Plant Science

Cytochrome P450 reductase (CPR) is a multi-domain protein that acts as a redox partner of cytochrome P450s. The CPR contains a flavin adenine dinucleotide (FAD)–binding domain, a flavin mononucleotide (FMN)-binding domain, and a connecting domain. To achieve catalytic events, the FMN-binding domain needs to move relative to the FAD-binding domain, and this high flexibility complicates structural determination in high-resolution by X-ray crystallography. Here, we demonstrate a seeding technique of sorghum CPR crystals for resolution improvement, which can be applied to other poorly diffracting protein crystals. Protein expression is completed using an E. coli cell line with a high protein yield and purified using chromatography techniques. Crystals are screened using an automated 96-well plating robot. Poorly diffracting crystals are originally grown using a hanging drop method from successful trials observed in sitting drops. A macro seeding technique is applied by transferring crystal clusters to fresh conditions without nucleation to increase crystal size. Prior to diffraction, a dehydration technique is applied by serial transfer to higher precipitant concentrations. Thus, an increase in resolution by 7 Å is achieved by limiting the inopportune effects of the flexibility inherent to the domains of CPR, and secondary structures of SbCPR2c are observed.

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In plant cells, galactolipids are predominant, representing up to 50% of the lipid content in photosynthetic tissues. Galactolipid synthesis is initiated by MGDG synthases (MGDs), which use UDP-galactose as a donor sugar and diacylglycerol (DAG) as acceptor, to form monogalactosyldiacylglycerol (MGDG). This protocol is used to produce a recombinant form of Arabidopsis thaliana (A. thaliana) monogalactosyldiacylglycerol synthase 1 (MGD1) protein, in Escherichia coli (E. coli), using a two-step chromatographic purification procedure. The protein is easily expressed and purified to milligram quantities, suitable for biochemical and structural studies. The crystallization of MGD1 is also described.

Heterologous expression of plant cellulose synthase (CESA) and its purification has remained a challenge for decades impeding detailed biophysical, biochemical and structural characterization of this key enzyme. An in-depth knowledge of structure and function of CESA proteins would enable us to better understand the hierarchical structure of the plant cell wall. Here, we report a detailed, and reproducible method of purification of catalytic domain of CESA1 from Arabidopsis thaliana that was recombinantly expressed in Escherichia coli. The method relies on a two stage purification procedure to obtain the catalytic domain in monomer and trimer forms. The biochemical and biophysical data including low resolution structures of the protein have been published (Vandavasi et al., 2016). Currently the crystallization studies of this protein are underway.

[Background] Cellulose is the most important structural component of plant cell walls and constitutes the Earth’s largest source of biorenewable material, yet the mechanism of its synthesis by plants is poorly understood. The plant cellulose synthesis complex (CSC), also called a ‘rosette’ because of its hexameric appearance in electron microscope images, is a large multi-subunit transmembrane protein complex responsible for synthesis of cellulose chains and their assembly into microfibrils. The number of cellulose synthase (CESA) proteins in the CSC and the number of cellulose chains in a microfibril have been debated for many years. Structural information about CESA proteins from plants is crucial to provide answers to some of the basic questions regarding the mechanism of cellulose synthesis. However, elucidation of the structure of CESA proteins has proved difficult because they are multi-domain proteins comprised of disordered, globular, and membrane associated domains. As an alternative to pursuing structural studies of CESA holoproteins, we are developing approaches for recombinant expression of individual CESA domains (e.g., N-terminal domain, central-cytosolic domain, C-terminal transmembrane domain) in large quantities suitable for structural studies. The current protocol has been optimized for isolation of the catalytic domain of A. thaliana CESA1 as reported (Vandavasi et al., 2016). Using this protocol, it is possible to control the oligomerization state of the protein enabling structural studies of the monomer and the trimeric form of the protein. The approach described may be broadly applicable to other systems.

Thylakoids are a formation of flattened membrane vesicles and protein complexes found in cyanobacteria, algae and plants. In the chloroplasts of land plants the thylakoid membrane systems form a network of densely packed stacks called grana lamellae, which are connected by unstacked stroma lamellae. Photosystem II is mainly localized in the appressed grana region, while photosystem I and the ATP synthase complexes are enriched in the stroma lamellae. The cytochrome b₆/f complex is distributed laterally throughout both stacked and unstacked membrane regions. The photosynthetic complexes consist of integral and peripheral proteins. The first part of this protocol (A) shows how to fractionate thylakoids into grana and stroma lamellae. The second part of this protocol (B) shows how to distinguish between strong hydrophobic integral membrane associations and weak electrostatic membrane and/or membrane complex associations. As it is necessary to specifically detect the protein of interest in the fractions, a specific antibody raised against the protein of interest or a complemented null mutant of a structural component expressing a tagged fusion protein would be of great advantage. The last part of this protocol (C) shows, how to investigate the topology of integral and peripheral proteins. This method requires a specific antibody for the protein of interest. For integral membrane proteins peptide-specific antibodies or epitope-tagged versions are required. The protocol is suitable for the investigation of low molecular weight proteins (LMW) below 5 kDa (Torabi et al., 2014).

2D diagonal redox SDS-PAGE of proteins is used to detect intramolecular or intermolecular disulfide bridges using Chlamydomonas in this example (Stroeher and Dietz, 2008; Schwarz et al., 2012). Both dimensions consist of a conventional SDS-PAGE, except that the sample buffer for the first dimension lacks a reducing agent. Intermolecular disulfide bridges increase the apparent molecular weight of a protein in the first dimension, whereas intramolecular bridges decrease the apparent weight of the protein.