the functional diversity, evolutionary mechanisms and biotechnological application of specialized ERDVs, which are associated with some of the highly remarkable characteristics important to plants.

A rich and bountiful harvest: Key discoveries in plant cell biology

The field of plant cell biology has a rich history of discovery, going back to Robert Hooke's discovery of cells themselves. The development of microscopes and preparation techniques has allowed for the visualization of subcellular structures, and the use of protein biochemistry, genetics, and molecular biology has enabled the identification of proteins and mechanisms that regulate key cellular processes. In this review, seven senior plant cell biologists reflect on the development of this research field in the past decades, including the foundational contributions that their teams have made to our rich, current insights into cell biology. Topics covered include signaling and cell morphogenesis, membrane trafficking, cytokinesis, cytoskeletal regulation, and cell wall biology. In addition, these scientists illustrate the pathways to discovery in this exciting research field.

An Apoplastic Defensin of Wheat Elicits the Production of Extracellular Polysaccharides in Snow Mold

TAD1 (Triticum aestivum defensin 1) is a plant defensin specifically induced by low temperature in winter wheat. In this study, we demonstrated that TAD1 accumulated in the apoplast during cold acclimation and displayed antifungal activity against the pink snow mold fungi Microdochium nivale. When M. nivale was treated with TAD1, Congo red-stainable extracellular polysaccharides (EPS) were produced. The EPS were degradable by cellulase treatment, suggesting the involvement of β-1,4 glucans. Interestingly, when the fungus was treated with FITC-labeled TAD1, fluorescent signals were observed within the EPS layer. Taken together, these results support the hypothesis that the EPS plays a role as a physical barrier against antimicrobial proteins secreted by plants. We anticipate that the findings from our study will have broad impact and will increase our understanding of plant–snow mold interactions under snow.

Protein bodies of the endoplasmic reticulum in Arabidopsis thaliana (Brassicaceae): origin, structural and biochemical features, functional significance

History of the discovery, formation, structural and biochemical traits of the protein bodies, derivatives of the granular endoplasmic reticulum (GER) that are known as ER-bodies, are reviewed. The functions of ER-bodies in cell vital activity mainly in Arabidopsis thaliana are reported. The highly specific component of ER-bodies, β-glucosidase enzyme, is described and its protecting role for plants under effect of abiotic and biotic factors is characterized. Based on the analytical review of the literature, it is shown that ER-bodies and the transcription factor NAI2 are unique to species of the family Brassicaceae. The specificity of the system GER – ER-bodies for Brassicaceae and thus the fundamental and applied importance of future research of mechanisms of its functioning in A. thaliana and other Brassicaceae species are emphasized.

Exploring Mechanisms of Quantitative Resistance to Leptosphaeria maculans (Blackleg) in the Cotyledons of Canola (Brassica napus) Based on Transcriptomic and Microscopic Analyses

Using resistant cultivars is a common approach to managing blackleg of canola/rapeseed caused by Leptosphaeria maculans (Lm). Quantitative resistance (QR), as opposed to major-gene resistance, is of interest because it is generally more durable, due to its multi-genetic basis. However, the mechanisms and genes underlying QR are mostly unknown. In this study, potential QR modes of action in “74-44 BL” was explored. This Canadian canola cultivar showed moderate but consistent race-nonspecific resistance at the cotyledon and adult-plant stages. A susceptible cultivar, “Westar”, was used as a control. After inoculation, the lesions developed more slowly on the cotyledons of 74-44 BL than those of Westar. We used RNA sequencing (-RNA-seq) to identify genes and their functions, putatively related to this resistance, and found that genes involved in programmed cell death (PCD), reactive oxygen species (ROS), signal transduction or intracellular endomembrane transport were most differentially expressed. ROS production was assessed in relation to Lm hyphal growth and lesion size; it occurred beyond the tissue colonized by Lm in 74-44 BL and appeared to trigger rapid cell death, limiting cotyledon colonization by Lm. In contrast, Lm grew more rapidly in Westar, often catching up with the ring of ROS and surpassing lesion boundaries. It appears that QR in 74-44 BL cotyledons is associated with limited colonization by Lm possibly mediated via ROS. The RNA-seq data also showed a link between ROS, signal transduction, and endomembrane vesicle trafficking, as well as PCD in the resistance. These results provide a starting point for a better understanding of the mechanisms behind QR against Lm in canola.

Red light delays programmed cell death in non-host interaction between Pseudomonas syringae pv tomato DC3000 and tobacco plants

Light modulates almost every aspect of plant physiology, including plant-pathogen interactions. Among these, the hypersensitive response (HR) of plants to pathogens is characterized by a rapid and localized programmed cell death (PCD), which is critical to restrict the spread of pathogens from the infection site. The aim of this work was to study the role of light in the interaction between Pseudomonas syringae pv. tomato DC3000 (Pto DC3000) and non-host tobacco plants. To this end, we examined the HR under different light treatments (white and red light) by using a range of well-established markers of PCD. The alterations found at the cellular level included: i) loss of membrane integrity and nuclei, ii) RuBisCo and DNA degradation, and iii) changes in nuclease profiles and accumulation of cysteine proteinases. Our results suggest that red light plays a role during the HR of tobacco plants to Pto DC3000 infection, delaying the PCD process.

Barley cysteine protease PAP14 plays a role in degradation of chloroplast proteins

Chloroplast protein degradation is known to occur both inside chloroplasts and in the vacuole. Genes encoding cysteine proteases have been found to be highly expressed during leaf senescence. However, it remains unclear where they participate in chloroplast protein degradation. In this study HvPAP14, which belongs to the C1A family of cysteine proteases, was identified in senescing barley (Hordeum vulgare L.) leaves by affinity enrichment using the mechanism-based probe DCG-04 targeting cysteine proteases and subsequent mass spectrometry. Biochemical analyses and expression of a HvPAP14:RFP fusion construct in barley protoplasts was used to identify the subcellular localization and putative substrates of HvPAP14. The HvPAP14:RFP fusion protein was detected in the endoplasmic reticulum and in vesicular bodies. Immunological studies showed that HvPAP14 was mainly located in chloroplasts, where it was found in tight association with thylakoid membranes. The recombinant enzyme was activated by low pH, in accordance with the detection of HvPAP14 in the thylakoid lumen. Overexpression of HvPAP14 in barley revealed that the protease can cleave LHCB proteins and PSBO as well as the large subunit of Rubisco. HvPAP14 is involved in the normal turnover of chloroplast proteins and may have a function in bulk protein degradation during leaf senescence.

Ipomoea batatas: papain propeptide inhibits cysteine protease in main plant parasites and enhances resistance of transgenic tomato to parasites

Different parasites cause severe lose in quantity and quality of crops. Many parasites develop haustorial cells and stylets that penetrate the host using secreted enzymes and mechanical pressure. Cysteine proteases are pre-pro-enzyme produced by parasites that are essential for normal parasitism. Papain is also a kind of cysteine proteases such that its propeptide segment has inhibitory properties and limits the protease activity of papain. To investigate the inhibitory effects of papain propeptide on some parasite proteases, we cloned inhibitory propeptide of papain of Ipomoea batatas, and enzymatic fragments of Diabrotica virgifera cathepsin L-like protease-1, Meloidogyne incognita cathepsin L-like protease 1, Heterodera glycines cysteine protease-1, Cuscuta chinesis cysteine protease and Orobanche cernua cysteine protease. After purification of recombinant inhibitory propeptide and enzymatic fragments, the inhibition activity of propeptide on cysteine proteases was measured. Finally inhibitory propeptide was transformed into tomato and transgenic plants resistance to parasites (bioassay) were examined. We demonstrated papain-propeptide inhibits cysteine protease of mentioned parasites. In transgenic tomato plants, papain-inhibitory propeptide effectively interrupted haustoria development. Haustoria-digitate cells of dodder could not differentiate and develop into the phloem and xylem hyphae on transgenic tomatoes. Parasites grown on transgenic tomatoes showed reduction in vigor and productivity due to defective connection of haustoria. Lower ratio of female nematodes and a decrease of nematode egg mass per transgenic line indicated biocontrol of nematode. The changes in growth factors of parasite challenged transgenic lines relative to controls, indicates the efficacy of papain propeptide in control of parasitism.

The role of KDEL-tailed cysteine endopeptidases of Arabidopsis (AtCEP2 and AtCEP1) in root development

Plants encode a unique group of papain-type cysteine endopeptidases (CysEP) characterized by a C-terminal KDEL endoplasmic reticulum retention signal (KDEL-CysEP) and an unusually broad substrate specificity. The three Arabidopsis KDEL-CysEPs (AtCEP1, AtCEP2, and AtCEP3) are differentially expressed in vegetative and generative tissues undergoing programmed cell death (PCD). While KDEL-CysEPs have been shown to be implicated in the collapse of tissues during PCD, roles of these peptidases in processes other than PCD are unknown. Using mCherry-AtCEP2 and EGFP-AtCEP1 reporter proteins in wild type versus atcep2 or atcep1 mutant plants, we explored the participation of AtCEP in young root development. Loss of AtCEP2, but not AtCEP1 resulted in shorter primary roots due to a decrease in cell length in the lateral root (LR) cap, and impairs extension of primary root epidermis cells such as trichoblasts in the elongation zone. AtCEP2 was localized to root cap corpses adherent to epidermal cells in the rapid elongation zone. AtCEP1 and AtCEP2 are expressed in root epidermis cells that are separated for LR emergence. Loss of AtCEP1 or AtCEP2 caused delayed emergence of LR primordia. KDEL-CysEPs might be involved in developmental tissue remodeling by supporting cell wall elongation and cell separation.

Endoplasmic reticulum retention motif fused to recombinant anti-cancer monoclonal antibody (mAb) CO17-1A affects mAb expression and plant stress response

The endoplasmic reticulum (ER) is the main site of protein synthesis, folding, and secretion to other organelles. The capacity of the ER to process proteins is limited, and excessive accumulation of unfolded and misfolded proteins can induce ER stress, which is associated with plant diseases. Here, a transgenic Arabidopsis system was established to express anti-cancer monoclonal antibodies (mAbs) that recognize the tumor-associated antigen GA733-2. Monoclonal antibody (mAb) CO17-1A recognize a tumor-associated epitope expressed on the colorectal cancer cell surface. The ER retention Lys-Asp-Glu-Leu (KDEL) motif sequence was added to the C-terminus of the heavy chain to retain anti-colorectal cancer mAbs in the ER, consequently boosting mAb production. Agrobacterium-mediated floral dip transformation was used to generate T₁ transformants, and homozygous T₄ seeds obtained from transgenic Arabidopsis plants expressing anti-colorectal cancer mAbs were used to confirm the physiological effects of KDEL tagging. Germination rates were not significantly different between both plants expressing mAb CO without KDEL mAb CO (CO plant) and mAb CO with KDEL mAb COK (COK plant). However, COK plants primary root lengths were shorter than those of CO plants and non-transgenic Arabidopsis plants in in vitro media. Most ER stress-related genes, with the exception of bZIP28 and IRE1a, were upregulated in COK plants compared to CO plants. Western blot and SDS-PAGE analyses showed that COK plants exhibited up to five times higher expression and mAb amounts than plants. Enhanced expression in mAb COK plants was confirmed by immunohistochemical analyses. mAb COK was distributed across most of the area of leaf tissues, whereas mAb CO was mainly distributed in extracellular areas. Surface plasmon resonance analyses revealed that mAb CO and mAb COK possessed equivalent or slightly better binding activities to antigen EpCAM compared to a commercially available parental antibody. N-glycosylation analysis showed that mAb CO had plant specific residues whereas mAb COK mainly showed an oligo-mannose N-glycan structure without the plant specific glycan residues. In this study, the reduction of plant growth and biomass induced by ER retention signal peptide might be only in in vitro conditions, and thus should be carefully considered for the initial screening for transgenic lines on culture media. Taken together, nevertheless the fusion of ER retention signal peptide is an effective approach for enhancing the yields of recombinant proteins in vivo.

Inhibitory properties of cysteine protease pro-peptides from barley confer resistance to spider mite feeding

C1A plant cysteine proteases are synthesized as pre-pro-enzymes that need to be processed to become active by the pro-peptide claves off from its cognate enzyme. These pro-sequences play multifunctional roles including the capacity to specifically inhibit their own as well as other C1A protease activities from diverse origin. In this study, it is analysed the potential role of C1A pro-regions from barley as regulators of cysteine proteases in target phytophagous arthropods (coleopteran and acari). The in vitro inhibitory action of these pro-sequences, purified as recombinant proteins, is demonstrated. Moreover, transgenic Arabidopsis plants expressing different fragments of HvPap-1 barley gene containing the pro-peptide sequence were generated and the acaricide function was confirmed by bioassays conducted with the two-spotted spider mite Tetranychus urticae. Feeding trials resulted in a significant reduction of leaf damage in the transgenic lines expressing the pro-peptide in comparison to non-transformed control and strongly correlated with an increase in mite mortality. Additionally, the analysis of the expression levels of a selection of potential mite targets (proteases and protease inhibitors) revealed a mite strategy to counteract the inhibitory activity produced by the C1A barley pro-prodomain. These findings demonstrate that pro-peptides can control mite pests and could be applied as defence proteins in biotechnological systems.

Silencing of the CaCP gene delays salt- and osmotic-induced leaf senescence in Capsicum annuum L

Cysteine proteinases have been known to participate in developmental processes and in response to stress in plants. Our present research reported that a novel CP gene, CaCP, was involved in leaf senescence in pepper (Capsicum annuum L.). The full-length CaCP cDNA is comprised of 1316 bp, contains 1044 nucleotides in open reading frame (ORF), and encodes a 347 amino acid protein. The deduced protein belongs to the papain-like cysteine proteases (CPs) superfamily, containing a highly conserved ERFNIN motif, a GCNGG motif and a conserved catalytic triad. This protein localized to the vacuole of plant cells. Real-time quantitative PCR analysis revealed that the expression level of CaCP gene was dramatically higher in leaves and flowers than that in roots, stems and fruits. Moreover, CaCP transcripts were induced upon during leaf senescence. CaCP expression was upregulated by plant hormones, especially salicylic acid. CaCP was also significantly induced by abiotic and biotic stress treatments, including high salinity, mannitol and Phytophthora capsici. Loss of function of CaCP using the virus-induced gene-silencing technique in pepper plants led to enhanced tolerance to salt- and osmotic-induced stress. Taken together, these results suggest that CaCP is a senescence-associated gene, which is involved in developmental senescence and regulates salt- and osmotic-induced leaf senescence in pepper.

Ex vivo processing for maturation of Arabidopsis KDEL-tailed cysteine endopeptidase 2 (AtCEP2) pro-enzyme and its storage in endoplasmic reticulum derived organelles

Ricinosomes are specialized ER-derived organelles that store the inactive pro-forms of KDEL-tailed cysteine endopeptidases (KDEL-CysEP) associated with programmed cell death (PCD). The Arabidopsis genome encodes three KDEL-CysEP (AtCEP1, AtCEP2, and AtCEP3) that are differentially expressed in vegetative and generative tissues undergoing PCD. These Arabidopsis proteases have not been characterized at a biochemical level, nor have they been localized intracellularly. In this study, we characterized AtCEP2. A 3xHA-mCherry-AtCEP2 gene fusion including pro-peptide and KDEL targeting sequences expressed under control of the endogenous promoter enabled us to isolate AtCEP2 "ex vivo". The purified protein was shown to be activated in a pH-dependent manner. After activation, however, protease activity was pH-independent. Analysis of substrate specificity showed that AtCEP2 accepts proline near the cleavage site, which is a rare feature specific for KDEL-CysEPs. mCherry-AtCEP2 was detected in the epidermal layers of leaves, hypocotyls and roots; in the root, it was predominantly found in the elongation zone and root cap. Co-localization with an ER membrane marker showed that mCherry-AtCEP2 was stored in two different types of ER-derived organelles: 10 μm long spindle shaped organelles as well as round vesicles with a diameter of approximately 1 μm. The long organelles appear to be ER bodies, which are found specifically in Brassicacae. The round vesicles strongly resemble the ricinosomes first described in castor bean. This study provides a first evidence for the existence of ricinosomes in Arabidopsis, and may open up new avenues of research in the field of PCD and developmental tissue remodeling.

Endoplasmic reticulum KDEL-tailed cysteine endopeptidase 1 of Arabidopsis (AtCEP1) is involved in pathogen defense

Programmed cell death (PCD) is a genetically determined process in all multicellular organisms. Plant PCD is effected by a unique group of papain-type cysteine endopeptidases (CysEP) with a C-terminal KDEL endoplasmic reticulum (ER) retention signal (KDEL CysEP). KDEL CysEPs can be stored as pro-enzymes in ER-derived endomembrane compartments and are released as mature CysEPs in the final stages of organelle disintegration. KDEL CysEPs accept a wide variety of amino acids at the active site, including the glycosylated hydroxyprolines of the extensins that form the basic scaffold of the cell wall. In Arabidopsis, three KDEL CysEPs (AtCEP1, AtCEP2, and AtCEP3) are expressed. Cell- and tissue-specific activities of these three genes suggest that KDEL CysEPs participate in the abscission of flower organs and in the collapse of tissues in the final stage of PCD as well as in developmental tissue remodeling. We observed that AtCEP1 is expressed in response to biotic stress stimuli in the leaf. atcep1 knockout mutants showed enhanced susceptibility to powdery mildew caused by the biotrophic ascomycete Erysiphe cruciferarum. A translational fusion protein of AtCEP1 with a three-fold hemaglutinin-tag and the green fluorescent protein under control of the endogenous AtCEP1 promoter (PCEP1::pre-pro-3xHA-EGFP-AtCEP1-KDEL) rescued the pathogenesis phenotype demonstrating the function of AtCEP1 in restriction of powdery mildew. The spatiotemporal AtCEP1-reporter expression during fungal infection together with microscopic inspection of the interaction phenotype suggested a function of AtCEP1 in controlling late stages of compatible interaction including late epidermal cell death. Additionally, expression of stress response genes appeared to be deregulated in the interaction of atcep1 mutants and E. cruciferarum. Possible functions of AtCEP1 in restricting parasitic success of the obligate biotrophic powdery mildew fungus are discussed.

Expression and localisation of a senescence-associated KDEL-cysteine protease from Lilium longiflorum tepals

Senescence is a tightly regulated process and both compartmentalisation and regulated activation of degradative enzymes is critical to avoid premature cellular destruction. Proteolysis is a key process in senescent tissues, linked to disassembly of cellular contents and nutrient remobilisation. Cysteine proteases are responsible for most proteolytic activity in senescent petals, encoded by a gene family comprising both senescence-specific and senescence up-regulated genes. KDEL cysteine proteases are present in senescent petals of several species. Isoforms from endosperm tissue localise to ricinosomes: cytosol acidification following vacuole rupture results in ricinosome rupture and activation of the KDEL proteases from an inactive proform. Here data show that a Lilium longiflorum KDEL protease gene (LlCYP) is transcriptionally up-regulated, and a KDEL cysteine protease antibody reveals post-translational processing in senescent petals. Plants over-expressing LlCYP lacking the KDEL sequence show reduced growth and early senescence. Immunogold staining and confocal analyses indicate that in young tissues the protein is retained in the ER, while during floral senescence it is localised to the vacuole. Our data therefore suggest that the vacuole may be the site of action for at least this KDEL cysteine protease during tepal senescence.

Involvement of autophagy in the direct ER to vacuole protein trafficking route in plants

Trafficking of proteins from the endoplasmic reticulum (ER) to the vacuole is a fundamental process in plants, being involved both in vacuole biogenesis as well as with plant growth and response to environmental stresses. Although the canonical transport of cellular components from the ER to the vacuole includes the Golgi apparatus as an intermediate compartment, there are multiple lines of evidence that support the existence of a direct ER-to-vacuole, Golgi-independent, trafficking route in plants that uses the autophagy machinery. Plant autophagy was initially described by electron microscopy, visualizing cellular structures that are morphologically reminiscent of autophagosomes. In some of these reports these structures were shown to transport vacuole residing proteins, particularly seed storage proteins, directly from the ER to the vacuole. More recently, following the discovery of the proteins of the core autophagy machinery, molecular tools were implemented in deciphering the involvement of autophagy in this special trafficking route. Here we review the relatively older and more recent scientific observations, supporting the involvement of autophagy in the special cellular trafficking pathways of plants.

Trafficking of endoplasmic reticulum-retained recombinant proteins is unpredictable in Arabidopsis thaliana

A wide variety of recombinant proteins has been produced in the dicot model plant, Arabidopsis thaliana. Many of these proteins are targeted for secretion by means of an N-terminal endoplasmic reticulum (ER) signal peptide. In addition, they can also be designed for ER retention by adding a C-terminal H/KDEL-tag. Despite extensive knowledge of the protein trafficking pathways, the final protein destination, especially of such H/KDEL-tagged recombinant proteins, is unpredictable. In this respect, glycoproteins are ideal study objects. Microscopy experiments reveal their deposition pattern and characterization of their N-glycans aids in elucidating the trafficking. Here, we combine microscopy and N-glycosylation data generated in Arabidopsis leaves and seeds, and highlight the lack of a decent understanding of heterologous protein trafficking.

The dynamic behavior of storage organelles in developing cereal seeds and its impact on the production of recombinant proteins

Cereal endosperm is a highly differentiated tissue containing specialized organelles for the accumulation of storage proteins, which are ultimately deposited either within protein bodies derived from the endoplasmic reticulum, or in protein storage vacuoles (PSVs). During seed maturation endosperm cells undergo a rapid sequence of developmental changes, including extensive reorganization and rearrangement of the endomembrane system and protein transport via several developmentally regulated trafficking routes. Storage organelles have been characterized in great detail by the histochemical analysis of fixed immature tissue samples. More recently, in vivo imaging and the use of tonoplast markers and fluorescent organelle tracers have provided further insight into the dynamic morphology of PSVs in different cell layers of the developing endosperm. This is relevant for biotechnological applications in the area of molecular farming because seed storage organelles in different cereal crops offer alternative subcellular destinations for the deposition of recombinant proteins that can reduce proteolytic degradation, allow control over glycan structures and increase the efficacy of oral delivery. We discuss how the specialized architecture and developmental changes of the endomembrane system in endosperm cells may influence the subcellular fate and post-translational modification of recombinant glycoproteins in different cereal species.

Comparative proteomic analysis on wild type and nitric oxide-overproducing mutant (nox1) of Arabidopsis thaliana

Nitric oxide (NO) as a ubiquitous signal molecule plays an important role in plant development and growth. Here, we compared the proteomic changes between NO-overproducing mutant (nox1) and wild-type (WT) of Arabidopsis thaliana using two-dimensional electrophoresis coupled with MALDI-TOF MS. We successfully identified 59 differentially expressed proteins in nox1 mutant, which are predicted to play potential roles in specific cellular processes, such as post-translational modification, energy production and conversion, metabolism, transcription and signal transduction, cell rescue and defense, development and differentiation. Particularly, expression levels of five anti-oxidative enzymes were altered by the mutation; and assays of their respective enzymatic activities indicated an enhanced level of oxidative stress in nox1 mutant. Finally, some important proteins were further confirmed at transcriptional level using quantitative real-time PCR revealing the systemic changes between WT and nox1. The result suggests that obvious morphological changes in the nox1 mutant may be regulated by different mechanisms and factors, while excess endogenous NO maybe one of the possible reasons.

The Arabidopsis exocyst subcomplex subunits involved in a golgi-independent transport into the vacuole possess consensus autophagy-associated atg8 interacting motifs

The exocyst complex is a multi-subunits evolutionary conserved complex, which was originally shown to be primarily associated with vesicular transport to the plasma membrane. A recent report (Kulich et al., 2013 Traffic; In Press) revealed that AtEXO70B1, one of the multiple subunits of the exocyst complex of Arabidopsis thaliana plants, is co-transported with the autophagy-associated Atg8f protein to the vacuole. This pathway does not involve the Golgi apparatus. The co-localization of AtEXO70B1 and Atg8f suggests either that both of these proteins are co-transported together to the vacuole or, alternatively, that Atg8 binds to a putative Atg8 interacting motif (AIM) located within the AtEXO70B1 polypeptide, apparently forming a tethering complex for an autophagic complex that is transported to the vacuole. In the present addendum, by tooling a bioinformatics approach, we show that AtEXO70B1 as well as the additional 20 paralogs of Arabidopsis EXO70 exocyst subunits each possess one or more AIMs whose consensus sequence implies their high fidelity binding to Atg8. This indicates that the autophagy machinery is strongly involved in the assembly, transport, and apparently also the function of AtEXO70B1 as well as the exocyst sub complex.

Plant species and organ influence the structure and subcellular localization of recombinant glycoproteins

Many plant-based systems have been developed as bioreactors to produce recombinant proteins. The choice of system for large-scale production depends on its intrinsic expression efficiency and its propensity for scale-up, post-harvest storage and downstream processing. Factors that must be considered include the anticipated production scale, the value and intended use of the product, the geographical production area, the proximity of processing facilities, intellectual property, safety and economics. It is also necessary to consider whether different species and organs affect the subcellular trafficking, structure and qualitative properties of recombinant proteins. In this article we discuss the subcellular localization and N-glycosylation of two commercially-relevant recombinant glycoproteins (Aspergillus niger phytase and anti-HIV antibody 2G12) produced in different plant species and organs. We augment existing data with novel results based on the expression of the same recombinant proteins in Arabidopsis and tobacco seeds, focusing on similarities and subtle differences in N-glycosylation that often reflect the subcellular trafficking route and final destination, as well as differences generated by unique enzyme activities in different species and tissues. We discuss the potential consequences of such modifications on the stability and activity of the recombinant glycoproteins.

Classification of protein motifs based on subcellular localization uncovers evolutionary relationships at both sequence and functional levels

Background

Most proteins have evolved in specific cellular compartments that limit their functions and potential interactions. On the other hand, motifs define amino acid arrangements conserved between protein family members and represent powerful tools for assigning function to protein sequences. The ideal motif would identify all members of a protein family but in practice many motifs identify both family members and unrelated proteins, referred to as True Positive (TP) and False Positive (FP) sequences, respectively.

Results

To address the relationship between protein motifs, protein function and cellular localization, we systematically assigned subcellular localization data to motif sequences from the comprehensive PROSITE sequence motif database. Using this data we analyzed relationships between localization and function. We find that TPs and FPs have a strong tendency to localize in different compartments. When multiple localizations are considered, TPs are usually distributed between related cellular compartments. We also identified cases where FPs are concentrated in particular subcellular regions, indicating possible functional or evolutionary relationships with TP sequences of the same motif.

Conclusions

Our findings suggest that the systematic examination of subcellular localization has the potential to uncover evolutionary and functional relationships between motif-containing sequences. We believe that this type of analysis complements existing motif annotations and could aid in their interpretation. Our results shed light on the evolution of cellular organelles and potentially establish the basis for new subcellular localization and function prediction algorithms.

C1A cysteine-proteases and their inhibitors in plants

Plant cysteine-proteases (CysProt) represent a well-characterized type of proteolytic enzymes that fulfill tightly regulated physiological functions (senescence and seed germination among others) and defense roles. This article is focused on the group of papain-proteases C1A (family C1, clan CA) and their inhibitors, phytocystatins (PhyCys). In particular, the protease-inhibitor interaction and their mutual participation in specific pathways throughout the plant's life are reviewed. C1A CysProt and PhyCys have been molecularly characterized, and comparative sequence analyses have identified consensus functional motifs. A correlation can be established between the number of identified CysProt and PhyCys in angiosperms. Thus, evolutionary forces may have determined a control role of cystatins on both endogenous and pest-exogenous proteases in these species. Tagging the proteases and inhibitors with fluorescence proteins revealed common patterns of subcellular localization in the endoplasmic reticulum-Golgi network in transiently transformed onion epidermal cells. Further in vivo interactions were demonstrated by bimolecular fluorescent complementation, suggesting their participation in the same physiological processes.

The formation, function and fate of protein storage compartments in seeds

Seed storage proteins (SSPs) have been studied for more than 250 years because of their nutritional value and their impact on the use of grain in food processing. More recently, the use of seeds for the production of recombinant proteins has rekindled interest in the behavior of SSPs and the question how they are able to accumulate as stable storage reserves. Seed cells produce vast amounts of SSPs with different subcellular destinations creating an enormous logistic challenge for the endomembrane system. Seed cells contain several different storage organelles including the complex and dynamic protein storage vacuoles (PSVs) and other protein bodies (PBs) derived from the endoplasmic reticulum (ER). Storage proteins destined for the PSV may pass through or bypass the Golgi, using different vesicles that follow different routes through the cell. In addition, trafficking may depend on the plant species, tissue and developmental stage, showing that the endomembrane system is capable of massive reorganization. Some SSPs contain sorting signals or interact with membranes or with other proteins en route in order to reach their destination. The ability of SSPs to form aggregates is particularly important in the formation or ER-derived PBs, a mechanism that occurs naturally in response to overloading with proteins that cannot be transported and that can be used to induce artificial storage bodies in vegetative tissues. In this review, we summarize recent findings that provide insight into the formation, function, and fate of storage organelles and describe tools that can be used to study them.

Coffee cysteine proteinases and related inhibitors with high expression during grain maturation and germination

BACKGROUND

Cysteine proteinases perform multiple functions in seeds, including participation in remodelling polypeptides and recycling amino acids during maturation and germination. Currently, few details exist concerning these genes and proteins in coffee. Furthermore, there is limited information on the cysteine proteinase inhibitors which influence the activities of these proteinases.

RESULTS

Two cysteine proteinase (CP) and four cysteine proteinase inhibitor (CPI) gene sequences have been identified in coffee with significant expression during the maturation and germination of coffee grain. Detailed expression analysis of the cysteine proteinase genes CcCP1 and CcCP4 in Robusta using quantitative RT-PCR showed that these transcripts accumulate primarily during grain maturation and germination/post germination. The corresponding proteins were expressed in E. coli and purified, but only one, CcCP4, which has a KDDL/KDEL C-terminal sequence, was found to be active after a short acid treatment. QRT-PCR expression analysis of the four cysteine proteinase inhibitor genes in Robusta showed that CcCPI-1 is primarily expressed in developing and germinating grain and CcCPI-4 is very highly expressed during the late post germination period, as well as in mature, but not immature leaves. Transcripts corresponding to CcCPI-2 and CcCPI-3 were detected in most tissues examined at relatively similar, but generally low levels.

CONCLUSIONS

Several cysteine proteinase and cysteine proteinase inhibitor genes with strong, relatively specific expression during coffee grain maturation and germination are presented. The temporal expression of the CcCP1 gene suggests it is involved in modifying proteins during late grain maturation and germination. The expression pattern of CcCP4, and its close identity with KDEL containing CP proteins, implies this proteinase may play a role in protein and/or cell remodelling during late grain germination, and that it is likely to play a strong role in the programmed cell death associated with post-germination of the coffee grain. Expression analysis of the cysteine proteinase inhibitor genes suggests that CcCPI-1 could primarily be involved in modulating the activity of grain CP activity; while CcCPI-4 may play roles modulating grain CP activity and in the protection of the young coffee seedlings from insects and pathogens. CcCPI-2 and CcCPI-3, having lower and more widespread expression, could be more general "house-keeping" CPI genes.

Unique defense strategy by the endoplasmic reticulum body in plants

The endoplasmic reticulum (ER) is a site for the production of secretory proteins. Plants have developed ER subdomains for protein storage. The ER body is one such structure, which is observed in Brassicaceae plants. ER bodies accumulate in seedlings and roots or in wounded leaves in Arabidopsis. ER bodies contain high amounts of the β-glucosidases PYK10/BGLU23 in seedlings and roots or BGLU18 in wounded tissues. These results suggest that ER bodies are involved in the metabolism of glycoside molecules, presumably to produce repellents against pests and fungi. When Arabidopsis roots are homogenized, PYK10 formed large protein aggregates that include other β-glucosidases (BGLU21 and BGLU22), GDSL lipase-like proteins (GLL22) and cytosolic jacalin-related lectins (PBP1/JAL30, JAL31, JAL33, JAL34 and JAL35). Glucosidase activity increases by the aggregate formation. NAI1, a basic helix-loop-helix transcription factor, regulates the expression of the ER body proteins PYK10 and NAI2. Reduced expression of NAI2, PYK10 and BGLU21 resulted in abnormal ER body formation, indicating that these components regulate ER body formation. PYK10, BGLU21 and BGLU22 possess hydrolytic activity for scopolin, a coumaroyl glucoside that accumulates in the roots of Arabidopsis, and nai1 and pyk10 mutants are more susceptible to the symbiotic fungus Piriformospora indica. Therefore, it appears that the ER body is a unique organelle of Brassicaceae plants that is important for defense against pests and fungi.

Isolation and characterization of a gene encoding a polyethylene glycol-induced cysteine protease in common wheat

Plant cysteine protease (CP) genes are induced by abiotic stresses such as drought, yet their functions remain largely unknown. We isolated the full-length cDNA encoding a Triticum aestivum CP gene, designated TaCP, from wheat by the rapid amplification of cDNA ends (RACE) method. Sequence analysis revealed that TaCP contains an open reading frame encoding a protein of 362 amino acids, which is 96% identical to barley cysteine protease HvSF42. The TaCP transcript level in wheat seedlings was upregulated during polyethylene glycol (PEG) stress, with a peak appearing around 12 h after treatment. TaCP expression level increased rapidly with NaCl treatment at 48 h. TaCP responded strongly to low temperature (4 degree C) treatment from 1 h post-treatment and reached a peak of about 40-fold at 72 h. However, it showed only a very slight response to abscisic acid (ABA). More than one copy of TaCP was present in each of the three genomes of hexaploid wheat and its diploid donors. TaCP fused with green fluorescent protein (GFP) was located in the plasma membrane of onion epidermis cells. Transgenic Arabidopsis plants overexpressing TaCP showed stronger drought tolerance and higher CP activity under water-stressed conditions than wild-type Arabidopsis plants. The results suggest that TaCP plays a role in tolerance to water deficit.

Expression of sweet potato cysteine protease SPCP2 altered developmental characteristics and stress responses in transgenic Arabidopsis plants

In this report a full-length cDNA, SPCP2, which encoded a putative papain-like cysteine protease was isolated from senescent leaves of sweet potato (Ipomoea batatas). SPCP2 contained 1101 nucleotides (366 amino acids) in its open reading frame, and exhibited high amino acid sequence identities (ca. 68% to 83%) with plant cysteine proteases, including Actinidia deliciosa, Arabidopsis thaliana, Brassica oleracea, Phaseolus vulgaris, Pisum sativa, Vicia faba, Vicia sativa and Vigna mungo. RT-PCR analysis showed that SPCP2 gene expression was enhanced significantly in natural senescent leaves and in dark-, abscisic acid- (ABA-), jasmonic acid- (JA-) and ethephon-induced senescent leaves, but was almost not detected in mature green leaves, stems, and roots. Transgenic Arabidopsis with constitutive SPCP2 expression exhibited earlier floral transition from vegetative to reproductive growth, higher percentage of incompletely developed siliques per plant, reduced average fresh weight and lower germination percentage of seed, and higher salt and drought stress tolerance compared to those of control. Based on these results we conclude that sweet potato papain-like cysteine protease, SPCP2, is a functional senescence-associated gene, and its expression causes altered developmental characteristics and stress responses in transgenic Arabidopsis plants.

{alpha}' Subunit of soybean {beta}-conglycinin forms complex with rice glutelin via a disulphide bond in transgenic rice seeds

The alpha' and beta subunits of soybean beta-conglycinin were expressed in rice seeds in order to improve the nutritional and physiological properties of rice as a food. The alpha' subunit accumulated in rice seeds at a higher level than the beta subunit, but no detectable difference in mRNA transcription level between subunits was observed. Sequential extraction results indicate that the alpha' subunit formed one or more disulphide bonds with glutelin. Electron microscopic analysis showed that the alpha' subunit and the beta subunit were transported to PB-II together with glutelin. In mature transgenic seeds, the beta subunit accumulated in low electron density regions in the periphery of PB-II, whereas the alpha' subunit accumulated together with glutelin in high-density regions of the periphery. The subcellular localization of mutated alpha' subunits lacking one cysteine residue in the N-terminal mature region (alpha'DeltaCys1) or five cysteine residues in the pro and N-terminal mature regions (alpha'DeltaCys5) were also examined. Low-density regions were formed in PB-II in mature seeds of transgenic rice expressing alpha'DeltaCys 5 and alpha'DeltaCys1. alpha'DeltaCys5 was localized only in the low-density regions, whereas alpha'DeltaCys1 was found in both low- and high-density regions. These results suggest that the alpha' subunit could make a complex via one or more disulphide bonds with glutelin and accumulate together in PB-II of transgenic rice seeds.

Wheat cysteine proteases triticain alpha, beta and gamma exhibit mutually distinct responses to gibberellin in germinating seeds

We cloned three novel papain-type cysteine proteases (CPs), triticain alpha, beta and gamma, from 1-d-germinating wheat seeds. Triticain alpha, beta and gamma were constituted with 461, 472 and 365 amino acid residues, respectively, and had Cys-His-Asn catalytic triads as well as signal and propeptide sequences. Triticain gamma contained a putative vacuole-sorting sequence. Phylogenetic analysis showed that these CPs were divided into mutually different clusters. Triticain alpha and gamma mRNAs were expressed in seeds at an early stage of maturation and at the stage of germination 2d after imbibition, while triticain beta mRNA appeared shortly after imbibition. The expression of mRNAs for triticain alpha and gamma was suppressed by uniconazol, a gibberellin synthesis inhibitor. All the three CP mRNAs were strongly expressed in both embryo and aleurone layers. These results suggest that triticain alpha, beta and gamma play differential roles in seed maturation as well as in digestion of storage proteins during germination.

NtbZIP60, an endoplasmic reticulum-localized transcription factor, plays a role in the defense response against bacterial pathogens in Nicotiana tabacum

A spermine-based signal transduction pathway plays a defensive role against incompatible pathogens. We identified a novel spermine-responsive cDNA from Nicotiana tabacum that encodes a basic region/leucine zipper protein with a putative transmembrane domain. Identity to Arabidopsis thaliana AtbZIP60 was sufficiently high to name the novel cDNA NtbZIP60. Expression analysis revealed that NtbZIP60 is a component of the spermine-signal pathway, and is also involved in the unfolded protein response (UPR), as demonstrated for AtbZIP60. The gene product, NtbZIP60, localizes to the endoplasmic reticulum (ER) in plant cells; once the putative transmembrane domain is eliminated from the intact protein, it targets the nucleus. The putative processed form of NtbZIP60 transactivates target genes through binding to plant-specific UPR cis-elements. Expression of NbbZIP60, an NtbZIP60 ortholog in Nicotiana benthamiana, was significantly up-regulated at 6 h and later time points upon infection with the non-host pathogen Pseudomonas cichorii, while it was unaffected by infection with the compatible pathogen Pseudomonas syringae pv. tabaci. Furthermore, NbbZIP60-silenced N. benthamiana plants allowed higher multiplication of P. cichorii compared to the control plants. Taken together, the results suggest that this ER-localized transcription factor is involved in the spermine-signal transduction pathway and plays an important role in plant innate immunity.

NAI2 is an endoplasmic reticulum body component that enables ER body formation in Arabidopsis thaliana

Plants develop various endoplasmic reticulum (ER)-derived structures, each of which has specific functions. The ER body found in Arabidopsis thaliana is a spindle-shaped structure that specifically accumulates high levels of PYK10/BGLU23, a beta-glucosidase that bears an ER-retention signal. The molecular mechanisms underlying the formation of the ER body remain obscure. We isolated an ER body-deficient mutant in Arabidopsis seedlings that we termed nai2. The NAI2 gene (At3g15950) encodes a member of a unique protein family that is only found in the Brassicaceae. NAI2 localizes to the ER body, and a reduction in NAI2 gene expression elongates ER bodies and reduces their numbers. NAI2 deficiency does not affect PYK10 mRNA levels but reduces the level of PYK10 protein, which becomes uniformly diffused throughout the ER. NAI1, a transcription factor responsible for ER body formation, regulates NAI2 gene expression. These observations indicate that NAI2 is a key factor that enables ER body formation and the accumulation of PYK10 in ER bodies of Arabidopsis. Interestingly, ER body-like structures are also restricted to the Brassicales, including the Brassicaceae. NAI2 homologs may have evolved specifically in Brassicales for the purpose of producing ER body-like structures.

Isolation and characterization of a cDNA encoding a papain-like cysteine protease from alfalfa

Protein hydrolyzation is activated and involved in response to various stress signals. In the present study, a full-length cDNA, named MsCP1, encoding a papain-like cysteine protease was obtained by degenerated primers and 3'- and 5'-RACE from salt-tolerant alfalfa. The cDNA contained an open reading frame encoding a deduced protein of 350 amino acids with a putative N-terminal signal peptide, NPIR vacuole-sorting signal sequence and potential N-linked glycosylation sites. The deduced sequence showed a high similarity to deduced proteins from pea, tobacco, tomato and ryegrass. Fusion expression analysis in Escherichia coli showed that the putative eukaryotic signal peptide prevented its expression in prokaryotic system. The integration and transcript of the expression elements in transgenic tobacco plants were detected with Southern blot and RT-PCR analysis.

Cysteine proteinases regulate chloroplast protein content and composition in tobacco leaves: a model for dynamic interactions with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) vesicular bodies

The roles of cysteine proteinases (CP) in leaf protein accumulation and composition were investigated in transgenic tobacco (Nicotiana tabacum L.) plants expressing the rice cystatin, OC-1. The OC-1 protein was present in the cytosol, chloroplasts, and vacuole of the leaves of OC-1 expressing (OCE) plants. Changes in leaf protein composition and turnover caused by OC-1-dependent inhibition of CP activity were assessed in 8-week-old plants using proteomic analysis. Seven hundred and sixty-five soluble proteins were detected in the controls compared to 860 proteins in the OCE leaves. A cyclophilin, a histone, a peptidyl-prolyl cis-trans isomerase, and two ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase isoforms were markedly altered in abundance in the OCE leaves. The senescence-related decline in photosynthesis and Rubisco activity was delayed in the OCE leaves. Similarly, OCE leaves maintained higher leaf Rubisco activities and protein than controls following dark chilling. Immunogold labelling studies with specific antibodies showed that Rubisco was present in Rubisco vesicular bodies (RVB) as well as in the chloroplasts of leaves from 8-week-old control and OCE plants. Western blot analysis of plants at 14 weeks after both genotypes had flowered revealed large increases in the amount of Rubisco protein in the OCE leaves compared to controls. These results demonstrate that CPs are involved in Rubisco turnover in leaves under optimal and stress conditions and that extra-plastidic RVB bodies are present even in young source leaves. Furthermore, these data form the basis for a new model of Rubisco protein turnover involving CPs and RVBs.

A KDEL-tagged monoclonal antibody is efficiently retained in the endoplasmic reticulum in leaves, but is both partially secreted and sorted to protein storage vacuoles in seeds

Transgenic plants are attractive biological systems for the large-scale production of pharmaceutical proteins. In particular, seeds offer special advantages, such as ease of handling and long-term stable storage. Nevertheless, most of the studies of the expression of antibodies in plants have been performed in leaves. We report the expression of a secreted (sec-Ab) or KDEL-tagged (Ab-KDEL) mutant of the 14D9 monoclonal antibody in transgenic tobacco leaves and seeds. Although the KDEL sequence has little effect on the accumulation of the antibody in leaves, it leads to a higher antibody yield in seeds. sec-Ab(Leaf) purified from leaf contains complex N-glycans, including Lewis(a) epitopes, as typically found in extracellular glycoproteins. In contrast, Ab-KDEL(Leaf) bears only high-mannose-type oligosaccharides (mostly Man 7 and 8) consistent with an efficient endoplasmic reticulum (ER) retention/cis-Golgi retrieval of the antibody. sec-Ab and Ab-KDEL gamma chains purified from seeds are cleaved by proteases and contain complex N-glycans indicating maturation in the late Golgi compartments. Consistent with glycosylation of the protein, Ab-KDEL(Seed) was partially secreted and sorted to protein storage vacuoles (PSVs) in seeds and not found in the ER. This dual targeting may be due to KDEL-mediated targeting to the PSV and to a partial saturation of the vacuolar sorting machinery. Taken together, our results reveal important differences in the ER retention and vacuolar sorting machinery between leaves and seeds. In addition, we demonstrate that a plant-made antibody with triantennary high-mannose-type N-glycans has similar Fab functionality to its counterpart with biantennary complex N-glycans, but the former antibody interacts with protein A in a stronger manner and is more immunogenic than the latter. Such differences could be related to a variable immunoglobulin G (IgG)-Fc folding that would depend on the size of the N-glycan.

Functional conservation of cold shock domains in bacteria and higher plants

In Escherichia coli, a family of cold shock proteins (CSPs) function as transcription antiterminators or translational enhancers at low temperature by destabilizing RNA secondary structure. A wheat nucleic acid-binding protein (WCSP1) was found to contain a cold shock domain (CSD) bearing high similarity to E. coli cold shock proteins. In the present study, a series of mutations were introduced into WCSP1, and its functionality was investigated by using in vivo and in vitro assays in the context of functional conservation with E. coli CSPs. Constitutive expression of WT WCSP1 in an E. coli cspA, cspB, cspE, cspG quadruple deletion mutant complemented its cold-sensitive phenotype, suggesting that WCSP1 shares a function with E. coli CSPs for cold adaptation. In addition, transcription antitermination activity was demonstrated for WCSP1 by using an E. coli strain that has a hairpin loop upstream of a chloramphenicol resistance gene. In vitro dsDNA melting assays clearly demonstrated that WCSP1 melts dsDNA, an activity that was positively correlated to the ability to bind ssDNA. When mutations were introduced at critical residues within the consensus RNA binding motifs (RNP1 and RNP2) of WCSP1, it failed to melt dsDNA. Studies with WCSP1-GFP fusion proteins documented patterns that are consistent with ER and nuclear localization. In vivo and in vitro functional analyses, coupled with subcellular localization data, suggest that WCSP1 may function as a RNA chaperone to destabilize secondary structure and is involved in the regulation of translation under low temperature.

Two new cysteine proteinases with specific expression patterns in mature and senescent tobacco (Nicotiana tabacum L.) leaves

Cysteine proteinases are involved in various physiological and developmental processes in plants. Two cDNAs from senescent and non-senescent tobacco leaves were isolated with degenerate primers designed from conserved regions of plant senescence-associated cysteine proteinases using rapid amplification of cDNA ends (RACE). Both sequences encode papain-like cysteine proteinases: the 833 bp fragment (NtCP1) encoding a C-terminus partial sequence of a putative tobacco cysteine proteinase gene whereas the 1300 bp fragment (NtCP2) is a full-length cysteine proteinase. On the amino acid sequence level, NtCP1 has a high similarity with other senescence-associated cysteine proteinases. It is expressed only in senescent leaves. It is not induced in mature green leaves upon exposure to drought or heat. These results suggest that it might be a good developmental senescence marker in tobacco. By contrast, NtCP2 has a high similarity to KDEL-tailed cysteine proteinases and is expressed in mature green leaves. Both drought and heat decreased NtCP2 transcript abundance in mature green leaves. It is concluded that NtCP1 is a senescence-specific cysteine proteinase whereas NtCP2 fulfils roles in green leaves that might be similar to those of KDEL-tailed cysteine proteinases involved, for example, in programmed cell death.

Optimization of Acidothermus cellulolyticus endoglucanase (E1) production in transgenic tobacco plants by transcriptional, post-transcription and post-translational modification

An attempt was made to obtain a high-level production of intact Acidothermus cellulolyticus endoglucanase (E1) in transgenic tobacco plants. The E1 expression was examined under the control of the constitutive and strong Mac promoter or light-inducible tomato Rubisco small sub-unit (RbcS-3C) promoter with its original or Alfalfa Mosaic Virus (AMV) RNA4 5'-untranslated leader (UTL) and targeted to different sub-cellular compartments via transit peptides. The transit peptides included native E1, endoplasmic reticulum, vacuole, apoplast, and chloroplast. E1 expression and its stability in transgenic plants were determined via E1 activity, protein immunoblotting, and RNA gel-blotting analyses. Effects of sub-cellular compartments on E1 production and its stability were determined in transgenic tobacco plants carrying one of six transgene expression vectors, where the E1 was under the control of Mac promoter, mannopine synthase transcription terminator, and one of the five transit peptides. Transgenic tobacco plants with an apoplastic transit peptide had the highest average E1 activity and protein accumulation, which was about 0.25% of total leaf soluble proteins estimated via E1 specific activity and protein gel blots. Intercellular fluid analyses confirmed that E1 signal peptide functioned properly in tobacco cells to secret E1 protein into the apoplast. By replacing RbcS-3C UTL with AMV RNA4 UTL E1 production was enhanced more than twofold, while it was less effective than the mannopine synthase UTL. It was observed that RbcS-3C promoter was more favorable for E1 expression in transgenic plants than the Mac promoter. E1 activity in dried tobacco seeds stored one year at room temperature was 45% higher than that observed immediately after harvesting, suggesting that E1 protein can be stored at room temperature for a long period. E1 stability in different sub-cellular compartments and the optimal combination of promoter, 5'-UTL, and sub-cellular compartmentation for heterologous protein production in transgenic plants are discussed.

A cut above the rest: the regulatory function of plant proteases

Proteolytic enzymes are intricately involved in many aspects of plant physiology and development. On the one hand, they are necessary for protein turnover. Degradation of damaged, misfolded and potentially harmful proteins provides free amino acids required for the synthesis of new proteins. Furthermore, the selective breakdown of regulatory proteins by the ubiquitin/proteasome pathway controls key aspects of plant growth, development, and defense. Proteases are, on the other hand, also responsible for the post-translational modification of proteins by limited proteolysis at highly specific sites. Limited proteolysis results in the maturation of enzymes, is necessary for protein assembly and subcellular targeting, and controls the activity of enzymes, regulatory proteins and peptides. Proteases are thus involved in all aspects of the plant life cycle ranging from the mobilization of storage proteins during seed germination to the initiation of cell death and senescence programs. This article reviews recent findings for the major catalytic classes, i.e. the serine, cysteine, aspartic, and metalloproteases, emphasizing the regulatory function of representative enzymes.

A novel C-terminal sequence from barley polyamine oxidase is a vacuolar sorting signal

Barley contains two different isoforms of flavin-containing polyamine oxidase (BPAO1 and BPAO2). We have previously demonstrated that BPAO2 is a symplastic protein in barley leaves. On the contrary, maize polyamine oxidase (MPAO), the best characterized member of this enzyme class, is apoplastic. Comparison of the derived amino-acid sequences of BPAO2 and MPAO has revealed that both precursor proteins include a cleavable N-terminal signal peptide of 25 amino acid residues, but the barley enzyme shows an extra C-terminal extension of eight amino acids. By means of MPAO engineering with BPAO2 C-terminal tail (MPAO-T) and exploiting transient expression in Nicotiana tabacum protoplasts, we demonstrate that this oligopeptide is a signal for protein sorting to the plant vacuole. The vacuolar sorting of MPAO-T was saturable. Specific mutations of the C-terminal tail were constructed to determine which amino acid residues of this novel propeptide affect proper protein sorting. No consensus sequence or common structural determinant is required for the intracellular retention of the MPAO-T protein, but a gradual lowering of the efficiency was observed as a result of progressive deletion of the C-terminus.

Endoplasmic reticulum-resident proteins are constitutively transported to vacuoles for degradation

Soluble endoplasmic reticulum (ER)-resident proteins have very long lives because of their ER residency. This residency depends largely on ER-retrieval signals at their C-terminus. We examined the long-term destiny of endogenous ER-resident proteins, a lumenal binding protein (BiP) and a protein disulfide isomerase (PDI), with cultured cells of Arabidopsis. ER residents, in contrast to vacuolar proteinases, were considerably degraded in cells at the stationary phase. A subcellular fractionation analysis suggested that ER residents were transported into the vacuoles, which accumulated the residents lacking the ER-retrieval signals. We showed that the PDI located in the vacuoles had high mannose glycans, but not complex glycans, which suggested that the ER resident was transported to the vacuoles independent of the medial/trans-Golgi complex. To visualize the pathway of transport of ER-resident proteins, tobacco BY-2 cells were transformed with a chimeric gene encoding an ER-targeted green fluorescent protein (30 kDa GFP-HDEL). In the transformed cells at the stationary phase, GFP fluorescence was observed in the vacuoles. A subcellular fractionation revealed that a trimmed form of 27 kDa GFP was localized in the vacuoles. Treatment with E-64d, an inhibitor of papain-type cysteine proteinases that inhibits the degradation of GFP in the vacuoles, resulted in a stable accumulation of 27 kDa GFP in the vacuoles, even in the logarithmic phase. Our results suggest that endogenous ER residents are transported constitutively to the vacuoles by bypassing the Golgi complex and are then degraded.