DNA aptamer-based affinity chromatography system for purification of recombinant proteins tagged with lysine tag.

Affinity chromatography (AC) is one of the techniques widely used for the purification of recombinant proteins. In our previous study, we presented a successful application of the Argi system [1] for the purification of recombinant proteins, based on the specific interaction between an arginine tag and a DNA aptamer. Exploring the possible application of positively charged peptide tags in the purification of recombinant proteins, in this study we developed and characterized an AC system based on the specific and reversible interaction between a DNA aptamer and a lysine tag (Lys-tag) comprising five lysine residues (5 K). We optimized the length of both the selected DNA aptamer and Lys-tag which were named B5K aptamer and 5K-tag, respectively. The results showed that the stability of the B5K aptamer and 5K-tag was dependent on the presence of potassium ions. The conditions for mild elution of 5K-tagged protein from B5K aptamer were determined. Our study proved that the developed system can be used for the purification of recombinant proteins from Escherichia coli total protein extracts.

Phototropin Interactions with SUMO Proteins.

The disruption of the sumoylation pathway affects processes controlled by the two phototropins (phots) of Arabidopsis thaliana, phot1 and phot2. Phots, plant UVA/blue light photoreceptors, regulate growth responses and fast movements aimed at optimizing photosynthesis, such as phototropism, chloroplast relocations and stomatal opening. Sumoylation is a posttranslational modification, consisting of the addition of a SUMO (SMALL UBIQUITIN-RELATED MODIFIER) protein to a lysine residue in the target protein. In addition to affecting the stability of proteins, it regulates their activity, interactions and subcellular localization. We examined physiological responses controlled by phots, phototropism and chloroplast movements, in sumoylation pathway mutants. Chloroplast accumulation in response to both continuous and pulse light was enhanced in the E3 ligase siz1 mutant, in a manner dependent on phot2. A significant decrease in phot2 protein abundance was observed in this mutant after blue light treatment both in seedlings and mature leaves. Using plant transient expression and yeast two-hybrid assays, we found that phots interacted with SUMO proteins mainly through their N-terminal parts, which contain the photosensory LOV domains. The covalent modification in phots by SUMO was verified using an Arabidopsis sumoylation system reconstituted in bacteria followed by the mass spectrometry analysis. Lys 297 was identified as the main target of SUMO3 in the phot2 molecule. Finally, sumoylation of phot2 was detected in Arabidopsis mature leaves upon light or heat stress treatment.

Inhibition of DNA replication by an anti-PCNA aptamer/PCNA complex.

Proliferating cell nuclear antigen (PCNA) is a multifunctional protein present in the nuclei of eukaryotic cells that plays an important role as a component of the DNA replication machinery, as well as DNA repair systems. PCNA was recently proposed as a potential non-oncogenic target for anti-cancer therapy. In this study, using the Systematic Evolution of Ligands by EXponential enrichment (SELEX) method, we developed a short DNA aptamer that binds human PCNA. In the presence of PCNA, the anti-PCNA aptamer inhibited the activity of human DNA polymerase δ and ϵ at nM concentrations. Moreover, PCNA protected the anti-PCNA aptamer against the exonucleolytic activity of these DNA polymerases. Investigation of the mechanism of anti-PCNA aptamer-dependent inhibition of DNA replication revealed that the aptamer did not block formation, but was a component of PCNA/DNA polymerase δ or ϵ complexes. Additionally, the anti-PCNA aptamer competed with the primer-template DNA for binding to the PCNA/DNA polymerase δ or ϵ complex. Based on the observations, a model of anti-PCNA aptamer/PCNA complex-dependent inhibition of DNA replication was proposed.

Selection and analysis of a DNA aptamer binding α-amanitin from Amanita phalloides.

Mushroom foraging is very popular in some regions of the world. Sometimes toxic and edible mushrooms are mistaken by mushroom collectors, leading to serious human poisoning. The group of mushrooms highly dangerous for human health includes Amanita phalloides. This mushroom produces a toxic octapeptide called α-amanitin which is an inhibitor of nuclear RNA polymerase II. The inhibition of this polymerase results in the abortion of mRNA synthesis. The ingestion of A. phalloides causes liver failure due to the fact that most of the toxin is uptaken by hepatocytes. The hospitalization of poisoned patients involves the removal of the toxin from the digestive tract, its dilution in the circulatory system and the administration of therapeutic adjuvants. Since there is no effective antidote against amanitin poisoning, in this study we developed a DNA aptamer exhibiting specific binding to α-amanitin. This aptamer was selected using the SELEX (Systematic Evolution of Ligands by Exponential Enrichment) method. Next, its ability of toxin removal from aqueous solution was confirmed by pull-down assay. The aptamer region sufficient for α-amanitin binding was determined. Finally, the dissociation constant of the α-amanitin/DNA aptamer complex was calculated.

The Argi system: one-step purification of proteins tagged with arginine-rich cell-penetrating peptides.

The discovery of cell penetrating peptides (CPPs) opened new perspectives for the delivery of proteins into human cells. It is considered that in the future CPP-mediated transport of therapeutic proteins may find applications in the treatment of human diseases. Despite this fact a fast and simple method for the purification of CPP-tagged proteins, free of additional tags, was not available to date. To fill this gap we developed the Argi system for one-step purification of proteins tagged with arginine rich CPPs.

High expression of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE coincides with initiation of various developmental pathways in in vitro culture of Trifolium nigrescens.

The aim of this study was to identify and examine the expression pattern of the ortholog of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE gene from Trifolium nigrescens (TnSERK) in embryogenic and non-regenerative cultures of immature cotyledonary-stage zygotic embryos (CsZEs). In the presence of 1-naphthaleneacetic acid and N(6)-[2-isopentenyl]-adenine, the CsZE regenerated embryoids directly and in a lengthy culture produced callus which was embryogenic or remained non-regenerative. As revealed by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), the TnSERK was expressed in both embryogenic and non-regenerative cultures, but the expression level was significantly higher in embryogenic ones. An in situ RNA hybridization assay revealed that the expression of TnSERK preceded the induction of cell division in explants, and then, it was maintained exclusively in actively dividing cells from which embryoids, embryo-like structures (ELSs), callus or tracheary elements were produced. However, the cells involved in different morphogenic events differed in intensity of hybridization signal which was the highest in embryogenic cells. The TnSERK was up-regulated during the development of embryoids, but in cotyledonary embryos, it was preferentially expressed in the regions of the apical meristems. The occurrence of morphological and anatomical abnormalities in embryoid development was preceded by a decline in TnSERK expression, and this coincided with the parenchymatization of the ground tissue in developing ELSs. TnSERK was also down-regulated during the maturation of parenchyma and xylem elements in CsZE and callus. Altogether, these data suggest the involvement of TnSERK in the induction of various developmental programs related to differentiation/transdifferentiation and totipotent state of cell(s).

Imidazole-free purification of His3-tagged recombinant proteins using ssDNA aptamer-based affinity chromatography.

Immobilized metal ion affinity chromatography (IMAC) is widely used for the purification of many different His6-tagged recombinant proteins. On the one hand, it is a powerful technique but on the other hand it has its disadvantages. In this report, we present the development of a unique ssDNA aptamer for the purification of His3-tagged recombinant proteins. Our study shows that stability of the His3-tag/H3T aptamer complex can be controlled by the sodium ion concentration. Based on this feature, we demonstrate that H3T aptamer resin was successfully employed for the purification of three out of four tested His3-tagged recombinant proteins from an E. coli total protein extract using imidazole-free buffers. Finally, we show that the purity of His3-tagged proteins is superior when purified with the help of the H3T aptamer in comparison with Ni-NTA resin.

RAD5a ubiquitin ligase is involved in ubiquitination of Arabidopsis thaliana proliferating cell nuclear antigen.

The proliferating cell nuclear antigen (PCNA) is post-translationally modified by ubiquitin in yeast and mammalian cells. It is widely accepted that in yeast mono- and polyubiquitinated PCNA is involved in distinct pathways of DNA postreplication repair. This study showed an interaction between plant ubiquitin and PCNA in the plant cell. Using different approaches, it was demonstrated that Arabidopsis RAD5a ubiquitin ligase is involved in the post-translational modification of plant PCNA. A detailed analysis of the properties of selected Arabidopsis ubiquitin-conjugating enzymes (AtUBC) has shown that a plant homologue of yeast RAD6 (AtUBC2) is sufficient to monoubiquitinate AtPCNA in the absence of ubiquitin ligase. Using different combinations of selected AtUBC proteins together with AtRAD5a, it was demonstrated that plants have potential to use different pathways to ubiquitinate PCNA. The analysis of Arabidopsis PCNA1 and PCNA2 did not demonstrate substantial differences in the ubiquitination pattern between these two proteins. The major ubiquitination target of Arabidopsis PCNA, conserved in eukaryotes, is lysine 164. Taken together, the presented results clearly demonstrate the involvement of Arabidopsis UBC and RAD5a proteins in the ubiquitination of plant PCNA at lysine 164. The data show the complexity of the plant ubiquitination system and open new questions about its regulation in the plant cell.

Arabidopsis thaliana proliferating cell nuclear antigen has several potential sumoylation sites.

Proliferating cell nuclear antigen (PCNA) is post-translationally modified in yeast and animal cells. Major studies carried out in the last decade have focused on the role of sumoylated and ubiquitinated PCNA. Using different approaches, an interaction between plant PCNA and SUMO both in vivo and in bacteria has been demonstrated for the first time. In addition, identical sumoylation patterns for both AtPCNA1 and 2 were observed in bacteria. The plant PCNA sumoylation pattern has been shown to differ significantly from that of Saccharomyces cerevisiae. This result contrasts with a common opinion based on previous structural analysis of yeast, human, and plant PCNAs, which treats PCNA as a highly conserved protein even between species. Analyses of AtPCNA post-translational modifications using different SUMO proteins (SUMO1, 2, 3, and 5) revealed similar modification patterns for each tested SUMO protein. Potential target lysine residues that might be sumoylated in vivo were identified on the basis of in bacteria AtPCNA mutational analyses. Taken together, these results clearly show that plant PCNA is post-translationally modified in bacteria and may be sumoylated in a plant cell at various sites. These data open up important new perspectives for further detailed studies on the role of PCNA sumoylation in plant cells.