Identifying and Visualizing Macromolecular Flexibility in Structural Biology

Structural biology comprises a variety of tools to obtain atomic resolution data for the investigation of macromolecules. Conventional structural methodologies including crystallography, NMR and electron microscopy often do not provide sufficient details concerning flexibility and dynamics, even though these aspects are critical for the physiological functions of the systems under investigation. However, the increasing complexity of the molecules studied by structural biology (including large macromolecular assemblies, integral membrane proteins, intrinsically disordered systems, and folding intermediates) continuously demands in-depth analyses of the roles of flexibility and conformational specificity involved in interactions with ligands and inhibitors. The intrinsic difficulties in capturing often subtle but critical molecular motions in biological systems have restrained the investigation of flexible molecules into a small niche of structural biology. Introduction of massive technological developments over the recent years, which include time-resolved studies, solution X-ray scattering, and new detectors for cryo-electron microscopy, have pushed the limits of structural investigation of flexible systems far beyond traditional approaches of NMR analysis. By integrating these modern methods with powerful biophysical and computational approaches such as generation of ensembles of molecular models and selective particle picking in electron microscopy, more feasible investigations of dynamic systems are now possible. Using some prominent examples from recent literature, we review how current structural biology methods can contribute useful data to accurately visualize flexibility in macromolecular structures and understand its important roles in regulation of biological processes

Optimized Recombinant Production of Secreted Proteins Using Human Embryonic Kidney (HEK293) Cells Grown in Suspension

Recombinant proteins are an essential milestone for a plethora of different applications ranging from pharmaceutical to clinical, and mammalian cell lines are among the currently preferred systems to obtain large amounts of proteins of interest due to their high level of post-translational modification and manageable large-scale production. In this regard, human embryonic kidney 293 (HEK293) cells constitute one of the main standard lab-scale mammalian hosts for recombinant protein production since these cells are relatively easy to handle, scale-up, and transfect. Here, we present a detailed protocol for the cost-effective, reproducible, and scalable implementation of HEK293 cell cultures in suspension (suitable for commercially available HEK293 cells, HEK293-F) for high-quantity recombinant production of secreted soluble multi-domain proteins. In addition, the protocol is optimized for a Monday-to-Friday maintenance schedule, thus simplifying and streamlining the work of operators responsible for cell culture maintenance

Inference of molecular structure for characterization and improvement of clinical grade immunocytokines

The use of immunomodulatory agents for the treatment of cancer is gaining a growing biopharmaceutical interest. Antibody-cytokine fusion proteins, namely immunocytokines, represent a promising solution for the regulation of the immune system at the site of disease. The three-dimensional arrangement of these molecules can profoundly influence their biological activity and pharmacokinetic properties. Structural techniques might provide important insight in the 3D arrangement of immunocytokines. Here, we performed structure investigations on clinical grade fusion proteins L19-IL2, IL12-L19L19 and L19L19-IL2 to elucidate their quaternary organization. Crystallographic characterization of the common L19 antibody fragment at a resolution of 2.0-Å was combined with low-resolution studies of the full-length chimeric molecules using small-angle synchrotron X-ray scattering (SAXS) and negative stain electron microscopy. Characterization of the full-length quaternary structures of the immunocytokines in solution by SAXS consistently supported the diabody structure in the L19-IL2 immunocytokine and allowed generation of low-resolution models of the chimeric proteins L19L19-IL2 and IL12-L19L19. Comparison with 3D reconstructions obtained from negative-stain electron microscopy revealed marked flexibility associated to the linker regions connecting the cytokine and the antibody components of the chimeric proteins. Collectively, our results indicate that low-resolution molecular structure characterizations provide useful complementary insights for the quality control of immunocytokines, constituting a powerful tool to guide the design and the subsequent optimization steps towards clinical enhancement of these chimeric protein reagents

Versatile medium-throughput strategies for recombinant expression screening in structural biology

The increasing complexity of biological targets subject to structural characterization constantly demands highly versatile approaches of recombinantly expression and purification. When targeting macromolecular complexes, the difficulties associated to the initial recombinant screening phase are further amplified by the need to identify suitable constructs to co-express or reconstitute in vitro the molecular interactions essential for complex stabilization. Large industry-scale high-throughput platforms offer extremely efficient and automated methods to obtain libraries of protein constructs for expression and purification scouting, whereas several academic research labs still rely on more conservative "one construct, one recombinant host, one target" approaches. Frequently, the choice depends on investments in automation, as the costs associated to creation of high-throughput facilities are not affordable to all research groups. Aiming at minimizing time and costs associated to the initial screening for recombinant expression; we optimized cloning and expression strategies to increase the throughput without the need of automation. Our system consists of two major components: 1) a large library of expression vectors, based on a limited number of commercial backbones with customized expression cassettes allowing rapid switch of combinations of expression hosts, affinity tags and protein fusions to enhance target stability and solubility, coupled to standardized sub-cloning sites for easy genes transfer from one expression vector to another; 2) digital tools to facilitate design of DNA constructs for expression scouting. In this talk, we will present the basic concepts behind the construction and functioning of our system, and we will briefly showcase it successful usage for the identification of optimal expression constructs for various ongoing structural biology projects in our lab, including extracellular enzymes, cytosolic macromolecular complexes, and membrane proteins