Assembly of water-soluble chlorophyll-binding proteins with native hydrophobic chlorophylls in water-in-oil emulsions

Brassica napus Drought-Induced 22-kD Protein (BnD22) Acts Simultaneously as a Cysteine Protease Inhibitor and Chlorophyll-Binding Protein

Class II water-soluble chlorophyll proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins that bind with chlorophyll (Chl) and its derivatives. The physiological function of WSCPs is still unclear, but it is assumed to be involved in stress responses, which is likely related to their Chl-binding and protease inhibition (PI) activities. Yet, the dual function and simultaneous functionality of WSCPs must still be better understood. Here, the biochemical functions of Brassica napus drought-induced 22-kDa protein (BnD22), a major WSCP expressed in B. napus leaves, were investigated using recombinant hexahistidine-tagged protein. We showed that BnD22 inhibited cysteine proteases, such as papain, but not serine proteases. BnD22 was able to bind with Chla or Chlb to form tetrameric complexes. Unexpectedly, BnD22-Chl tetramer displays higher inhibition toward cysteine proteases, indicating (i) simultaneous Chl-binding and PI activities and (ii) Chl-dependent activation of PI activity of BnD22. Moreover, the photostability of BnD22-Chl tetramer was reduced upon binding with the protease. Using three-dimensional structural modeling and molecular docking, we revealed that Chl binding favors interaction between BnD22 and proteases. Despite its Chl-binding ability, the BnD22 was not detected in chloroplasts but rather in the endoplasmic reticulum and vacuole. In addition, the C-terminal extension peptide of BnD22, which cleaved off post-translationally in vivo, was not implicated in subcellular localization. Instead, it drastically promoted the expression, solubility and stability of the recombinant protein.

New insights into chlorophyll-WSCP (water-soluble chlorophyll proteins) interactions : The case study of BnD22 (Brassica napus drought-induced 22 kDa)

The water-soluble chlorophyll-proteins (WSCP) of class II from Brassicaceae are non-photosynthetic proteins that bind chlorophylls (Chls) and chlorophyll derivatives. Their physiological roles, biochemical functions and mode of action are still unclear. It is assumed that the WSCPs have a protection function against Chl photodamage during stressful conditions. WSCPs are subdivided into class IIA and class IIB according to their apparent Chla/b binding ratio. Although their Chla/Chlb binding selectivity has been partly characterized, their Chl affinities are not yet precisely defined. For instance, WSCPs IIA do not show any Chl binding preference while WSCPs IIB have greater affinity to Chlb. In this study, we present a novel method for assessment of Chl binding to WSCPs based on the differences of Chl photobleaching rates in a large range of Chl/protein ratios. The protein we have chosen to study WSCP is BnD22, a WSCP IIA induced in the leaves of Brassica napus under water deficit. BnD22 formed oligomeric complexes upon binding to Chla and/or Chlb allowing a protective effect against photodamage. The binding constants indicate that BnD22 binds with high affinity the Chls and with a strong selectivity to Chla. Moreover, dependending of Chl/protein ratio upon reconstitution, two distinct binding events were detected resulting from difference of Chl stoichiometry inside oligomeric complexes.

Controlling synthetic membraneless organelles by a red-light-dependent singlet oxygen-generating protein

Membraneless organelles (MLOs) formed via protein phase separation have great implications for both physiological and pathological processes. However, the inability to precisely control the bioactivities of MLOs has hindered our understanding of their roles in biology, not to mention their translational applications. Here, by combining intrinsically disordered domains such as RGG and mussel-foot proteins, we create an in cellulo protein phase separation system, of which various biological activities can be introduced via metal-mediated protein immobilization and further controlled by the water-soluble chlorophyll protein (WSCP)—a remarkably stable, red-light-responsive singlet oxygen generator. The WSCP-laden protein condensates undergo a liquid-to-solid phase transition on light exposure, due to oxidative crosslinking, providing a means to control catalysis within synthetic MLOs. Moreover, these photoresponsive condensates, which retain the light-induced phase-transition behavior in living cells, exhibit marked membrane localization, reminiscent of the semi-membrane-bound compartments like postsynaptic densities in nervous systems. Together, this engineered system provides an approach toward controllable synthetic MLOs and, alongside its light-induced phase transition, may well serve to emulate and explore the aging process at the subcellular or even molecular level.

Membraneless organelles play vital cellular roles, and control over their formation and state could have varied applications. Here, the authors develop photoresponsive synthetic condensates whose activity can be controlled through the use of light to trigger liquid-to-solid phase transition.

Direct Assembly in Aqueous Solutions of Stable Chlorophyllide Complexes with Type II Water-soluble Chlorophyll Proteins

Water-soluble chlorophyll-binding proteins (WSCPs) from Brassicaceae constitute a small family of non-photosynthetic proteins that may provide a useful benchmark and model system for studying molecular aspects of chlorophyll-protein interactions such as the tuning of absorption and emission spectra, and binding selectivity. WSCP apo-proteins are readily expressed by recombinant DNA techniques and can be assembled in vitro with natural and synthetic chlorophyll derivatives. The complexes with native chlorophylls are exceptionally stable toward thermal dissociation and protein denaturation due to hydrophobic interactions with the chlorophyll's phytyl chains that stabilize the core of the WSCP tetrameric complexes. However, assembly requires the use of detergents or water-in-oil emulsions to introduce the hydrophobic pigments into the water-soluble apo-proteins. Here, we explore the direct assembly of recombinant WSCPs with the water-soluble phytyl-free chlorophyll analogue chlorophyllide a in aqueous solutions. We show that the complexes formed by mixing chlorophyllide and WSCP apo-proteins are exclusively tetrameric, and while they lack the extreme thermostability of the respective chlorophyll complexes, they are still thermostable up to around 60°C. Their absorption and CD spectra are very similar to the chlorophyll complexes albeit slight peak shifts and broadening of the bands indicate variations in pigment and protein conformations, and less rigid structures. Simplifying the assembly process of WSCPs opens new possibilities for their use in modelling natural chlorophyll-protein complexes, and as templates for designing novel artificial protein-pigment complexes.

How water-soluble chlorophyll protein extracts chlorophyll from membranes

Water-soluble chlorophyll proteins (WSCPs) found in Brassicaceae are non-photosynthetic proteins that bind only a small number of chlorophylls. Their biological function remains unclear, but recent data indicate that WSCPs are involved in stress response and pathogen defense as producers of reactive oxygen species and/or Chl-regulated protease inhibitors. For those functions, WSCP apoprotein supposedly binds Chl to become physiologically active or inactive, respectively. Thus, Chl-binding seems to be a pivotal step for the biological function of WSCP. WSCP can extract Chl from the thylakoid membrane but little is known about the mechanism of how Chl is sequestered from the membrane into the binding sites. Here, we investigate the interaction of WSCP with the thylakoid membrane in detail. The extraction of Chl from the thylakoid by WSCP apoprotein is a slow and inefficient reaction, because WSCP presumably does not directly extract Chl from other Chl-binding proteins embedded in the membrane. WSCP apoprotein interacts with model membranes that contain the thylakoid lipids MGDG, DGDG or PG, and can extract Chl from those. Furthermore, the WSCP-Chl complex, once formed, no longer interacts with membranes. We concluded that the surroundings of the WSCP pigment-binding site are involved in the WSCP-membrane interaction and identified a ring of hydrophobic amino acids with two conserved Trp residues around the Chl-binding site. Indeed, WSCP variants, in which one of the Trp residues was exchanged for Phe, still interact with the membrane but are no longer able to extract Chl.

Conversion of NaHCO3 to Na2 CO3 with a growth of Arthrospira platensis cells in 660 m2 raceway ponds with a CO2 bicarbonation absorber

The weight ratio of Na2 CO3 /NaHCO3 was investigated in order to improve microalgal productivity in large-scale industrial operations by converting NaHCO3 to Na2 CO3 with a growth of Arthrospira platensis cells in 660 m2 raceway ponds. Two microalgal cultivation systems with a NaHCO3 by-product (SPBP) and a CO2 bicarbonation absorber (CBAP) were firstly thoroughly introduced. There was a 13.3% decrease in the initial weight ratio of Na2 CO3 /NaHCO3 resulting in a 25.3% increase in the biomass growth rate with CBAP, compared to that of SPBP. Increased sunlight intensity, solution temperature and pH all resulted in both a higher HCO 3 - absorbance and CO 3 2 - release, thereby increasing the weight ratio of Na2 CO3 /NaHCO3 during the growth of A. platensis. The biomass growth rate was peaked at 39.9 g m-2  day-1 when the weight ratio of Na2 CO3 /NaHCO3 was 3.7. Correspondingly, the cell pigments (chlorophyll a and carotenoid) and trichome size (helix pitch and trichome length) reached to a maximum state of 8.47 mg l-1 , 762 μg l-1 , 57 and 613 μm under the CBAP system.

How water-mediated hydrogen bonds affect chlorophyll a/b selectivity in Water-Soluble Chlorophyll Protein

The Water-Soluble Chlorophyll Protein (WSCP) of Brassicaceae is a remarkably stable tetrapyrrole-binding protein that, by virtue of its simple design, is an exceptional model to investigate the interactions taking place between pigments and their protein scaffold and how they affect the photophysical properties and the functionality of the complexes. We investigated variants of WSCP from Lepidium virginicum (Lv) and Brassica oleracea (Bo), reconstituted with Chlorophyll (Chl) b, to determine the mechanisms by which the different Chl binding sites control their Chl a/b specificities. A combined Raman and crystallographic investigation has been employed, aimed to characterize in detail the hydrogen-bond network involving the formyl group of Chl b. The study revealed a variable degree of conformational freedom of the hydrogen bond networks among the WSCP variants, and an unexpected mixed presence of hydrogen-bonded and not hydrogen-bonded Chls b in the case of the L91P mutant of Lv WSCP. These findings helped to refine the description of the mechanisms underlying the different Chl a/b specificities of WSCP versions, highlighting the importance of the structural rigidity of the Chl binding site in the vicinity of the Chl b formyl group in granting a strong selectivity to binding sites.

Designed for life: biocompatible de novo designed proteins and components

A principal goal of synthetic biology is the de novo design or redesign of biomolecular components. In addition to revealing fundamentally important information regarding natural biomolecular engineering and biochemistry, functional building blocks will ultimately be provided for applications including the manufacture of valuable products and therapeutics. To fully realize this ambitious goal, the designed components must be biocompatible, working in concert with natural biochemical processes and pathways, while not adversely affecting cellular function. For example, de novo protein design has provided us with a wide repertoire of structures and functions, including those that can be assembled and function in vivo. Here we discuss such biocompatible designs, as well as others that have the potential to become biocompatible, including non-protein molecules, and routes to achieving full biological integration.

New homologues of Brassicaceae water-soluble chlorophyll proteins shed light on chlorophyll binding, spectral tuning, and molecular evolution

Type-II water-soluble chlorophyll (Chl) proteins (WSCPs) of Brassicaceae are promising models for understanding how protein sequence and structure affect Chl binding and spectral tuning in photosynthetic Chl-protein complexes. However, to date, their use has been limited by the small number of known WSCPs, which also limited understanding their physiological roles. To overcome these limitations, we performed a phylogenetic analysis to compile a more comprehensive and complete set of natural type-II WSCP homologues. The identified homologues were heterologously expressed in Escherichia coli, purified, tested for assembly with chlorophylls, and spectroscopically characterized. The analyses led to the discovery of previously unrecognized type-IIa and IIb subclass WSCPs, as well as of a new subclass that did not bind chlorophylls. Further analysis by ancestral sequence reconstruction yielded sequences of putative ancestors of the three subclasses, which were subsequently recombinantly expressed in E. coli, purified and characterized. Combining the phylogenetic and spectroscopic data with molecular structural information revealed distinct Chl-binding motifs, and identified residues critically impacting spectral tuning. The distinct Chl-binding properties of the WSCP archetypes suggest that the non-Chl-binding subclass evolved from a Chl-binding ancestor that most likely lost its Chl-binding capacity upon localization in the plant tissues with low Chl content. This dual evolutionary trajectory is consistent with WSCPs association with the Kunitz-type protease inhibitors superfamily, and indications of their inhibitory activity in response to various forms of stress in plants. These findings suggest new directions for exploring the physiological roles of WSCPs and the correlation, if any, between Chl-binding and protease inhibition functionality.

A chloroplast-inspired nanoplatform for targeting cancer and synergistic photodynamic/photothermal therapy

A bioinspired nanoplatform composed of Cu(ii)-chlorophyll–hyaluronic acid nanoparticles (Cu(ii)Chl–HA NPs) was developed for targeting cancer and combined photodynamic/photothermal therapy.

Chlorophyll a/b binding-specificity in water-soluble chlorophyll protein

We altered the chlorophyll (Chl) binding sites in various versions of water-soluble chlorophyll protein (WSCP) by amino acid exchanges to alter their preferences for either Chl a or Chl b. WSCP is ideally suited for this mutational analysis since it forms a tetrameric complex with only four identical Chl binding sites. A loop of 4-6 amino acids is responsible for Chl a versus Chl b selectivity. We show that a single amino acid exchange within this loop changes the relative Chl a/b affinities by a factor of 40. We obtained crystal structures of this WSCP variant binding either Chl a or Chl b. The Chl binding sites in these structures were compared with those in the major light-harvesting complex (LHCII) of the photosynthetic apparatus in plants to search for similar structural features involved in Chl a/b binding specificity.

Sensory, Physico-Chemical and Water Sorption Properties of Corn Extrudates Enriched with Spirulina

This study compares the quality of extrudates made from corn grits with the addition of up to 8% of spirulina powder. The sensory properties (shape, color, aroma, taste and crispness), chemicals (content of water, protein, fat, ash, fiber, carbohydrates, carotenoids, chlorophyll and phycocyanin) and physical properties (color, water absorption index, expansion indices, texture and water sorption properties) were determined. It has been found that spirulina-enriched extrudates had slightly lower sensory scores, but the addition of spirulina improved their nutritional value. The contents of protein, ash, fiber and β-carotene increased in extrudates with 8% of spirulina by 34, 36, 140 and 1,260%, respectively. The increasing addition of spirulina caused a decrease in extrudates lightness, an increase in their greenness and yellowness accompanied by a decrease of expansion indices and an increase of softness. Only small differences were found in water sorption properties, suggesting a similar behavior of spirulina-enriched extrudates during storage.

An unusual role for the phytyl chains in the photoprotection of the chlorophylls bound to Water-Soluble Chlorophyll-binding Proteins

Water-Soluble Chlorophyll Proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins which tetramerize upon binding four chlorophyll (Chl) molecules. The bound Chls are highly photostable, despite the lack of bound carotenoids known, in Chl-containing photosynthetic proteins, to act as singlet oxygen and Chl triplet (3Chl) quenchers. Although the physiological function of WSCPs is still unclear, it is likely to be related to their biochemical stability and their resistance to photodegradation. To get insight into the origin of this photostability, the properties of the 3Chl generated in WSCPs upon illumination were investigated. We found that, unlike the excited singlet states, which are excitonic states, the triplet state is localized on a single Chl molecule. Moreover, the lifetime of the 3Chl generated in WSCPs is comparable to that observed in other Chl-containing systems and is reduced in presence of oxygen. In contrast to previous observations, we found that WSCP actually photosensitizes singlet oxygen with an efficiency comparable to that of Chl in organic solvent. We demonstrated that the observed resistance to photooxidation depends on the conformation of the phytyl moieties, which in WSCP are interposed between the rings of Chl dimers, hindering the access of singlet oxygen to the oxidizable sites of the pigments.

Water-Soluble Chlorophyll Protein (WSCP) Stably Binds Two or Four Chlorophylls

Water-soluble chlorophyll proteins (WSCPs) of class IIa from Brassicaceae form tetrameric complexes containing one chlorophyll (Chl) per apoprotein but no carotenoids. The complexes are remarkably stable toward dissociation and protein denaturation even at 100 °C and extreme pH values, and the Chls are partially protected against photooxidation. There are several hypotheses that explain the biological role of WSCPs, one of them proposing that they function as a scavenger of Chls set free upon plant senescence or pathogen attack. The biochemical properties of WSCP described in this paper are consistent with the protein acting as an efficient and flexible Chl scavenger. At limiting Chl concentrations, the recombinant WSCP apoprotein binds substoichiometric amounts of Chl (two Chls per tetramer) to form complexes that are as stable toward thermal dissociation, denaturation, and photodamage as the fully pigmented ones. If more Chl is added, these two-Chl complexes can bind another two Chls to reach the fully pigmented state. The protection of WSCP Chls against photodamage has been attributed to the apoprotein serving as a diffusion barrier for oxygen, preventing its access to triplet excited Chls and, thus, the formation of singlet oxygen. By contrast, the sequential binding of Chls by WSCP suggests a partially open or at least flexible structure, raising the question of how WSCP photoprotects its Chls without the help of carotenoids.

Harnessing the Profinity eXact™ System for Expression and Purification of Heterologous Proteins in E. coli

Highly purified recombinant proteins in large quantities are valuable material for biochemical and structural studies. To achieve this goal, versatile tools were developed to increase the expression of the recombinant proteins and to facilitate the purification process. Fusion tags are commonly used for enhancing expression and solubility and some can be used in the purification process. However, these tags may need to be removed by treatment with specific proteases in order to obtain the tag-free protein. The Profinity eXact™ system provides an alternative system for a fusion tag, enhancing expression and purification in one-step. Here we describe a set of new vectors in which the Profinity eXact™ tag, in addition to a 6× His-tag, with or without additional expression-enhancing sequences, could be used in the Profinity eXact™ system. We show that the solubility enhancing tags (Trx, GST, GB1) increase the yield of the purified tested protein compared to the vector containing only a His-tag upstream of the Profinity eXact™ fusion tag.

Fine Tuning of Chlorophyll Spectra by Protein-Induced Ring Deformation

The ability to tune the light-absorption properties of chlorophylls by their protein environment is the key to the robustness and high efficiency of photosynthetic light-harvesting proteins. Unfortunately, the intricacy of the natural complexes makes it very difficult to identify and isolate specific protein-pigment interactions that underlie the spectral-tuning mechanisms. Herein we identify and demonstrate the tuning mechanism of chlorophyll spectra in type II water-soluble chlorophyll binding proteins from Brassicaceae (WSCPs). By comparing the molecular structures of two natural WSCPs we correlate a shift in the chlorophyll red absorption band with deformation of its tetrapyrrole macrocycle that is induced by changing the position of a nearby tryptophan residue. We show by a set of reciprocal point mutations that this change accounts for up to 2/3 of the observed spectral shift between the two natural variants.

Water in Oil Emulsions: A New System for Assembling Water-soluble Chlorophyll-binding Proteins with Hydrophobic Pigments

Chlorophylls (Chls) and bacteriochlorophylls (BChls) are the primary cofactors that carry out photosynthetic light harvesting and electron transport. Their functionality critically depends on their specific organization within large and elaborate multisubunit transmembrane protein complexes. In order to understand at the molecular level how these complexes facilitate solar energy conversion, it is essential to understand protein-pigment, and pigment-pigment interactions, and their effect on excited dynamics. One way of gaining such understanding is by constructing and studying complexes of Chls with simple water-soluble recombinant proteins. However, incorporating the lipophilic Chls and BChls into water-soluble proteins is difficult. Moreover, there is no general method, which could be used for assembly of water-soluble proteins with hydrophobic pigments. Here, we demonstrate a simple and high throughput system based on water-in-oil emulsions, which enables assembly of water-soluble proteins with hydrophobic Chls. The new method was validated by assembling recombinant versions of the water-soluble chlorophyll binding protein of Brassicaceae plants (WSCP) with Chl a. We demonstrate the successful assembly of Chl a using crude lysates of WSCP expressing E. coli cell, which may be used for developing a genetic screen system for novel water-soluble Chl-binding proteins, and for studies of Chl-protein interactions and assembly processes.