Coordinated protein co-expression in plants by harnessing the synergy between an intein and a viral 2A peptide.

A novel approach is developed for coordinated expression of multiple proteins from a single transgene in plants. An Ssp DnaE mini-intein variant engineered for hyper-N-terminal autocleavage is covalently linked to the foot-and-mouth disease virus 2A (F2A) peptide with unique ribosome skipping property, via a peptide linker, to create an 'IntF2A' self-excising fusion protein domain. This IntF2A domain acts, in cis, to direct highly effective release of its flanking proteins of interest (POIs) from a 'polyprotein' precursor in plants. This is successfully demonstrated in stably transformed cultured tobacco cells as well as in different organs of transgenic tobacco plants. Highly efficient polyprotein processing mediated by the IntF2A domain was also demonstrated in lettuce and Nicotiana benthamiana based on transient expression. Protein constituents released from the polyprotein precursor displayed proper function and accumulated at similar levels inside the cells. Importantly, no C-terminal F2A extension remains on the released POIs. We demonstrated co-expression of as many as three proteins in plants without compromising expression levels when compared with those using single-protein vectors. Accurate differential cellular targeting of released POIs is also achieved. In addition, we succeeded in expressing a fully assembled and functional chimeric anti-His Tag antibody in N. benthamiana leaves. The IntF2A-based polyprotein transgene system overcomes key impediments of existing strategies for multiprotein co-expression in plants, which is particularly important for gene/trait stacking.

Coordinate expression of multiple proteins in plant cells by exploiting endogenous kex2p-like protease activity.

Simultaneous expression of multiple proteins in plants finds ample applications. Here, we examined the biotechnological application of native kex2p-like protease activity in plants for coordinate expression of multiple secretory proteins from a single transgene encoding a cleavable polyprotein precursor. We expressed a secretory red fluorescent protein (DsRed) or human cytokine (GMCSF), fused to a downstream green fluorescent protein (GFP) by a linker containing putative recognition sites of the kex2p-like protease in tobacco cells and referred to them as RKG and GKG cells, respectively. Our analyses showed that GFP is cleaved off the fusion proteins and secreted into the media by both RKG and GKG cells. The cleaved GFP product displayed the expected fluorescence characteristics. Using GFP immunoprecipitation and fluorescence analysis, the cleaved DsRed product in the RKG cells was found to be functional as well. However, DsRed was not detected in the RKG culture medium, possibly due to its tetramer formation. Cleaved and biologically active GMCSF could also be detected in GKG cell extracts, but secreted GMCSF was found to be only at a low level, likely because of instability of GMCSF protein in the medium. Processing of polyprotein precursors was observed to be similarly effective in tobacco leaf, stem and root tissues. Importantly, we also demonstrated that, via agroinfiltration, polyprotein precursors can be efficiently processed in plant species other than tobacco. Collectively, our results demonstrate the utility of native kex2p-like protease activity for the expression of multiple secretory proteins in plant cells using cleavable polyprotein precursors containing kex2p linker(s).

Euglena gracilis ascorbate peroxidase forms an intramolecular dimeric structure: its unique molecular characterization.

Euglena gracilis lacks a catalase and contains a single APX (ascorbate peroxidase) and enzymes related to the redox cycle of ascorbate in the cytosol. In the present study, a full-length cDNA clone encoding the Euglena APX was isolated and found to contain an open reading frame encoding a protein of 649 amino acids with a calculated molecular mass of 70.5 kDa. Interestingly, the enzyme consisted of two entirely homologous catalytic domains, designated APX-N and APX-C, and an 102 amino acid extension in the N-terminal region, which had a typical class II signal proposed for plastid targeting in Euglena. A computer-assisted analysis indicated a novel protein structure with an intramolecular dimeric structure. The analysis of cell fractionation showed that the APX protein is distributed in the cytosol, but not the plastids, suggesting that Euglena APX becomes mature in the cytosol after processing of the precursor. The kinetics of the recombinant mature FL (full-length)-APX and the APX-N and APX-C domains with ascorbate and H2O2 were almost the same as that of the native enzyme. However, the substrate specificity of the mature FL-APX and the native enzyme was different from that of APX-N and APX-C. The mature FL-APX, but not the truncated forms, could reduce alkyl hydroperoxides, suggesting that the dimeric structure is correlated with substrate recognition. In Euglena cells transfected with double-stranded RNA, the silencing of APX expression resulted in a significant increase in the cellular level of H2O2, indicating the physiological importance of APX to the metabolism of H2O2.

A dual-intein autoprocessing domain that directs synchronized protein co-expression in both prokaryotes and eukaryotes.

Being able to coordinate co-expression of multiple proteins is necessary for a variety of important applications such as assembly of protein complexes, trait stacking, and metabolic engineering. Currently only few options are available for multiple recombinant protein co-expression, and most of them are not applicable to both prokaryotic and eukaryotic hosts. Here, we report a new polyprotein vector system that is based on a pair of self-excising mini-inteins fused in tandem, termed the dual-intein (DI) domain, to achieve synchronized co-expression of multiple proteins. The DI domain comprises an Ssp DnaE mini-intein N159A mutant and an Ssp DnaB mini-intein C1A mutant connected in tandem by a peptide linker to mediate efficient release of the flanking proteins via autocatalytic cleavage. Essentially complete release of constituent proteins, GFP and RFP (mCherry), from a polyprotein precursor, in bacterial, mammalian, and plant hosts was demonstrated. In addition, successful co-expression of GFP with chloramphenicol acetyltransferase, and thioredoxin with RFP, respectively, further substantiates the general applicability of the DI polyprotein system. Collectively, our results demonstrate the DI-based polyprotein technology as a highly valuable addition to the molecular toolbox for multi-protein co-expression which finds vast applications in biotechnology, biosciences, and biomedicine.

Light regulation of ascorbate biosynthesis is dependent on the photosynthetic electron transport chain but independent of sugars in Arabidopsis.

It has been known that leaves exposed to high light contain more L-ascorbic acid (AsA) than those in the shade. However, the mechanism of the light regulation of the AsA pool size in plants is largely unknown. In this work, the relationship between gene expression levels related to AsA biosynthesis and photosynthesis have been studied. When 2-week-old Arabidopsis plants grown under a 16 h daily photoperiod were moved into the dark, the AsA level in the leaves was decreased by 91% in 72 h, whereas it increased by 171% in the leaves of plants exposed to continuous light during the same period. Among the several enzymes of the AsA biosynthesis pathway, the transcript levels of GDP-D-mannose pyrophosphorylase, L-galactose 1-P phosphatase, L-galactono-1,4-lactone dehydrogenase, and the VTC2 gene were down-regulated in the dark. Treatment with inhibitors of photosynthesis, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine, arrested a rise in the AsA pool size accompanying the decrease in the transcript levels of the genes of the above enzyme in the leaves. When the plants were transferred to a medium containing 0.5% (w/v) sucrose, the photosynthesis activities and the leaf AsA levels were lowered even under exposure to light compared with those in plants on the medium without sucrose. In contrast, the AsA level in leaves of the sugar-insensitive Arabidopsis mutant abi4/sun6 was unaffected by external sucrose. No significant difference in the expression profiles for AsA biosynthesis enzymes was observed between the wild-type and mutant plants by sucrose feeding. The results suggest that photosynthetic electron transport of chloroplasts is closely related to AsA pool size regulation in leaves.

Feedback inhibition of spinach L-galactose dehydrogenase by L-ascorbate.

We have studied the enzymological properties of L-galactose dehydrogenase (l-GalDH), a key enzyme in the biosynthetic pathway of l-ascorbate (AsA) in plants. L-GalDH was purified approximately 560-fold from spinach leaves. The enzyme was a homodimer with a subunit mass of 36 kDa. We also cloned the full-length cDNA of spinach L-GalDH, which contained an open reading frame encoding 322 amino acid residues with a calculated molecular mass of 35,261 Da. The deduced amino acid sequence of the cDNA showed 82, 79 and 75% homology to L-GalDH from kiwifruit, apple and Arabidopsis, respectively. Recombinant enzyme expressed from the cDNA in Escherichia coli showed L-GalDH activity. Southern blot analysis revealed that the spinach L-GalDH gene occurs in a single copy. Northern blot analysis suggests that L-GalDH is expressed in different organs of spinach. The purified native L-GalDH showed high specificity for L-galactose with a Km of 116.2+/-3.2 microM. Interestingly, spinach L-GalDH exhibited reversible inhibition by AsA, the end-product of the biosynthetic pathway. The inhibition kinetics indicated a linear-competitive inhibition with a Ki of 133.2+/-7.2 microM, suggesting feedback regulation in AsA synthesis in the plant.