Water in Oil Emulsions: A New System for Assembling Water-soluble Chlorophyll-binding Proteins with Hydrophobic Pigments
Summary
This manuscript describes a simple and high-throughput method for assembling water-soluble proteins with hydrophobic pigments that is based on water-in-oil emulsions. We demonstrate the effectiveness of the method in the assembly of native chlorophylls with four variants of recombinant water-soluble-chlorophyll binding proteins (WSCPs) of Brassica plants expressed in E. coli.
Abstract
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.
Introduction
Hydrophobic pigments such as chlorophylls (Chls), bacteriochlorophylls (BChls) and carotenoids are the primary cofactors in photosynthetic reaction centers and light harvesting proteins that carry out electron transport, and light energy capture and transfer. The reaction centers and most of the Chl-binding light harvesting complexes are transmembrane proteins. The Fenna-Matthews-Olson (FMO) protein of non-oxygenic photosynthetic green-sulfur bacteria 1,2, and the peridinin-Chl protein (PCP) of dinoflagellates 3 are exceptional examples of water soluble light harvesting proteins. The water-soluble chlorophyll binding proteins (WSCPs) of Brassicaceae, Polygonaceae, Chenopodiaceae and Amaranthaceae plants 4,5 are another unique example, yet in contrast to FMO and PCP, these are neither involved in light harvesting nor in any of the primary photosynthetic reaction, and their precise physiological functions are yet unclear 5-8. Their high Chl-binding affinity have led to a suggested function as transient carriers of Chls and Chl derivatives 9,10. Alternatively, it was hypothesized that WSCP plays a role in scavenging Chls in damaged cells and protects against Chl-induced photooxidative damage 7,11-13. More recently, it was suggested that WSCP functions as a protease inhibitor and plays a role during herbivore resistance as well regulates cell death during flower development 14. WSCPs are divided into two main classes according to their photophysical properties. The first class (class I, e.g. from Chenopodium album) may undergo photoconversion upon illumination. Class II WSCPs from Brassica plants, that do not undergo photoconversion 5,10, are further subdivided into class IIa (e.g., from Brassica oleracea, Raphanus sativus) and IIb (e.g., from Lepidium virginicum). The structure of class IIb WSCP from Lepidium virginicum was solved by X-ray crystallography at 2.0 Å resolution 8. It reveals a symmetric homotetramer in which the protein subunits form a hydrophobic core. Each subunit binds a single Chl which results in a tight arrangement of four closely packed Chls within the core.This simple all Chl arrangement makes WSCPs a potentially useful model system for studying binding and assembly of Chl-protein complexes, and the effects of neighboring Chls and protein environments on the spectral and electronic properties of individual Chls. Furthermore, it may provide templates for constructing artificial Chl-binding proteins that may be used for light-harvesting modules in artificial photosynthetic devices.
Rigorous studies of native WSCPs are not feasible because the complexes purified from plants always contain a heterogeneous mixture of tetramers with different combinations of Chl a and Chl b 9. Thus, a method for assembling recombinantly expressed WSCPs with Chls in vitro is required. This is challenged by the negligible water-solubility of Chls which makes it impossible to assemble the complex in vitro by simply mixing the water-soluble apoproteins with pigments in aqueous solutions. In vitro assembly by mixing the apoproteins with thylakoid membranes 15 was demonstrated, but this method is limited to the native Chls present in the thylakoids. Schmidt et al. reported on assembling several Chl and BChl derivatives with WSCP from cauliflower (CaWSCP) by recombinantly expressing a histidine-tagged protein in E. coli immobilizing it onto a Ni-affinity column and introducing Chl derivatives solubilized in detergents 11. Successfully reconstitution of recombinant WSCPs from A. thaliana 6, and Brussels sprouts (BoWSCP), Japanese wild radish (RshWSCP) and Virginia pepperweed (LvWSCP) by a similar method were also reported.
Here, we present a novel, general, straightforward method for assembling Chls with WSCP that does not require tagging or immobilizing the proteins. It relies on preparing emulsions from their aqueous solutions of the water-soluble apoproteins in mineral oil. The proteins are thus encapsulated in water-in-oil (W/O) microdroplets with very high surface to volume ratio 16. The hydrophobic cofactors are then dissolved in the oil and are readily introduced into the droplets from the oil phase. We report on using the method for assembling of several variants of WSCP apoproteins recombinantly expressed in E. coli with Chl a. We demonstrate the assembly from crude lysate of WSCP-overexpressing bacteria which may be used as a screening system for developing novel Chl binding proteins.
Protocol
Representative Results
Discussion
Our goal was to develop a new general system for assembly of water-soluble chlorophyll-binding proteins with hydrophobic pigments. Here it is shown that the new reconstitution system based on W/O emulsion is a general approach proven to work for assembly of WSCP apoproteins from Brussels sprouts, cauliflower, Japanese horseradish and Virginia pepperweed recombinantly expressed in E. coli. Here results are presented from reconstitution of 1 mg of WSCP with 10-fold molar excess of Chl a. However it is als…
Divulgaciones
The authors have nothing to disclose.
Acknowledgements
DN acknowledges support from EU FP7 projects PEPDIODE (GA 256672) and REGPOT-2012-2013-1(GA 316157), and a personal research grant (No. 268/10) from the Israel Science Foundation. We thank Prof. Shmuel Rubinstein, School of Engineering and Applied Sciences, Harvard University, Cambridge MA, USA for taking the confocal microscopy images.
Materials
| Mineral oil | Sigma | M5904 | |
| Span80 | Sigma | 85548 | |
| Tween80 | Sigma | P8074 | |
| Bio-Scale Mini Profinity eXact Cartridges | Bio Rad | 10011164 | Affinity chromatography for WSCP purification with native sequence. |
| His Trap HF column | GE Healthcare Life Science | 17-5248-02 | Affinity chromatography for WSCP purification with His-tag |
| DEAE Sepharose Fast Flow | GE Healthcare Life Science | 17-0709-01 | Chromatography medium for chlorophyll purification |
Referencias
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