New trends in protein expression.
In order to benefit from the wealth of genomics data and to make new discoveries in the protein world, genes need to be converted to proteins that, in turn, must be characterized at the molecular level. This has galvanized researchers from across a broad range of biological and biomedical disciplines to study the form and function of thousands of proteins. This task can be achieved by production of stable and pure proteins for their biophysical and biochemical analysis and/or for their 3D structure determination. The broad hierarchy of structures in biological systems necessitates integration of a wide range of high and low-resolution structural determination techniques. These include X-ray and neutron crystallography, NMR, electron microscopy and tomography, small angle X-ray and neutron scattering, mass spectroscopy and advanced light microscopy techniques. Common to all these techniques is the requirement for highly pure protein samples. Consequently, high throughput protein expression and purification, as well as more versatile expression systems and techniques, are now in even greater demand to support the pressing requirements of the research pipeline. However, protein expression is not only a technology, but also an art. Each protein is unique and thus requires special and often tedious treatment. Therefore, the expression of high levels of stable and functional proteins remains a bottleneck in many scientific endeavors, including the determination of structures in a high-throughput fashion or the screening for novel active compounds in modern drug discovery. Recently, numerous developments have substantially improved the procedures for production of soluble and active proteins in heterologous expression systems. These include parallel construct design, new and more robust expression systems, modifications to the expression constructs, introduction of new and/or improved prokaryotic and eukaryotic expression systems, and the development of improved cell-free protein synthesis systems. This has driven technological developments striving for miniaturization and automation, and methods to characterize proteins prior to structural characterization, in order to achieve a better understanding of how to design constructs generating proteins more amenable to structural characterization. While many of these developments have gained momentum from the various ‘‘structural genomics/proteomics initiatives”, they are now being applied to study the form and function of proteins by scientists from a broad range of biological and biomedical disciplines. We present here a collection of papers in a special issue entitled ‘‘New Trends in Protein Expression”. Our goal was to gather papers concentrating on current developments of the methods and tools that are being used in the production of proteins for structural analysis. In assembling the articles appearing in this volume of the Journal of Structural Biology, an attempt has been made to focus on those principles, strategies, and technologies that currently