High-molecular-weight polymers for protein crystallization: poly-gamma-glutamic acid-based precipitants

Poly (γ) glutamic acid: a unique microbial biopolymer with diverse commercial applicability

Microbial biopolymers have emerged as promising solutions for environmental pollution-related human health issues. Poly-γ-glutamic acid (γ-PGA), a natural anionic polymeric compound, is composed of highly viscous homo-polyamide of D and L-glutamic acid units. The extracellular water solubility of PGA biopolymer facilitates its complete biodegradation and makes it safe for humans. The unique properties have enabled its applications in healthcare, pharmaceuticals, water treatment, foods, and other domains. It is applied as a thickener, taste-masking agent, stabilizer, texture modifier, moisturizer, bitterness-reducing agent, probiotics cryoprotectant, and protein crystallization agent in food industries. γ-PGA is employed as a biological adhesive, drug carrier, and non-viral vector for safe gene delivery in tissue engineering, pharmaceuticals, and medicine. It is also used as a moisturizer to improve the quality of hair care and skincare cosmetic products. In agriculture, it serves as an ideal stabilizer, environment-friendly fertilizer synergist, plant-growth promoter, metal biosorbent in soil washing, and animal feed additive to reduce body fat and enhance egg-shell strength.

Proline/alanine-rich sequence (PAS) polypeptides as an alternative to PEG precipitants for protein crystallization

Proline/alanine-rich sequence (PAS) polypeptides represent a novel class of biosynthetic polymers comprising repetitive sequences of the small proteinogenic amino acids L-proline, L-alanine and/or L-serine. PAS polymers are strongly hydrophilic and highly soluble in water, where they exhibit a natively disordered conformation without any detectable secondary or tertiary structure, similar to polyethylene glycol (PEG), which constitutes the most widely applied precipitant for protein crystallization to date. To investigate the potential of PAS polymers for structural studies by X-ray crystallography, two proteins that were successfully crystallized using PEG in the past, hen egg-white lysozyme and the Fragaria × ananassa O-methyltransferase, were subjected to crystallization screens with a 200-residue PAS polypeptide. The PAS polymer was applied as a precipitant using a vapor-diffusion setup that allowed individual optimization of the precipitant concentration in the droplet in the reservoir. As a result, crystals of both proteins showing high diffraction quality were obtained using the PAS precipitant. The genetic definition and precise macromolecular composition of PAS polymers, both in sequence and in length, distinguish them from all natural and synthetic polymers that have been utilized for protein crystallization so far, including PEG, and facilitate their adaptation for future applications. Thus, PAS polymers offer potential as novel precipitants for biomolecular crystallography.

Structure of the GH9 glucosidase/glucosaminidase from Vibrio cholerae

Glycoside hydrolase family 9 (GH9) of carbohydrate-processing enzymes primarily consists of inverting endoglucanases. A subgroup of GH9 enzymes are believed to act as exo-glucosidases or exo-glucosaminidases, with many being found in organisms of the family Vibrionaceae, where they are proposed to function within the chitin-catabolism pathway. Here, it is shown that the GH9 enzyme from the pathogen Vibrio cholerae (hereafter referred to as VC0615) is active on both chitosan-derived and β-glucoside substrates. The structure of VC0615 at 3.17 Å resolution is reported from a crystal form with poor diffraction and lattice disorder. VC0615 was highly refractory to crystallization efforts, with crystals only appearing using a high protein concentration under conditions containing the precipitant poly-γ-glutamic acid (PGA). The structure is highly mobile within the crystal lattice, which is likely to reflect steric clashes between symmetry molecules which destabilize crystal packing. The overall tertiary structure of VC0615 is well resolved even at 3.17 Å resolution, which has allowed the structural basis for the exo-glucosidase/glucosaminidase activity of this enzyme to be investigated.

Structure of Rubisco from Arabidopsis thaliana in complex with 2-carboxyarabinitol-1,5-bisphosphate

The crystal structure of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from Arabidopsis thaliana is reported at 1.5 Å resolution. In light of the importance of A. thaliana as a model organism for understanding higher plant biology, and the pivotal role of Rubisco in photosynthetic carbon assimilation, there has been a notable absence of an A. thaliana Rubisco crystal structure. A. thaliana Rubisco is an L8S8 hexadecamer comprising eight plastome-encoded catalytic large (L) subunits and eight nuclear-encoded small (S) subunits. A. thaliana produces four distinct small-subunit isoforms (RbcS1A, RbcS1B, RbcS2B and RbcS3B), and this crystal structure provides a snapshot of A. thaliana Rubisco containing the low-abundance RbcS3B small-subunit isoform. Crystals were obtained in the presence of the transition-state analogue 2-carboxy-D-arabinitol-1,5-bisphosphate. A. thaliana Rubisco shares the overall fold characteristic of higher plant Rubiscos, but exhibits an interesting disparity between sequence and structural relatedness to other Rubisco isoforms. These results provide the structural framework to understand A. thaliana Rubisco and the potential catalytic differences that could be conferred by alternative A. thaliana Rubisco small-subunit isoforms.

An integrated pipeline for sample preparation and characterization at the EMBL@PETRA3 synchrotron facilities

The characterization of macromolecular samples at synchrotrons has traditionally been restricted to direct exposure to X-rays, but beamline automation and diversification of the user community has led to the establishment of complementary characterization facilities off-line. The Sample Preparation and Characterization (SPC) facility at the EMBL@PETRA3 synchrotron provides synchrotron users access to a range of biophysical techniques for preliminary or parallel sample characterization, to optimize sample usage at the beamlines. Here we describe a sample pipeline from bench to beamline, to assist successful structural characterization using small angle X-ray scattering (SAXS) or macromolecular X-ray crystallography (MX). The SPC has developed a range of quality control protocols to assess incoming samples and to suggest optimization protocols. A high-throughput crystallization platform has been adapted to reach a broader user community, to include chemists and biologists that are not experts in structural biology. The SPC in combination with the beamline and computational facilities at EMBL Hamburg provide a full package of integrated facilities for structural biology and can serve as model for implementation of such resources for other infrastructures.

Post-polymerization modification of poly(L-glutamic acid) with D-(+)-glucosamine

Carboxyl functional groups of poly(L-glutamic acid) (PGlu) were modified with a D-(+)-glucosamine (GlcN) by amidation using 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) as a coupling reagent. The coupling reaction was performed in aqueous medium without protection of hydroxyl functional groups of D-(+)-glucosamine. Poly(L-glutamic acid) and GlcN functionalized polyglutamates (P(Glu-GlcN)) were thoroughly characterized by 1D and 2D NMR spectroscopy and SEC-MALS to gain detailed information on their structure, composition and molar mass characteristics. The results reveal successful functionalization with GlcN through the amide bond and also to a minor extent through ester bond formation in position 1 of GlcN. In addition, a ratio between the α- and β-form of glucosamine substituent coupled to polyglutamate repeating units as well as the content of residual dimethoxy triazinyl active ester moiety in the samples were evaluated.

Poly-γ-glutamic acid: production, properties and applications

Poly-γ-glutamic acid (γ-PGA) is a naturally occurring biopolymer made up of repeating units of l-glutamic acid, d-glutamic acid or both. γ-PGA can exhibit different properties (conformational states, enantiomeric properties and molecular mass). Owing to its biodegradable, non-toxic and non-immunogenic properties, it has been used successfully in the food, medical and wastewater industries. Amongst other novel applications, it has the potential to be used for protein crystallization, as a soft tissue adhesive and a non-viral vector for safe gene delivery. This review focuses on the production, properties and applications of γ-PGA. Each application of γ-PGA utilizes specific properties attributed to various forms of γ-PGA. As a result of its growing applications, more strains of bacteria need to be investigated for γ-PGA production to obtain high yields of γ-PGA with different properties. Many medical applications (especially drug delivery) have exploited α-PGA. As γ-PGA is essentially different from α-PGA (i.e. it does not involve a chemical modification step and is not susceptible to proteases), it could be better utilized for such medical applications. Optimization of γ-PGA with respect to cost of production, molecular mass and conformational/enantiomeric properties is a major step in making its application practical. Analyses of γ-PGA production and knowledge of the enzymes and genes involved in γ-PGA production will not only help increase productivity whilst reducing the cost of production, but also help to understand the mechanism by which γ-PGA is effective in numerous applications.

Structural basis for kinesin-1:cargo recognition

Kinesin-mediated cargo transport is required for many cellular functions and plays a key role in pathological processes. Structural information on how kinesins recognize their cargoes is required for a molecular understanding of this fundamental and ubiquitous process. Here, we present the crystal structure of the tetratricopeptide repeat domain of kinesin light chain 2 in complex with a cargo peptide harboring a "tryptophan-acidic" motif derived from SKIP (SifA-kinesin interacting protein), a critical host determinant in Salmonella pathogenesis and a regulator of lysosomal positioning. Structural data together with biophysical, biochemical, and cellular assays allow us to propose a framework for intracellular transport based on the binding by kinesin-1 of W-acidic cargo motifs through a combination of electrostatic interactions and sequence-specific elements, providing direct molecular evidence of the mechanisms for kinesin-1:cargo recognition.

The crystallization and structural analysis of cellulases (and other glycoside hydrolases): strategies and tactics

The three-dimensional (3-D) structures of cellulases, and other glycoside hydrolases, are a central feature of research in carbohydrate chemistry and biochemistry. 3-D structure is used to inform protein engineering campaigns, both academic and industrial, which are typically used to improve the stability or activity of an enzyme. Examples of classical protein engineering goals include higher thermal stability, reduced metal-ion dependency, detergent and protease resistance, decreased product inhibition, and altered specificity. 3-D structure may also be used to interpret the behavior of enzyme variants that are derived from screening or random mutagenesis approaches, with a view to establishing an iterative design process. In other areas, 3-D structure is used as one of the many tools to probe enzymatic catalysis, typically dovetailing with physical organic chemistry approaches to provide complete reaction mechanisms for enzymes by visualizing catalytic site interactions at different stages of the reaction. Such mechanistic insight is not only fundamentally important, impacting on inhibitor and drug design approaches with ramifications way beyond cellulose hydrolysis, but also provides the framework for the design of enzyme variants to use as biocatalysts for the synthesis of bespoke oligosaccharides. Here we review some of the strategies and tactics that may be applied to the X-ray structure solution of cellulases (and other carbohydrate-active enzymes). The general approach is first to decide why you are doing the work, then to establish correct domain boundaries for truncated constructs (typically the catalytic domain only), and finally to pursue crystallization of pure, homogeneous, and monodisperse protein with appropriate ligand and additive combinations. Cellulase-specific strategies are important for the delineation of domain boundaries, while glycoside hydrolases generally also present challenges and opportunities for the selection and optimization of ligands to both aid crystallization, and also provide structural and mechanistic insight. As the many roles for plant cell wall degrading enzymes increase, so does the need for rapid high-quality structure determination to provide a sound structural foundation for understanding mechanism and specificity, and for future protein engineering strategies.

Structure of Leishmania major cysteine synthase

A crystallographic and biochemical study of L. major cysteine synthase, which is a pyridoxyl phosphate-dependent enzyme, is reported. The structure was determined to 1.8 Å resolution and revealed that the cofactor has been lost and that a fragment of γ-poly-d-glutamic acid, a crystallization ingredient, was bound in the active site. The enzyme was inhibited by peptides.

Cysteine biosynthesis is a potential target for drug development against parasitic Leishmania species; these protozoa are responsible for a range of serious diseases. To improve understanding of this aspect of Leishmania biology, a crystallographic and biochemical study of L. major cysteine synthase has been undertaken, seeking to understand its structure, enzyme activity and modes of inhibition. Active enzyme was purified, assayed and crystallized in an orthorhombic form with a dimer in the asymmetric unit. Diffraction data extending to 1.8 Å resolution were measured and the structure was solved by molecular replacement. A fragment of γ-poly-d-glutamic acid, a constituent of the crystallization mixture, was bound in the enzyme active site. Although a d-­glutamate tetrapeptide had insignificant inhibitory activity, the enzyme was competitively inhibited (K_i = 4 µM) by DYVI, a peptide based on the C-­terminus of the partner serine acetyltransferase with which the enzyme forms a complex. The structure surprisingly revealed that the cofactor pyridoxal phosphate had been lost during crystallization.

Alternative polymer precipitants for protein crystallization

A set of 16 inexpensive and commercially available polymer precipitants were tested for protein crystallization. Eight of them were found suitable: polyethylene glycol dimethyl ether of molecular weight (MW) 500, 1000 and 2000; di[poly(ethylene glycol)] adipate, MW 900; poly(ethylene glycol-ran-propylene glycol), MW 2500 and 12000; poly(acrylic acid) sodium salt, MW 2100; and polyethylene glycol methyl ether methacrylate, MW 1100. Two new crystallization screens, PolyA and PolyB, were formulated using these eight polymers, each containing 96 solutions – four polymers in combination with 24 common salts and buffers, covering pH values from 4.5 to 9.0. The screens were tested on 29 proteins, 21 of which were crystallized. The tests confirmed the applicability of the eight polymers as precipitants for protein crystallization.

A crystallization screen based on alternative polymeric precipitants

Most commercially available crystallization screens are sparse-matrix screens with a predominance of inorganic salts and polyethylene glycols (PEGs) as precipitants. It was noted that commercially available screens are largely unsatisfactory for the purpose of the crystallization of multimeric protein and protein-nucleic acid complexes. This was reasoned to be a consequence of the redundancy in screening crystallization parameter space by the predominance of PEG as a precipitant in standard screens and it was suggested that this limitation could be overcome by introducing a variety of other organic polymers. Here, a set of 288 crystallization conditions was devised based on alternative polymeric precipitants and tested against a set of 20 different proteins/complexes; finally, a screen comprising the 96 most promising conditions designed to complement PEG- and salt-based commercial screens was proposed.