Novel Hydrophobin Fusion Tags for Plant-Produced Fusion Proteins

Production and characterization of novel Anti-HIV Fc-fusion proteins in plant-based systems: Nicotiana benthamiana & tobacco BY-2 cell suspension

Multifunctional anti-HIV Fc-fusion proteins aim to tackle HIV efficiently through multiple modes of action. Although results have been promising, these recombinant proteins are hard to produce. This study explored the production and characterization of anti-HIV Fc-fusion proteins in plant-based systems, specifically Nicotiana benthamiana plants and tobacco BY-2 cell suspension. Fc-fusion protein expression in plants was optimized by incorporating codon optimization, ER retention signals, and hydrophobin fusion elements. Successful transient protein expression was achieved in N. benthamiana, with notable improvements in expression levels achieved through N-terminal hydrophobin fusion and ER retention signals. Stable expression in tobacco BY-2 resulted in varying accumulation levels being at highest 2.2.mg/g DW. The inclusion of hydrophobin significantly enhanced accumulation, providing potential benefits for downstream processing. Mass spectrometry analysis confirmed the presence of the ER retention signal and of N-glycans. Functional characterization revealed strong binding to CD64 and CD16a receptors, the latter being important for antibody-dependent cellular cytotoxicity (ADCC). Interaction with HIV antigens indicated potential neutralization capabilities. In conclusion, this research highlights the potential of plant-based systems for producing functional anti-HIV Fc-fusion proteins, offering a promising avenue for the development of these novel HIV therapies.

Plant Expression of Hydrophobin Fused K39 Antigen for Visceral Leishmaniasis Immunodiagnosis

Visceral leishmaniasis is a Neglected Tropical Disease of high mortality caused by the protozoan Leishmania infantum. Its transmission cycle is complex, and it has in the domestic dog its main reservoir. The diagnostic tests currently used rely on prokaryotic systems' proteins, but their low sensitivity increases the disease's burden. The plant transient expression of recombinant proteins allows the production of complex antigens. However, this system has limited competitiveness against the bacterial production of purified antigens. Thus, we have shown that the L. infantum K39 antigen's fusion to a hydrophobin allows its production for diagnostic tests without the need for intensive purification. The sera of naturally infected dogs specifically detect the semi-purified rK39-HFBI protein. The test validation against a panel of 158 clinical samples demonstrates the platform's viability, resulting in sensitivity and specificity of 90.7 and 97.5%, respectively. Thus, the use of semi-purified antigens fused to hydrophobins can become the standard platform for large-scale antigens production to expand diagnostic tests for other human and veterinary diseases worldwide.

Advances in Plant-Derived Scaffold Proteins

Scaffold proteins form critical biomatrices that support cell adhesion and proliferation for regenerative medicine and drug screening. The increasing demand for such applications urges solutions for cost effective and sustainable supplies of hypoallergenic and biocompatible scaffold proteins. Here, we summarize recent efforts in obtaining plant-derived biosynthetic spider silk analogue and the extracellular matrix protein, collagen. Both proteins are composed of a large number of tandem block repeats, which makes production in bacterial hosts challenging. Furthermore, post-translational modification of collagen is essential for its function which requires co-transformation of multiple copies of human prolyl 4-hydroxylase. We discuss our perspectives on how the GAANTRY system could potentially assist the production of native-sized spider dragline silk proteins and prolyl hydroxylated collagen. The potential of recombinant scaffold proteins in drug delivery and drug discovery is also addressed.

Enhancing chimeric hydrophobin II-vascular endothelial growth factor A165 expression in Pichia pastoris and its efficient purification using hydrophobin counterpart

We report cloning and expressing of recombinant human VEGF-A₁₆₅, fused at the N-terminal with Hydrophobin II (HFBII) from Trichoderma reseei, in yeast Pichia pastoris. We validated the construct using SDS-PAGE and ELISA against VEGF-A₁₆₅ and efficiently performed protein purification and enrichment based on HFBII counterpart and using an aqueous two-phase system (ATPS) with nonionic surfactant X-114. We studied the effects of various culture medium additives and interaction effects of positive factors to increase the recombinant HFBII-VEGF-A₁₆₅ production. Supplementing the Pichia pastoris cell culture medium with Mg²⁺, Polysorbate 20 (PS 20), and 4-phenylbutyrate (PBA) improved the expression of the chimeric protein. Orthogonal experiments showed that the optimal condition to achieve maximal HFBII-VEGF-A₁₆₅ production was with the addition of PBA, PS 20, and MgSO₄. Under this condition, the production of the target protein was 4.5 times more than that in the medium without the additives. Overall, our approach to produce chimeric HFBII-VEGF-A₁₆₅ and selectively capture it in ATPS is promising for large-scale protein production without laborious downstream processing.

Hydrophobins: multifunctional biosurfactants for interface engineering

Hydrophobins are highly surface-active proteins that have versatile potential as agents for interface engineering. Due to the large and growing number of unique hydrophobin sequences identified, there is growing potential to engineer variants for particular applications using protein engineering and other approaches. Recent applications and advancements in hydrophobin technologies and production strategies are reviewed. The application space of hydrophobins is large and growing, including hydrophobic drug solubilization and delivery, protein purification tags, tools for protein and cell immobilization, antimicrobial coatings, biosensors, biomineralization templates and emulsifying agents. While there is significant promise for their use in a wide range of applications, developing new production strategies is a key need to improve on low recombinant yields to enable their use in broader applications; further optimization of expression systems and yields remains a challenge in order to use designed hydrophobin in commercial applications.

Engineered Fusion Proteins for Efficient Protein Secretion and Purification of a Human Growth Factor from the Green Microalga Chlamydomonas reinhardtii

Light-driven recombinant protein (RP) production in eukaryotic microalgae offers a sustainable alternative to other established cell-culture systems. RP production via secretion into the culture medium enables simple product separation from the cells adding a layer of process value in addition to the algal biomass, which can be separately harvested. For the model microalga Chlamydomonas reinhardtii, a broad range of molecular tools have been established to enable heterologous gene expression; however, low RP production levels and unreliable purification from secretion concepts have been reported. Domesticated C. reinhardtii strains used for genetic engineering are often cell-wall deficient. These strains nevertheless secrete cell-wall components such as insoluble (hydroxy)proline-rich glycoproteins into the culture media, which hinder downstream purification processes. Here, we attempted to overcome limitations in secretion titers and improve protein purification by combining fusion partners that enhance RP secretion and enable alternative aqueous two-phase (ATPS) RP extraction from the culture medium. Protein fusions were strategically designed to contain a stably folded peptide, which enhanced secretion capacities and gave insights into (some) regulatory mechanisms responsible for this process in the algal host. The elevated protein titers mediated by this fusion were then successfully applied in combination with a fungal hydrophobin tag, which enabled protein purification from the complex microalgal extracellular environment by ATPS, to yield functional recombinant human epidermal growth factor (hEGF) from the algal host.

What We Do Not Know about Fungal Cell Adhesion Molecules

There has been extensive research on structure and function of fungal cell adhesion molecules, but the most of the work has been about adhesins in Candida albicans and Saccharomyces cerevisiae. These yeasts are members of a single ascomycete order, and adhesion molecules from the six other fungal phyla are only sparsely described in the literature. In these other phyla, most of the research is at the cellular level, rather than at the molecular level, so there has been little characterization of the adhesion molecules themselves. A catalog of known adhesins shows some common features: high Ser/Thr content, tandem repeats, N- and O-glycosylations, GPI anchors, dibasic sequence motifs, and potential amyloid-forming sequences. However, none of these features is universal. Known ligands include proteins and glycans on homologous cells and host cells. Existing and novel tools can exploit the availability of genome sequences to identify and characterize new fungal adhesins. These include bioinformatics tools and well-established yeast surface display models, which could be coupled with an adhesion substrate array. Thus, new knowledge could be exploited to answer key questions in fungal ecology, animal and plant pathogenesis, and roles of biofilms in infection and biomass turnover.

A Plant-Produced Candidate Subunit Vaccine Reduces Shedding of Enterohemorrhagic Escherichia coli in Ruminants

Enterohemorrhagic Escherichia coli (EHEC) are commonly present in the gastrointestinal tract of cattle and cause serious infectious disease in humans. Immunizing cattle against EHEC is a promising strategy to decrease the risk of food contamination; however, veterinary vaccines against EHEC such as Econiche have not been widely adopted by the agricultural industry, and have been discontinued, prompting the need for more cost-effective EHEC vaccines. The objective of this project is to develop a platform to produce plant-made antigens for oral vaccination of ruminants against EHEC. Five recombinant proteins were designed as vaccine candidates and expressed transiently in Nicotiana benthamiana and transplastomically in Nicotiana tabacum. Three of these EHEC proteins, NleA, Stx2b, and a fusion of EspA accumulated when transiently expressed. Transient protein accumulation was the highest when EHEC proteins were fused to an elastin-like polypeptide (ELP) tag. In the transplastomic lines, EspA accumulated up to 479 mg kg-1 in lyophilized leaf material. Sheep that were administered leaf tissue containing recombinant EspA shed less E. coli O157:H7 when challenged, as compared to control animals. These results suggest that plant-made, transgenic EspA has the potential to reduce EHEC shedding in ruminants.

Coating Nanoparticles with Plant-Produced Transferrin-Hydrophobin Fusion Protein Enhances Their Uptake in Cancer Cells

The encapsulation of drugs to nanoparticles may offer a solution for targeted delivery. Here, we set out to engineer a self-assembling targeting ligand by combining the functional properties of human transferrin and fungal hydrophobins in a single fusion protein. We showed that human transferrin can be expressed in Nicotiana benthamiana plants as a fusion with Trichoderma reesei hydrophobins HFBI, HFBII, or HFBIV. Transferrin-HFBIV was further expressed in tobacco BY-2 suspension cells. Both partners of the fusion protein retained their functionality; the hydrophobin moiety enabled migration to a surfactant phase in an aqueous two-phase system, and the transferrin moiety was able to reversibly bind iron. Coating porous silicon nanoparticles with the fusion protein resulted in uptake of the nanoparticles in human cancer cells. This study provides a proof-of-concept for the functionalization of hydrophobin coatings with transferrin as a targeting ligand.

Cytosolic and Nuclear Co-localization of Betalain Biosynthetic Enzymes in Tobacco Suggests that Betalains Are Synthesized in the Cytoplasm and/or Nucleus of Betalainic Plant Cells

Betalains replace anthocyanins as color pigments in most families of Caryophyllales. Unlike anthocyanins, betalains are derived from tyrosine via three enzymatic steps: hydroxylation of L-tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA; step 1), and conversion of L-DOPA to betalamic acid (step 2), and to cyclo-DOPA (cDOPA; step 3). The principal enzymes responsible for these reactions have been elucidated at the molecular level, but their subcellular localizations have not been explored; hence, the intracellular compartments wherein betalains are biosynthesized remain unknown. Here, we report on the subcellular localization of these principal enzymes. Bioinformatic predictors and N- and C-terminal GFP tagging in transgenic tobacco, showed that Beta vulgaris CYP76AD1 which mediates both steps 1 and 3, DODA1 that catalyzes step 2, and CYP76AD6 which also mediates step 1, were similarly localized to the cytoplasm and nucleus (although the P450s were also weakly present in the endoplasmic reticulum). These two compartments were also the principal locations of Mirabilis jalapa cDOPA5GT. The cytoplasmic and nuclear co-localization of these key enzymes in tobacco suggests that betalains are biosynthesized in the cytoplasm and/or nucleus of betalain-containing plant cells. Elucidation of the subcellular compartmentation of betalain biosynthesis will facilitate the bioengineering of the betalain biosynthetic pathway in non-betalain-containing plants.

Subcellular Localization of a Plant Catalase-Phenol Oxidase, AcCATPO, from Amaranthus and Identification of a Non-canonical Peroxisome Targeting Signal

AcCATPO is a plant catalase-phenol oxidase recently identified from red amaranth. Its physiological function remains unexplored. As the starting step of functional analysis, here we report its subcellular localization and a non-canonical targeting signal. Commonly used bioinformatics programs predicted a peroxisomal localization for AcCATPO, but failed in identification of canonical peroxisomal targeting signals (PTS). The C-terminal GFP tagging led the fusion protein AcCATPO-GFP to the cytosol and the nucleus, but N-terminal tagging directed the GFP-AcCATPO to peroxisomes and nuclei, in transgenic tobacco. Deleting the tripeptide (PTM) at the extreme C-terminus almost ruled out the peroxisomal localization of GFP-AcCATPOΔ3, and removing the C-terminal decapeptide completely excluded peroxisomes as the residence of GFP-AcCATPOΔ10. Furthermore, this decapeptide as a targeting signal could import GFP-10aa to the peroxisome exclusively. Taken together, these results demonstrate that AcCATPO is localized to the peroxisome and the nucleus, and its peroxisomal localization is attributed to a non-canonical PTS1, the C-terminal decapeptide which contains an internal SRL motif and a conserved tripeptide P-S/T-I/M at the extreme of C-terminus. This work may further the study as to the physiological function of AcCATPO, especially clarify its involvement in betalain biosynthesis, and provide a clue to elucidate more non-canonic PTS.