High-throughput protein crystallization

The combinatorial chemistry industry has made major advances in the handling and mixing of small volumes, and in the development of robust liquid-handling systems. In addition, developments have been made in the area of material handling for the high-throughput drug screening and combinatorial chemistry fields. Lastly, improvements in beamline optics at synchrotron sources have enabled the use of flash-frozen micron-sized (10-50 microm) crystals. The combination of these and other recent advances will make high-throughput protein crystallography possible. Further advances in high-throughput methods of protein crystallography will require application of the above developments and the accumulation of success/failure data in a more systematic manner. Major changes in crystallography technology will emerge based on the data collected by first-generation high-throughput systems.     

Membrane protein crystallization

The need for high-resolution structure information on membrane proteins is immediate and growing. Currently, the only reliable way to get it is crystallographically. The rate-limiting step from protein to structure is crystal production. An overview of the current ideas and experimental approaches prevailing in the area of membrane protein crystallization is presented. The long-established surfactant-based method has been reviewed extensively and is not examined in detail here. The focus instead is on the latest methods, all of which exploit the spontaneous self-assembling properties of lipids and detergent as vesicles (vesicle-fusion method), discoidal micelles (bicelle method), and liquid crystals or mesophases (in meso or cubic-phase method). In the belief that a knowledge of the underlying phase science is integral to understanding the molecular basis of these assorted crystallization strategies, the article begins with a brief primer on lipids, mesophases, and phase science, and the related issue of form and function as applied to lipids is addressed. The experimental challenges associated with and the solutions for procuring adequate amounts of homogeneous membrane proteins, or parts thereof, are examined. The cubic-phase method is described from the following perspectives: how it is done in practice, its general applicability and successes to date, and the nature of the mesophases integral to the process. Practical aspects of the method are examined with regard to salt, detergent, and screen solution effects; crystallization at low temperatures; tailoring the cubic phase to suit the target protein; different cubic-phase types; dealing with low-protein samples, colorless proteins, microcrystals, and radiation damage; transport within the cubic phase for drug design, cofactor retention, and phasing; using spectroscopy for quality control; harvesting crystals; and miniaturization and robotization for high-throughput screening. The section ends with a hypothesis for nucleation and growth of membrane protein crystals in meso. Thus far, the bicelle and vesicle-fusion methods have produced crystals of one membrane protein, bacteriorhodopsin. The experimental details of both methods are reviewed and their general applicability in the future is commented on. The three new methods are rationalized by analogy to crystallization in microgravity and with respect to epitaxy. A list of Web resources in the area of membrane protein crystallogenesis is included.     

High-throughput protein crystallization

High-throughput structural biology is a focus of a number of academic and pharmaceutical laboratories around the world. The use of X-ray crystallography in these efforts is critically dependent on high-throughput protein crystallization. The application of current protocols yields crystal leads for approximately 30% of the input proteins and well-diffracting crystals for a smaller fraction. Increasing the success rate will require a multidisciplinary approach that must invoke techniques from molecular biology, protein biochemistry, biophysics, artificial intelligence, and automation.     

Introduction to protein crystallization

Biological macromolecules can be crystallized by a variety of techniques, and using a wide range of reagents which produce supersaturated mother liquors. These may, in turn, be applied under different physical conditions such as temperature. The fundamental approaches to devising successful crystallization conditions and the factors that influence them are summarized here. For the Novice, it is hoped that this brief review might serve as a useful introduction and a stepping-stone to a successful X-ray structure determination. In addition, it may provide a framework in which to place the articles that follow.     

Predicting protein crystallization propensity from protein sequence

The high-throughput structure determination pipelines developed by structural genomics programs offer a unique opportunity for data mining. One important question is how protein properties derived from a primary sequence correlate with the protein’s propensity to yield X-ray quality crystals (crystallizability) and 3D X-ray structures. A set of protein properties were computed for over 1,300 proteins that expressed well but were insoluble, and for ~720 unique proteins that resulted in X-ray structures. The correlation of the protein’s iso-electric point and grand average hydropathy (GRAVY) with crystallizability was analyzed for full length and domain constructs of protein targets. In a second step, several additional properties that can be calculated from the protein sequence were added and evaluated. Using statistical analyses we have identified a set of the attributes correlating with a protein’s propensity to crystallize and implemented a Support Vector Machine (SVM) classifier based on these. We have created applications to analyze and provide optimal boundary information for query sequences and to visualize the data. These tools are available via the web site .     

Predictive models for protein crystallization

Crystallization of proteins is a nontrivial task, and despite the substantial efforts in robotic automation, crystallization screening is still largely based on trial-and-error sampling of a limited subset of suitable reagents and experimental parameters. Funding of high throughput crystallography pilot projects through the NIH Protein Structure Initiative provides the opportunity to collect crystallization data in a comprehensive and statistically valid form. Data mining and machine learning algorithms thus have the potential to deliver predictive models for protein crystallization. However, the underlying complex physical reality of crystallization, combined with a generally ill-defined and sparsely populated sampling space, and inconsistent scoring and annotation make the development of predictive models non-trivial. We discuss the conceptual problems, and review strengths and limitations of current approaches towards crystallization prediction, emphasizing the importance of comprehensive and valid sampling protocols. In view of limited overlap in techniques and sampling parameters between the publicly funded high throughput crystallography initiatives, exchange of information and standardization should be encouraged, aiming to effectively integrate data mining and machine learning efforts into a comprehensive predictive framework for protein crystallization. Similar experimental design and knowledge discovery strategies should be applied to valid analysis and prediction of protein expression, solubilization, and purification, as well as crystal handling and cryo-protection.     

Rationalizing alpha-helical membrane protein crystallization

X-ray crystallography is currently the most successful method for determining the three-dimensional structure of membrane proteins. Nevertheless, growing the crystals required for this technique presents one of the major bottlenecks in this area of structural biology. This is especially true for the alpha-helical type membrane proteins that are of particular interest due to their medical relevance. To address this problem we have undertaken a detailed analysis of the crystallization conditions from 121 alpha-helical membrane protein structures deposited in the Protein Data Bank. This information has been analyzed so that the success of different parameters can be easily compared for different membrane protein families. Concurrent with this analysis, we also present the new sparse matrix crystallization screen MemGold.     

Widening the protein crystallization bottleneck

Two recent reports describe simple methods that may help expand the range of proteins amenable to crystallization.     

Detergent-free membrane protein crystallization.

A comprehensive understanding of structure-function relationships of proteins requires their structures to be elucidated to high resolution. With most membrane proteins this has not been accomplished so far, mainly because of their notoriously poor crystallizability. Here we present a completely detergent-free procedure for the incorporation of a native purple membrane into a monoolein-based lipidic cubic phase, and subsequent crystallization of three-dimensional bacteriorhodopsin crystals therein. These crystals exhibit comparable X-ray diffraction quality and mosaicity, and identical crystal habit and space group to those of bacteriorhodopsin crystals that are grown from detergent-solubilized protein in cubic phase.     

Automated systems for protein crystallization

Growth of high quality crystals is often the most difficult step in the determination of protein structures by X-ray diffraction. Automation can improve the success of this process both by reducing the amount of protein required for each screen and by relieving the tedium of setting up crystallization experiments by hand. We have been using an automated system for the design and execution of hanging drop crystallization experiments for the last two years. The system includes robots for the preparation of solutions, setup of hanging drops, and automated imaging, as well as a new software package (RoCKS) for managing all phases of the crystallization process. Here, we review the fundamentals of automated protein crystallization and present results from our comparisons of various approaches to screening.     

Advanced protein crystallization using water-soluble ionic liquids as crystallization additives

The application of five water-soluble, halogen-free, alkylammonium-based ionic liquids (ILs) as additives for advanced crystallization of lysozyme was investigated. Their biocompatibility was determined by long-term measurement of the overall mean relative enzyme activities. These were maximally reduced by about 10–15% when up to 200 g IL l⁻¹ was added. Sitting-drop vapor diffusion crystallization experiments revealed that the addition of some of the ILs led to less crystal polymorphism and precipitation was avoided reliably even at larger NaCl concentrations. The addition of ILs tended to result in larger crystals. The kinetics of lysozyme crystallization were significantly enhanced using ILs as crystallization additives, e.g. by a factor of 5.5 when 100 g ethanolammonium formate l⁻¹ was added. ILs with “soft” anions, such as formate or glycolate, were superior to ILs with “hard” anions, like nitrate.     

The protein as a variable in protein crystallization

Strategies for growing protein crystals have for many years been essentially empirical, the protein, once purified to a certain homogeneity, being mixed with a selection of crystallization agents selected in a more or less trial-and-error fashion. Screening for the correct conditions has been made easier through automation and by the introduction of commercially available crystallization kits. Many parameters can be changed in these experiments, such as temperature, pH, and ionic strength, but perhaps the most important variable has been ignored, namely the protein. The crystallization properties of a protein vary greatly: some crystallize readily, whereas others have proven extremely difficult or even impossible to obtain in a crystalline state. The possibility of altering the intrinsic characteristics of a protein for crystallization has become a feasible strategy. Some historical perspectives and advances in this area will be reviewed.     

Insights into outer membrane protein crystallization

Outer membrane proteins are structurally distinct from those that reside in the inner membrane and play important roles in bacterial pathogenicity and human metabolism. X-ray crystallography studies on >40 different outer membrane proteins have revealed that the transmembrane portion of these proteins can be constructed from either β-sheets or less commonly from α-helices. The most common architecture is the β-barrel, which can be formed from either a single anti-parallel sheet, fused at both ends to form a barrel or from multiple peptide chains. Outer membrane proteins exhibit considerable rigidity and stability, making their study through x-ray crystallography particularly tractable. As the number of structures of outer membrane proteins increases a more rational approach to their crystallization can be made. Herein we analyse the crystallization data from 53 outer membrane proteins and compare the results to those obtained for inner membrane proteins. A targeted sparse matrix screen for outer membrane protein crystallization is presented based on the present analysis.     

Enhancing protein crystallization through precipitant synergy

Suitable conditions for protein crystallization are commonly identified by screening combinations of independent factors that affect crystal formation. Because precipitating agents are prime determinants of crystallization, we investigated whether a systematic exploration of combinations of mechanistically distinct precipitants would enhance crystallization. A crystallization screen containing 64 precipitant mixtures was devised. Tests with ten HIV envelope-related proteins demonstrated that use of precipitant mixtures significantly enhanced both the probability of crystallization as well as the quality of optimized crystals. Tests with hen egg white lysozyme generated a novel C2 crystal from a salt/organic solvent mixture; structure solution at 2 A resolution revealed a lattice held together by both hydrophobic and electrostatic dyad interactions. The results indicate that mechanistically distinct precipitants can synergize, with precipitant combinations adding unique dimensions to protein crystallization.     

Managing and mining protein crystallization data

The crystallization of macromolecules remains a major bottleneck in structural biology. The routine screening of more than one thousand crystallization conditions and subsequent optimization by fine screening presents a challenge to conventional laboratory notebook keeping. In addition, the development of high-throughput robotic crystallization and imaging systems presents a pressing need for low-cost laboratory information management system (LIMS). Here we describe CLIMS2, a crystallization LIMS that features a simple, user-friendly graphical interface, allowing the storage, management, retrieval and mining of crystallization data. The CLIMS2 executable and documentation is freely available at http://clims.med.monash.edu.au.     

Supported phospholipid bilayers for two-dimensional protein crystallization

Phospholipid bilayers, supported on UV irradiated carbon shadowed nitrocellulose electron microscope grids, have been used to induce two-dimensional crystal growth of IgE and IgG anti-DNP monoclonal antibodies. The UV irradiation renders the grids hydrophilic in a very uniform fashion and allows for the transfer of phospholipid monolayers from an air/water interface in a sequential dipping procedure. The surface coverage achieved was nearly 100% as measured by antibody binding and by the formation of protein arrays on the bilayer covered grids. The supported bilayers appear to be stably held and are appropriate for slow binding conditions and long incubation times with low concentrations of binding protein.