Recent excitements in protein NMR: Large proteins and biologically relevant dynamics

John PC, Bomble YJ. Advances in integrative structural biology: Towards understanding protein complexes in their cellular context

Microorganisms rely on protein interactions to transmit signals, react to stimuli, and grow. One of the best ways to understand these protein interactions is through structural characterization. However, in the past, structural knowledge was limited to stable, high-affinity complexes that could be crystallized. Recent developments in structural biology have revolutionized how protein interactions are characterized. The combination of multiple techniques, known as integrative structural biology, has provided insight into how large protein complexes interact in their native environment. In this mini-review, we describe the past, present, and potential future of integrative structural biology as a tool for characterizing protein interactions in their cellular context.

Using NMR spectroscopy to investigate the role played by copper in prion diseases

Prion diseases are a group of rare neurodegenerative disorders that develop as a result of the conformational conversion of normal prion protein (PrPC) to the disease-associated isoform (PrPSc). The mechanism that actually causes disease remains unclear. However, the mechanism underlying the conformational transformation of prion protein is partially understood-in particular, there is strong evidence that copper ions play a significant functional role in prion proteins and in their conformational conversion. Various models of the interaction of copper ions with prion proteins have been proposed for the Cu (II)-binding, cell-surface glycoprotein known as prion protein (PrP). Changes in the concentration of copper ions in the brain have been associated with prion diseases and there is strong evidence that copper plays a significant functional role in the conformational conversion of PrP. Nevertheless, because copper ions have been shown to have both a positive and negative effect on prion disease onset, the role played by Cu (II) ions in these diseases remains a topic of debate. Because of the unique properties of paramagnetic Cu (II) ions in the magnetic field, their interactions with PrP can be tracked even at single atom resolution using nuclear magnetic resonance (NMR) spectroscopy. Various NMR approaches have been utilized to study the kinetic, thermodynamic, and structural properties of Cu (II)-PrP interactions. Here, we highlight the different models of copper interactions with PrP with particular focus on studies that use NMR spectroscopy to investigate the role played by copper ions in prion diseases.

Intact Transition Epitope Mapping - Targeted High-Energy Rupture of Extracted Epitopes (ITEM-THREE)

Epitope mapping, which is the identification of antigenic determinants, is essential for the design of novel antibody-based therapeutics and diagnostic tools. ITEM-THREE is a mass spectrometry-based epitope mapping method that can identify epitopes on antigens upon generating an immune complex in electrospray-compatible solutions by adding an antibody of interest to a mixture of peptides from which at least one holds the antibody's epitope. This mixture is nano-electrosprayed without purification. Identification of the epitope peptide is performed within a mass spectrometer that provides an ion mobility cell sandwiched in-between two collision cells and where this ion manipulation setup is flanked by a quadrupole mass analyzer on one side and a time-of-flight mass analyzer on the other side. In a stepwise fashion, immune-complex ions are separated from unbound peptide ions and dissociated to release epitope peptide ions. Immune complex-released peptide ions are separated from antibody ions and fragmented by collision induced dissociation. Epitope-containing peptide fragment ions are recorded, and mass lists are submitted to unsupervised data base search thereby retrieving both, the amino acid sequence of the epitope peptide and the originating antigen. ITEM-THREE was developed with antiTRIM21 and antiRA33 antibodies for which the epitopes were known, subjecting them to mixtures of synthetic peptides of which one contained the respective epitope. ITEM-THREE was then successfully tested with an enzymatic digest of His-tagged recombinant human β-actin and an antiHis-tag antibody, as well as with an enzymatic digest of recombinant human TNFα and an antiTNFα antibody whose epitope was previously unknown.

Role of Computational Methods in Going beyond X-ray Crystallography to Explore Protein Structure and Dynamics

Protein structural biology came a long way since the determination of the first three-dimensional structure of myoglobin about six decades ago. Across this period, X-ray crystallography was the most important experimental method for gaining atomic-resolution insight into protein structures. However, as the role of dynamics gained importance in the function of proteins, the limitations of X-ray crystallography in not being able to capture dynamics came to the forefront. Computational methods proved to be immensely successful in understanding protein dynamics in solution, and they continue to improve in terms of both the scale and the types of systems that can be studied. In this review, we briefly discuss the limitations of X-ray crystallography in studying protein dynamics, and then provide an overview of different computational methods that are instrumental in understanding the dynamics of proteins and biomacromolecular complexes.

Characterization of the interaction interface and conformational dynamics of human TGIF1 homeodomain upon the binding of consensus DNA

The TG interacting factor-1 homeodomain (TGIF1-HD) binds with the consensus DNA motif 5'-TGTCA-3' in gene promoters through its three-amino acid loop extension (TALE) type homeodomain, and then recruits co-regulators to regulate gene expression. Although the solution NMR structure of human TGIF1-HD has been reported previously, little is known about its DNA binding mechanism. NMR titrations have been extensively used to study mechanisms of ligand binding to target proteins; however, an intermediate exchange occurred predominantly between TGIF1-HD in the free and bound states when titrated with the consensus DNA, which resulted in poor-quality NMR spectra and precluded further exploration of its interaction interface and conformational dynamics. Here, the helix α3 of TGIF1-HD was speculated as the specific DNA binding interface by hydrogen-deuterium exchange mass spectrometry (HDX-MS) experiments, and subsequently confirmed by chemical exchange saturation transfer (CEST) spectroscopy. In addition, simultaneous conformational changes in other regions, including α1 and α2, were induced by DNA binding, explaining the observation of chemical shift perturbations from extensive residues besides those located in α3. Further, low-populated DNA-bound TGIF1-HD undergoing a slow exchange at a rate of 130.2 ± 3.6 s^-1 was derived from the analysis of the CEST data, and two residues, R220 and R221, located in the middle of α3 were identified to be crucial for DNA binding. Our study provides structural and dynamic insights into the mechanisms of TGIF1-HD recognition of extensive promoter DNA.