We show that fluorine NMR can be used to monitor the insertion and change in conformation of a 19F-labeled cell-penetrating peptide upon interacting with the cellular plasma membrane. α-Synuclein and a construct comprising a cell-penetrating peptide covalently attached to its N-terminus were studied. Important information about the interaction of the proteins with CHO-K1 cells was obtained by monitoring the diminution of 19F resonances of 3-fluoro-L-tyrosine labeled proteins. For α-synuclein, a decrease in the resonance from position 39 was observed indicating that only the N-terminal third region of the protein interacts with plasma membrane. However, when the fusion construct was incubated with the cells, a decrease in the resonance from the fusion peptide region was noted with no change in the resonances from α-synuclein region. Longer incubation, studied by using confocal fluorescence microscopy, revealed that the fusion construct translocates into the cells, but α-synuclein alone did not cross the membrane in significant amounts.
cell-penetrating peptide; delivery system; eukaryotic cell; 19F NMR; α-synuclein
α-Synuclein, an intrinsically-disordered protein associated with Parkinson’s disease, interacts with mitochondria, but the details of this interaction are unknown. We probed the interaction of α-synuclein and its A30P variant with lipid vesicles by using fluorescence anisotropy and 19F nuclear magnetic resonance. Both proteins interact strongly with large unilamellar vesicles whose composition is similar to that of the inner mitochondrial membrane, which contains cardiolipin. However, the proteins have no affinity for vesicles mimicking the outer mitochondrial membrane, which lacks cardiolipin. The 19F data show that the interaction involves α-synuclein’s N-terminal region. These data indicate that the middle of the N-terminal region, which contains the KAKEGVVAAAE repeats, is involved in binding, probably via electrostatic interactions between the lysines and cardiolipin. We also found that the strength of α-synuclein binding depends on the nature of the cardiolipin acyl side chains. Eliminating one double bond increases affinity, while complete saturation dramatically decreases affinity. Increasing the temperature increases the binding of wild-type, but not the A30P variant. The data are interpreted in terms of the properties of the protein, cardiolipin demixing within the vesicles upon binding of α-synuclein, and packing density. The results advance our understanding of α-synuclein’s interaction with mitochondrial membranes.
Intrinsically disordered proteins are important in signaling, regulation, and translocation. Understanding their diffusion under physiologically relevant conditions will yield insight into their functions. We used NMR to quantify the translational diffusion of a globular and a disordered protein in dilute solution and under crowded conditions. In dilute solution, the globular protein chymotrypsin inhibitor 2 (CI2, 7.4 kDa) diffuses faster than the disordered protein α-synuclein (14 kDa). Surprisingly, the opposite occurs under crowded conditions; α-synuclein diffuses faster than CI2, even though α-synuclein is larger than CI2. These data show that shape is a key parameter determining protein diffusion under crowded conditions, adding to the properties known to be affected by macromolecular crowding. The results also offer a clue about why many signaling proteins are disordered.
19F NMR; Crowding; Diffusion; Disordered proteins
The feasibility of using solid-state MAS NMR for in situ structural characterization of the LR11 (sorLA) transmembrane domain in native Escherichia coli (E. coli) membranes is presented. LR11 interacts with the human amyloid precursor protein (APP), a central player in the pathology of Alzheimer's disease. The background signals from E. coli lipids and membrane proteins had only minor effects on LR11 TM resonances. Approximately 50% of the LR11 TM residues were assigned by using 13C PARIS data. These assignments allow comparisons of the secondary structure of LR11 TM in native membrane environments and in commonly used membrane mimics (e.g. micelles). In situ spectroscopy bypasses several obstacles in the preparation of membrane proteins for structural analysis, and offers an opportunity to investigate the consequences of membrane heterogeneity, bilayer asymmetry, chemical gradients, and macromolecular crowding on the protein structure.
Proteins perform their function in cells where macromolecular solutes reach concentrations of >300 g/L and occupy >30% of the volume. The volume excluded by these macromolecules will stabilize globular proteins because the native state occupies less space than the denatured state. Theory predicts that crowding can increase the ratio of folded to unfolded protein by a factor of 100, amounting to 3 kcal/mol of stabilization at room temperature. We tested the idea that volume exclusion dominates the crowding effect in cells with a variant of protein L, a 7-kDa globular protein with seven lysine residues replaced by glutamic acids. Eighty-four percent of the variant molecules populate the denatured state in dilute buffer at room temperature, compared to 0.1% for the wild-type protein. We then used in-cell nuclear magnetic resonance spectroscopy to show that the cytoplasm of Escherichia coli does not overcome even this modest (~1 kcal/mol) free energy deficit. The data are consistent with the idea that non-specific interactions between cytoplasmic components can overcome the excluded volume effect. Evidence for these interactions is provided by the observation that adding simple salts folds the variant in dilute solution, but increasing the salt concentration inside E. coli does not fold the protein. Our data are consistent with other studies of protein stability in cells, and suggest that stabilizing excluded volume effects, which must be present under crowded conditions, can be ameliorated by non-specific interactions between cytoplasmic components.
In-cell nuclear magnetic resonance spectroscopy is a tool for studying proteins under physiologically relevant conditions. In some instances, however, protein signals from leaked protein are observed in the liquid surrounding the cells. Here, we examine the expression of four proteins in Escherichia coli. We describe the controls that should be used for in-cell NMR experiments, and show that leakage is likely when the protein being studied exceeds approximately 20% of the total cellular protein.
in-cell NMR; protein expression; protein leakage
Protein aggregation; 19F NMR; Unnatural amino acid; SDS Micelle
Most proteins function in nature under crowded conditions, and crowding can change protein properties. Quantification of crowding effects, however, is difficult because solutions containing hundreds of grams per liter of macromolecules often interfere with observing the protein being studied. Models for macromolecular crowding tend to focus on the steric effects of crowders, neglecting potential chemical interactions between the crowder and the test protein. Here, we report the first systematic, quantitative, residue-level study of crowding effects on the equilibrium stability of a globular protein. We used a system comprising poly(vinylpyrrolidone)s (PVPs) of varying molecular weights as crowding agents and chymotrypsin inhibitor 2 (CI2) as a small globular test protein. Stability was quantified with NMR-detected amide 1H exchange. We analyze the data in terms of hard particle exclusion, confinement, and soft interactions. For all crowded conditions, nearly every observed residue experiences a stabilizing effect. The exceptions are residues where stabilities are unchanged. At a PVP concentration of 100 g/L, the data are consistent with theories of hard particle exclusion. At higher concentrations, the data are more consistent with confinement. The data show that the crowder also stabilizes the test protein by weakly binding its native state. We conclude that the role of native-state binding and other soft interactions need to be seriously considered when applying both theory and experiment to studies of macromolecular crowding.
Despite increased attention, little is known about how the crowded intracellular environment affects basic phenomena like protein diffusion. Here, we use NMR to quantify the rotational and translational diffusion of a 7.4-kDa test protein, chymotrypsin inhibitor 2 (CI2), in solutions of glycerol, synthetic polymers, proteins, and cell lysates. As expected, translational diffusion and rotational diffusion decrease with increasing viscosity. In glycerol, for example, the decrease follows the Stokes-Einstein and Stokes-Einstein-Debye laws. Synthetic polymers cause negative deviation from the Stokes Laws and affect translation more than rotation. Surprisingly, however, protein crowders have the opposite effect, causing positive deviation and reducing rotational diffusion more than translational diffusion. Indeed, bulk proteins severely attenuate the rotational diffusion of CI2 in crowded protein solutions. Similarly, CI2 diffusion in cell lysates is comparable to its diffusion in crowded protein solutions, supporting the biological relevance of the results. The rotational attenuation is independent of the size and total charge of the crowding protein, suggesting that the effect is general. The difference between the behavior of synthetic polymers and protein crowders suggests that synthetic polymers may not be suitable mimics of the intracellular environment. NMR relaxation data reveal that the source of the difference between synthetic polymers and proteins is the presence of weak interactions between the proteins and CI2. In summary, weak but non-specific, non-covalent chemical interactions between proteins appear to fundamentally impact protein diffusion in cells.
In-cell NMR; Macromolecular crowding; Protein diffusion; Weak interactions
Solutions containing high macromolecule concentrations are predicted to affect a number of protein properties compared to those properties in dilute solution. In cells, these macromolecular crowders have a large range of sizes and can occupy 30% or more of the available volume. We chose to study the stability and ps-ns internal dynamics of a globular protein whose radius is ~2 nm when crowded by a synthetic microgel composed of poly(N-isopropylacrylamide-co-acrylic acid) with particle radii of ~300 nm.
Our studies revealed no change in protein rotational or ps-ns backbone dynamics and only mild (~0.5 kcal/mol at 37°C, pH 5.4) stabilization at a volume occupancy of 70%, which approaches the occupancy of closely packing spheres. The lack of change in rotational dynamics indicates the absence of strong crowder-protein interactions.
Our observations are explained by the large size discrepancy between the protein and crowders and by the internal structure of the microgels, which provide interstitial spaces and internal pores where the protein can exist in a dilute solution-like environment. In summary, microgels that interact weakly with proteins do not strongly influence protein dynamics or stability because these large microgels constitute an upper size limit on crowding effects.
The inside of the cell is a complex environment that is difficult to simulate when studying proteins and other molecules in vitro. We have developed a device and system that provides a controlled environment for Nuclear Magnetic Resonance (NMR) experiments involving living cells. Our device comprises two main parts, an NMR detection region and a circulation system. The flow of medium from the bottom of the device pushes alginate encapsulated cells into the circulation chamber. In the chamber, the exchange of oxygen and nutrients occurs between the media and the encapsulated cells. When the media flow is stopped, the encapsulated cells fall back into the NMR detection region, and spectra can be acquired. We have utilized the bioreactor to study the expression of the natively disordered protein α-synuclein, inside Escherichia coli cells.
bioreactor; in-cell nuclear magnetic resonance (NMR)
Although over expression and 15N enrichment facilitate the observation of resonances from disordered proteins in Escherichia coli, 15N enrichment alone is insufficient for detecting most globular proteins. Here we explain this dichotomy and overcome the problem while extending the capability of in-cell NMR by using 19F labeled proteins. Resonances from small (~10 kDa) globular proteins containing the amino acid analog 3-fluoro-tyrosine can be observed in cells, but for larger proteins the 19F resonances are broadened beyond detection. Incorporating the amino acid analog trifluoromethyl-L-phenylalanine allows larger proteins (up to 100 kDa) to be observed in cells. We also show that site specific structural and dynamic information about both globular and disordered proteins can be obtained inside cells by using 19F NMR.
19F NMR; in-cell NMR; intrinsically disordered proteins; site specific labeling; protein structure; protein dynamics
Protein protein interaction is the fundamental step of biological signal transduction. Interacting proteins find each other by diffusion. To gain insight into diffusion under the crowded conditions found in cells, we used nuclear magnetic resonance spectroscopy (NMR) to measure the effects of solvent additives on the translational and rotational diffusion of the 7.4 kDa globular protein, chymotrypsin inhibitor 2. The additives were glycerol and the macromolecular crowding agent, polyvinylpyrrolidone (PVP). Both translational diffusion and rotational diffusion decrease with increasing solution viscosity. For glycerol, the decrease obeys the Stokes Einstein and Stokes Einstein Debye laws. Three types of deviation are observed for PVP: the decrease in diffusion with increased viscosity is less than predicted, this negative deviation is greater for rotational diffusion, and the negative deviation increases with increasing PVP molecular weight. We discuss our results in terms of other studies on the effects of macromolecules on globular protein diffusion.
macromolecular crowding; nuclear magnetic resonance; viscosity
Fluorescence recovery after photobleaching and fluorescence correlation spectroscopy are the primary means for studying translational diffusion in biological systems. Both techniques, however, present numerous obstacles for measuring translational mobility in structures only slightly larger than optical resolution. We report a new method using through-prism total internal reflection fluorescence microscopy with continuous photobleaching (TIR-CP) to overcome these obstacles. Small structures, such as prokaryotic cells or isolated eukaryotic organelles, containing fluorescent molecules are adhered to a surface. This surface is continuously illuminated by an evanescent wave created by total internal reflection. The characteristic length describing the decay of the evanescent intensity with distance from the surface is smaller than the structures. The fluorescence decay rate resulting from continuous evanescent illumination is monitored as a function of the excitation intensity. The data at higher excitation intensities provide apparent translational diffusion coefficients for the fluorescent molecules within the structures because the decay results from two competing processes (the intrinsic photobleaching propensity and diffusion in the small structures). We present the theoretical basis for the technique and demonstrate its applicability by measuring the diffusion coefficient, 6.3 ± 1.1 µm2/sec, of green fluorescent protein (GFP) in Escherichia coli cells.
fluorescence microscopy; fluorescent proteins; protoplasmic mobility; total internal reflection
Conventional NMR approaches to detect weak protein binding and aggregation are hindered by the increased viscosity brought about by crowding. We describe a simple and reliable NMR method to distinguish viscosity effects from binding and aggregation under crowded conditions.
Almost everything we know about protein biophysics comes from studies on purified proteins in dilute solution. Most proteins, however, operate inside cells where the concentration of macromolecules can be >300 mg per mL. Although reductionism-based approaches have served protein science well for over a century, biochemists now have the tools to study proteins under these more physiologically-relevant conditions. We review a part of this burgeoning post-reductionist landscape by focusing on high-resolution protein NMR spectroscopy, the only method that provides atomic-level information over an entire protein under the crowded conditions found in cells.
Oxidative stress and aggregation of the protein α-synuclein are thought to be key factors in Parkinson’s disease. Previous work shows that cytochrome c plus H2O2 causes tyrosine-dependent in vitro peroxidative aggregation of proteins, including α-synuclein. Here, we examine the role of each of α-synuclein’s four tyrosine residues and how the protein’s conformation affects covalent oxidative aggregation. When α-synuclein adopts a collapsed conformation, tyrosine 39 is essential for wild-type-like covalent aggregation. This lone N-terminal tyrosine, however, is not required for wild type-like covalent aggregation in the presence of a denaturant or when α-synuclein is present in non-covalent fibrils. We also show that pre-formed oxidative aggregates are not incorporated into non-covalent fibrils. These data provide insight as to how dityrosine may be formed in Lewy bodies seen in Parkinson’s disease.
Theory predicts that macromolecular crowding affects protein behavior, but experimental confirmation is scant. Herein, we report the first residue-level interrogation of the effects of macromolecular crowding on protein stability. We observe up to a 100-fold increase in the stability, as measured by the equilibrium constant for folding, for the globular protein chymotrypsin inhibitor 2 (CI2) in concentrations of the cosolute poly(vinylpyrrolidone) (PVP) that mimic the protein concentration in cells. We show that the increased stability is caused by the polymeric nature of PVP and that the degree of stabilization depends on both the location of the individual residue in the protein structure and the PVP concentration. Our data reinforce the assertion that macromolecular crowding stabilizes the protein by destabilizing its unfolded states.