A similar in vitro and in cell lysate folding intermediate for the FF domain

Protein Conformational Exchanges Modulated by the Environment of Outer Membrane Vesicles

Protein function, in many cases, is strongly coupled to the dynamics and conformational equilibria of the protein. The environment surrounding proteins is critical for their dynamics and can dramatically affect the conformational equilibria and subsequently the activities of proteins. However, it is unclear how protein conformational equilibria are modulated by their crowded native environments. Here we reveal that outer membrane vesicle (OMV) environments modulate the conformational exchanges of Im7 protein at its local frustrated sites and shift the conformation toward its ground state. Further experiments show both macromolecular crowding and quinary interactions with the periplasmic components stabilize the ground state of Im7. Our study highlights the key role that the OMV environment plays in the protein conformational equilibria and subsequently the conformation-related protein functions. Furthermore, the long-lasting nuclear magnetic resonance measurement time of proteins within OMVs indicates that they could serve as a promising system for investigating protein structures and dynamics in situ via nuclear magnetic spectroscopy.

Liquid-Liquid Phase Separation of an Intrinsically Disordered Region of a Germ Cell-Specific Protein Modulates the Stability and Conformational Exchange Rate of SH3 Domain

The phenomenon of liquid-liquid phase separation is found in numerous biological processes. The biomolecules enveloped in the phase-separated droplets experience an obviously different environment from those in cellular or aqueous solution. Herein, we quantitatively characterized the thermodynamics and exchange kinetics of a model protein SH3 domain in the condensed phase of an intrinsically disordered region of a germ cell-specific protein DDX4N1 by using ¹⁹F-NMR spectroscopy. The stability and exchange rate of the SH3 domain are different from those in buffer and macromolecular crowding conditions. Our finding indicates that the local transient ordered microstructure and heterogeneity in the condensates play significant roles in modulating the biophysical properties of the enveloped proteins, and this finding may be essential to further our understanding how phase separation regulates the function of proteins in cells.

Methyl-Based NMR Spectroscopy Methods for Uncovering Structural Dynamics in Large Proteins and Protein Complexes

NMR spectroscopy is particularly adept at site-specifically monitoring dynamic processes in proteins, such as protein folding, domain movements, ligand binding, and side-chain rotations. By coupling the favorable spectroscopic properties of highly dynamic side-chain methyl groups with transverse-relaxation-optimized spectroscopy (TROSY), it is now possible to routinely study such dynamic processes in high-molecular-weight proteins and complexes approaching 1 MDa. In this Perspective, we describe many elegant methyl-based NMR experiments that probe slow (second) to fast (picosecond) dynamics in large systems. To demonstrate the power of these methods, we also provide interesting examples of studies that utilized each methyl-based NMR technique to uncover functionally important dynamics. In many cases, the NMR experiments are paired with site-directed mutagenesis and/or other biochemical assays to put the dynamics and function into context. Our vision of the future of structural biology involves pairing methyl-based NMR spectroscopy with biochemical studies to advance our knowledge of the motions large proteins and macromolecular complexes use to choreograph complex functions. Such studies will be essential in elucidating the critical structural dynamics that underlie function and characterizing alterations in these processes that can lead to human disease.

Solution NMR views of dynamical ordering of biomacromolecules

BACKGROUND

To understand the mechanisms related to the 'dynamical ordering' of macromolecules and biological systems, it is crucial to monitor, in detail, molecular interactions and their dynamics across multiple timescales. Solution nuclear magnetic resonance (NMR) spectroscopy is an ideal tool that can investigate biophysical events at the atomic level, in near-physiological buffer solutions, or even inside cells.

SCOPE OF REVIEW

In the past several decades, progress in solution NMR has significantly contributed to the elucidation of three-dimensional structures, the understanding of conformational motions, and the underlying thermodynamic and kinetic properties of biomacromolecules. This review discusses recent methodological development of NMR, their applications and some of the remaining challenges.

MAJOR CONCLUSIONS

Although a major drawback of NMR is its difficulty in studying the dynamical ordering of larger biomolecular systems, current technologies have achieved considerable success in the structural analysis of substantially large proteins and biomolecular complexes over 1MDa and have characterised a wide range of timescales across which biomolecular motion exists. While NMR is well suited to obtain local structure information in detail, it contributes valuable and unique information within hybrid approaches that combine complementary methodologies, including solution scattering and microscopic techniques.

GENERAL SIGNIFICANCE

For living systems, the dynamic assembly and disassembly of macromolecular complexes is of utmost importance for cellular homeostasis and, if dysregulated, implied in human disease. It is thus instructive for the advancement of the study of the dynamical ordering to discuss the potential possibilities of solution NMR spectroscopy and its applications. This article is part of a Special Issue entitled "Biophysical Exploration of Dynamical Ordering of Biomolecular Systems" edited by Dr. Koichi Kato.

Crowding in Cellular Environments at an Atomistic Level from Computer Simulations

The effects of crowding in biological environments on biomolecular structure, dynamics, and function remain not well understood. Computer simulations of atomistic models of concentrated peptide and protein systems at different levels of complexity are beginning to provide new insights. Crowding, weak interactions with other macromolecules and metabolites, and altered solvent properties within cellular environments appear to remodel the energy landscape of peptides and proteins in significant ways including the possibility of native state destabilization. Crowding is also seen to affect dynamic properties, both conformational dynamics and diffusional properties of macromolecules. Recent simulations that address these questions are reviewed here and discussed in the context of relevant experiments.

Towards understanding cellular structure biology: In-cell NMR

To watch biological macromolecules perform their functions inside the living cells is the dream of any biologists. In-cell nuclear magnetic resonance is a branch of biomolecular NMR spectroscopy that can be used to observe the structures, interactions and dynamics of these molecules in the living cells at atomic level. In principle, in-cell NMR can be applied to different cellular systems to achieve biologically relevant structural and functional information. In this review, we summarize the existing approaches in this field and discuss its applications in protein interactions, folding, stability and post-translational modifications. We hope this review will emphasize the effectiveness of in-cell NMR for studies of intricate biological processes and for structural analysis in cellular environments.

NMR Method for Characterizing Microsecond-to-Millisecond Chemical Exchanges Utilizing Differential Multiple-Quantum Relaxation in High Molecular Weight Proteins

Chemical exchange processes of proteins on the order of microseconds (μs) to milliseconds (ms) play critical roles in biological functions. Developments in methyl-transverse relaxation optimized spectroscopy (methyl-TROSY), which observes the slowly relaxing multiple quantum (MQ) coherences, have enabled the studies of biologically important large proteins. However, the analyses of μs to ms chemical exchange processes based on the methyl-TROSY principle are still challenging, because the interpretation of the chemical exchange contributions to the MQ relaxation profiles is complicated, as significant chemical shift differences occur in both (1)H and (13)C nuclei. Here, we report a new methyl-based NMR method for characterizing chemical exchanges, utilizing differential MQ relaxation rates and a heteronuclear double resonance pulse technique. The method enables quantitative evaluations of the chemical exchange processes, in which significant chemical shift differences exist in both the (1)H and (13)C nuclei. The versatility of the method is demonstrated with the application to KirBac1.1, with an apparent molecular mass of 200 kDa.

The molecular mechanism of nuclear transport revealed by atomic-scale measurements

Nuclear pore complexes (NPCs) form a selective filter that allows the rapid passage of transport factors (TFs) and their cargoes across the nuclear envelope, while blocking the passage of other macromolecules. Intrinsically disordered proteins (IDPs) containing phenylalanyl-glycyl (FG)-rich repeats line the pore and interact with TFs. However, the reason that transport can be both fast and specific remains undetermined, through lack of atomic-scale information on the behavior of FGs and their interaction with TFs. We used nuclear magnetic resonance spectroscopy to address these issues. We show that FG repeats are highly dynamic IDPs, stabilized by the cellular environment. Fast transport of TFs is supported because the rapid motion of FG motifs allows them to exchange on and off TFs extremely quickly through transient interactions. Because TFs uniquely carry multiple pockets for FG repeats, only they can form the many frequent interactions needed for specific passage between FG repeats to cross the NPC.

DOI:http://dx.doi.org/10.7554/eLife.10027.001

Eukaryotic cells have a nucleus that contains most of the organism's genetic material. Two layers of membrane form an envelope around the nucleus and protect its contents from the rest of the cell's interior. However, this protective barrier must also allow certain proteins and nucleic acids(collectively called ‘cargo’) to move in and out of the nucleus.

Cargo molecules can pass through channel-like structures called nuclear pore complexes, which are embedded in the nuclear envelope. However, transport across this barrier is highly selective. While small molecules can pass freely through nuclear pore complexes, larger cargo can only be transported when they are bound to so-called transport factors. The nuclear pore complex is a large structure made up of more than 30 different proteins called nucleoporins. Like all proteins, nucleoporins are built from amino acids. Many nucleoporins contain repeating units of two amino acids, namely phenylalanine (which is often referred to as ‘F’) and glycine (or ‘G’). These ‘FG nucleoporins’ are found on the inside of the nuclear pore complex and interact with transport factors to allow them to transit across the nuclear envelope.

Several models have been put forward to explain how FG nucleoporins block the passage of most molecules. But it was unclear from these models how these nucleoporins could do this while simultaneously allowing the selective and fast transport of nuclear transport receptors. There was also a major lack of experimental data that probed the behavior of FG nucleoporins in detail.

Hough, Dutta et al. have now used a technique called nuclear magnetic resonance spectroscopy (or NMR for short) to address this issue. NMR can be used to analyze the structure of proteins and how they interact with other molecules. This analysis revealed that FG nucleoporins never adopt an ordered three-dimensional shape, even briefly; instead they remain unfolded or disordered, moving constantly. Nevertheless, and unlike many other unfolded proteins, FG nucleoporins do not aggregate into clumps. This is because they are constantly changing and continuously interacting with other molecules present inside the cell, which prevents them from aggregating.

Hough, Dutta et al. also observed that the repeating units in the FG nucleoporins engaged briefly with a large number of sites or pockets present on the transport factors. These FG repeats can bind and then release the transport factors at unusually high speeds, which enables the transport factors to move quickly through the nuclear pore complex. This transit is specific because only transport factors have a high capacity for interacting with the FG repeats. These findings provide an explanation for how the nuclear pore complex achieves fast and selective transport. Further work is needed to see whether certain FG nucleoporins specifically interact with a particular type of transport factor, to provide preferred transport routes through the nuclear pore complex.

DOI:http://dx.doi.org/10.7554/eLife.10027.002

Probing protein quinary interactions by in-cell nuclear magnetic resonance spectroscopy

Historically introduced by McConkey to explain the slow mutation rate of highly abundant proteins, weak protein (quinary) interactions are an emergent property of living cells. The protein complexes that result from quinary interactions are transient and thus difficult to study biochemically in vitro. Cross-correlated relaxation-induced polarization transfer-based in-cell nuclear magnetic resonance allows the characterization of protein quinary interactions with atomic resolution inside live prokaryotic and eukaryotic cells. We show that RNAs are an important component of protein quinary interactions. Protein quinary interactions are unique to the target protein and may affect physicochemical properties, protein activity, and interactions with drugs.

Hydrogen exchange of disordered proteins in Escherichia coli

A truly disordered protein lacks a stable fold and its backbone amide protons exchange with solvent at rates predicted from studies of unstructured peptides. We have measured the exchange rates of two model disordered proteins, FlgM and α-synuclein, in buffer and in Escherichia coli using the NMR experiment, SOLEXSY. The rates are similar in buffer and cells and are close to the rates predicted from data on small, unstructured peptides. This result indicates that true disorder can persist inside the crowded cellular interior and that weak interactions between proteins and macromolecules in cells do not necessarily affect intrinsic rates of exchange.

NMR studies of protein folding and binding in cells and cell-like environments

Proteins function in cells where the concentration of macromolecules can exceed 300g/L. The ways in which this crowded environment affects the physical properties of proteins remain poorly understood. We summarize recent NMR-based studies of protein folding and binding conducted in cells and in vitro under crowded conditions. Many of the observations can be understood in terms of interactions between proteins and the rest of the intracellular environment (i.e. quinary interactions). Nevertheless, NMR studies of folding and binding in cells and cell-like environments remain in their infancy. The frontier involves investigations of larger proteins and further efforts in higher eukaryotic cells.