Models for excluded volume interaction between an unfolded protein and rigid macromolecular cosolutes: macromolecular crowding and protein stability revisited

Applying macromolecular crowding to enhance extracellular matrix deposition and its remodeling in vitro for tissue engineering and cell-based therapies

With the advent of multicellular organisms, the exterior of the cells evolved dramatically from highly aqueous surroundings into an extracellular matrix and space crowded with macromolecules. Cell-based therapies require removal of cells from their crowded physiological context and propagating them in dilute culture medium to attain therapeutically relevant numbers whilst preserving their phenotype. However, bereft of their microenvironment, cells under perform and lose functionality. Major efforts currently aim to modify cell culture surfaces and build three dimensional scaffolds to improve this situation. We discuss here alternative strategies that enable cells to re-create their own microenvironment in vitro, using carbohydrate-based macromolecules as culture media additives that create an excluded volume effect at defined fraction volume occupancies. This biophysical approach dramatically enhances extracellular matrix deposition by differentiated cells and stem cells, and boosts progenitor cell differentiation and proliferation. We begin to understand how well cells really can perform ex vivo if given the chance.

Effects of macromolecular crowding on an intrinsically disordered protein characterized by small-angle neutron scattering with contrast matching

Small-angle neutron scattering was used to examine the effects of molecular crowding on an intrinsically disordered protein, the N protein of bacteriophage λ, in the presence of high concentrations of a small globular protein, bovine pancreatic trypsin inhibitor (BPTI). The N protein was labeled with deuterium, and the D(2)O concentration of the solvent was adjusted to eliminate the scattering contrast between the solvent and unlabeled BPTI, leaving only the scattering signal from the unfolded protein. The scattering profile observed in the absence of BPTI closely matched that predicted for an ensemble of random conformations. With BPTI added to a concentration of 65 mg/mL, there was a clear change in the scattering profile representing an increase in the mass fractal dimension of the unfolded protein, from 1.7 to 1.9, as expected if crowding favors more compact conformations. The crowding protein also inhibited aggregation of the unfolded protein. At 130 mg/mL BPTI, however, the fractal dimension was not significantly different from that measured at the lower concentration, contrary to the predictions of models that treat the unfolded conformations as convex particles. These results are reminiscent of the behavior of polymers in concentrated melts, suggesting that these synthetic mixtures may provide useful insights into the properties of unfolded proteins under crowding conditions.

Macromolecular crowding tunes folding landscape of parallel α/β protein, apoflavodoxin

Proteins normally fold in crowded cellular environments. Here we use a set of Desulfovibrio desulfuricans apoflavodoxin variants to assess--with residue-specific resolution--how apoflavodoxin's folding landscape is tuned by macromolecular crowding. We find that, under crowded conditions, initial topological frustration is reduced, subsequent folding requires less ordering in the transition state, and β-strand 1 becomes more important in guiding the process. We propose that conditions more closely mimicking the cellular environment make the ensemble of unfolded conformations less expanded, resulting in a folding funnel that is smoother and narrower.

Hydration is a major determinant of the G-quadruplex stability and conformation of the human telomere 3' sequence of d(AG3(TTAG3)3)

The factors that determine the conformation and stability of G-quadruplex forming sequences remain poorly understood. Here we demonstrate the influence of cosolvents on the conformation and stability of the human telomeric sequence d(A(GGGTTA)3GGG)) in both K(+) and Na(+) containing solutions using a combination of circular dichroism, NMR, and thermodynamics. Molecular crowding arguments have previously been used to suggest that the parallel quadruplex form may be biologically relevant. However, the small cosolvents previously used, PEG 200 and 400, are actually dehydrating agents. We have used acetonitrile as a non-hydrogen-bonding dehydrating agent; similar conformational transitions were observed in K(+) solution. Moreover, NMR analysis shows that the resulting structure contains non-anti guanine glycosyl torsion angles suggesting that the conformation present in acetonitrile is not identical to the all-parallel crystal structure, despite the supposed parallel type CD spectrum.

Structure, function, and folding of phosphoglycerate kinase are strongly perturbed by macromolecular crowding

We combine experiment and computer simulation to show how macromolecular crowding dramatically affects the structure, function, and folding landscape of phosphoglycerate kinase (PGK). Fluorescence labeling shows that compact states of yeast PGK are populated as the amount of crowding agents (Ficoll 70) increases. Coarse-grained molecular simulations reveal three compact ensembles: C (crystal structure), CC (collapsed crystal), and Sph (spherical compact). With an adjustment for viscosity, crowded wild-type PGK and fluorescent PGK are about 15 times or more active in 200 mg/ml Ficoll than in aqueous solution. Our results suggest a previously undescribed solution to the classic problem of how the ADP and diphosphoglycerate binding sites of PGK come together to make ATP: Rather than undergoing a hinge motion, the ADP and substrate sites are already located in proximity under crowded conditions that mimic the in vivo conditions under which the enzyme actually operates. We also examine T-jump unfolding of PGK as a function of crowding experimentally. We uncover a nonmonotonic folding relaxation time vs. Ficoll concentration. Theory and modeling explain why an optimum concentration exists for fastest folding. Below the optimum, folding slows down because the unfolded state is stabilized relative to the transition state. Above the optimum, folding slows down because of increased viscosity.

Polymeric crowding agents improve passive biomacromolecule encapsulation in lipid vesicles

Large solutes such as high molecular weight proteins can be difficult to encapsulate in lipid vesicles. Passive trapping of these macromolecular solutes during vesicle formation typically results in concentrations inside the vesicles that are much lower than in the external solution. Here, we investigated the effect of macromolecular crowding on passive encapsulation of biological macromolecules with molecular weights ranging from 52 kDa to 660 kDa within both individual giant lipid vesicles (GVs, > 3 microm diameter) and populations of 200 nm diameter large unilamellar vesicles (LUVs). Fluorescently labeled biomacromolecules were encapsulated during vesicle formation in the presence or absence of three weight percent poly(ethylene glycol) (PEG; 8 kDa) or dextran 500 kDa, which served as crowding agents. Encapsulation efficiency of the labeled biomolecules was higher for the lower molecular weight solutes, with internal concentrations essentially equal to external concentrations for labeled biomacromolecules with hydrodynamic radii (r(h)) less than 10 nm. In contrast, internal concentrations were reduced markedly for larger solutes with r(h) > or = 10 nm. Addition of PEG or dextran during vesicle formation improved encapsulation of these larger proteins up to the same levels as observed for the smaller proteins, such that internal and external concentrations were equal. This observation is consistent with PEG and dextran acting as volume excluders, reducing the hydrodynamic radius of the biomacromolecules and increasing their encapsulation. This work demonstrates a simple and general route to improved encapsulation of otherwise poorly encapsulated macromolecular solutes in both GV and LUVs up to their concentration in the solution present during vesicle formation.

Macromolecular crowding remodels the energy landscape of a protein by favoring a more compact unfolded state

The interior of cells is highly crowded with macromolecules, which impacts all physiological processes. To explore how macromolecular crowding may influence cellular protein folding, we interrogated the folding landscape of a model beta-rich protein, cellular retinoic acid-binding protein I (CRABP I), in the presence of an inert crowding agent (Ficoll 70). Urea titrations revealed a crowding-induced change in the water-accessible polar amide surface of its denatured state, based on an observed ca. 15% decrease in the change in unfolding free energy with respect to urea concentration (the m-value), and the effect of crowding on the equilibrium stability of CRABP I was less than our experimental error (i.e., < or = 1.2 kcal/mol). Consequently, we directly probed the effect of crowding on the denatured state of CRABP I by measuring side-chain accessibility using iodide quenching of tryptophan fluorescence and chemical modification of cysteines. We observed that the urea-denatured state is more compact under crowded conditions, and the observed extent of reduction of the m-value by crowding agent is fully consistent with the extent of reduction of the accessibility of the Trp and Cys probes, suggesting a random and nonspecific compaction of the unfolded state. The thermodynamic consequences of crowding-induced compaction are discussed. In addition, over a wide range of Ficoll concentration, crowding significantly retarded the unfolding kinetics of CRABP I without influencing the urea dependence of the unfolding rate, arguing for no appreciable change in the nature of the transition state. Our results demonstrate how macromolecular crowding may influence protein folding by effects on both the unfolded state ensemble and unfolding kinetics.

A didactic model of macromolecular crowding effects on protein folding

A didactic model is presented to illustrate how the effect of macromolecular crowding on protein folding and association is modeled using current analytical theory and discrete molecular dynamics. While analytical treatments of crowding may consider the effect as a potential of average force acting to compress a polypeptide chain into a compact state, the use of simulations enables the presence of crowding reagents to be treated explicitly. Using an analytically solvable toy model for protein folding, an approximate statistical thermodynamic method is directly compared to simulation in order to gauge the effectiveness of current analytical crowding descriptions. Both methodologies are in quantitative agreement under most conditions, indication that both current theory and simulation methods are capable of recapitulating aspects of protein folding even by utilizing a simplistic protein model.

Dependence of protein folding stability and dynamics on the density and composition of macromolecular crowders

We investigate the effect of macromolecular crowding on protein folding, using purely repulsive crowding particles and a self-organizing polymer model of protein folding. We find that the variation in folding stability with crowder size for typical alpha-, beta-, and alpha/beta-proteins is well described by an adaptation of the scaled particle theory. The native state, the transition state, and the unfolded protein are treated as effective hard spheres, with the folded and transition state radii independent of the size and concentration of the crowders. Remarkably, we find that, as the effective unfolded state radius is very weakly dependent on the crowder concentration, it can also be approximated by a single size. The same model predicts the effect of crowding on the folding barrier and therefore refolding rates with no adjustable parameters. A simple extension of the scaled-particle theory model, assuming additivity, can also describe the behavior of mixtures of crowding particles.

Compression of random coils due to macromolecular crowding: scaling effects

The addition of a macromolecular crowding agent to a dilute solution of polymer exerts a compressive force that tends to reduce the size of the chain. We study here the effect of changing the size ratio between the random coil and the crowding agent. The compression occurs at lower crowding agent concentration, Φ when polymer molecular weight increases. The Flory exponent ν(Φ) decreases from ν(0)≃0.48 in water down to 0.3 with macromolecular crowding. The effective polymer-polymer interactions change from repulsive to strongly attractive inducing aggregation of the chains. This effect changes drastically for larger polymer sizes, being much more pronounced at high molecular weights.

Macromolecular crowding induces polypeptide compaction and decreases folding cooperativity

A cell's interior is comprised of macromolecules that can occupy up to 40% of its available volume. Such crowded environments can influence the stability of proteins and their rates of reaction. Using discrete molecular dynamics simulations, we investigate how both the size and number of neighboring crowding reagents affect the thermodynamic and folding properties of structurally diverse proteins. We find that crowding induces higher compaction of proteins. We also find that folding becomes less cooperative with the introduction of crowders into the system. The crowders may induce alternative non-native protein conformations, thus creating barriers for protein folding in highly crowded media.

Models of macromolecular crowding effects and the need for quantitative comparisons with experiment

In recent years significant effort has been devoted to exploring the potential effects of macromolecular crowding on protein folding and association phenomena. Theoretical calculations and molecular simulations have, in particular, been exploited to describe aspects of protein behavior in crowded and confined conditions and many aspects of the simulated behavior have reflected, at least at a qualitative level, the behavior observed in experiments. One major and immediate challenge for the theorists is to now produce models capable of making quantitatively accurate predictions of in vitro behavior. A second challenge is to derive models that explain results obtained from experiments performed in vivo, the results of which appear to call into question the assumed dominance of excluded-volume effects in vivo.

Diffusion, crowding & protein stability in a dynamic molecular model of the bacterial cytoplasm

A longstanding question in molecular biology is the extent to which the behavior of macromolecules observed in vitro accurately reflects their behavior in vivo. A number of sophisticated experimental techniques now allow the behavior of individual types of macromolecule to be studied directly in vivo; none, however, allow a wide range of molecule types to be observed simultaneously. In order to tackle this issue we have adopted a computational perspective, and, having selected the model prokaryote Escherichia coli as a test system, have assembled an atomically detailed model of its cytoplasmic environment that includes 50 of the most abundant types of macromolecules at experimentally measured concentrations. Brownian dynamics (BD) simulations of the cytoplasm model have been calibrated to reproduce the translational diffusion coefficients of Green Fluorescent Protein (GFP) observed in vivo, and “snapshots” of the simulation trajectories have been used to compute the cytoplasm's effects on the thermodynamics of protein folding, association and aggregation events. The simulation model successfully describes the relative thermodynamic stabilities of proteins measured in E. coli, and shows that effects additional to the commonly cited “crowding” effect must be included in attempts to understand macromolecular behavior in vivo.

The interior of a typical bacterial cell is a highly crowded place in which molecules must jostle and compete with each other in order to carry out their biological functions. The conditions under which such molecules are typically studied in vitro, however, are usually quite different: one or a few different types of molecules are studied as they freely diffuse in a dilute, aqueous solution. There is therefore a significant disconnect between the conditions under which molecules can be most usefully studied and the conditions under which such molecules usually “live”, and developing ways to bridge this gap is likely to be important for properly understanding molecular behavior in vivo. Toward this end, we show in this work that computer simulations can be used to model the interior of bacterial cells at a near atomic level of detail: the rates of diffusion of proteins are matched to known experimental values, and their thermodynamic stabilities are found to be in good agreement with the few measurements that have so far been performed in vivo. While the simulation approach is certainly not free of assumptions, it offers a potentially important complement to experimental techniques and provides a vivid illustration of molecular behavior inside a biological cell that is likely to be of significant educational value.

Atomistic modeling of macromolecular crowding predicts modest increases in protein folding and binding stability

Theoretical models predict that macromolecular crowding can increase protein folding stability, but depending on details of the models (e.g., how the denatured state is represented), the level of stabilization predicted can be very different. In this study, we represented the native and denatured states atomistically, with conformations sampled from explicit-solvent molecular dynamics simulations at room temperature and high temperature, respectively. We then designed an efficient algorithm to calculate the allowed fraction, f, when the protein molecule is placed inside a box of crowders. That a fraction of placements of the protein molecule is disallowed because of volume exclusion by the crowders leads to an increase in chemical potential, given by Deltamu = -k(B)T lnf. The difference in Deltamu between the native and denatured states predicts the effect of crowding on the folding free energy. Even when the crowders occupied 35% of the solution volume, the stabilization reached only 1.5 kcal/mol for cytochrome b562. The modest stabilization predicted is consistent with experimental studies. Interestingly, a mixture of different sized crowders was found to exert a greater effect than the sum of the individual species of crowders. The stabilization of crowding on the binding stability of barnase and barstar, based on atomistic modeling of the proteins, was similarly modest. These findings have profound implications for macromolecular crowding inside cells.

Nonadditive effects of mixed crowding on protein stability

The crowded environments inside cells can have significant effects on the folding stability and other biophysical properties of proteins. In this study on how macromolecular crowding affects protein folding, we took a significant step toward realistically mimicking intracellular environments by using a mixture of two crowding agents, Ficoll and dextran. We found that the mixed crowding exerts a greater stabilizing effect than the sum of the two individual crowding agents. Therefore, the composition of crowders, not just the total concentration, has a significant influence on the effects of crowding on protein folding. Since the composition of intracellular macromolecules varies within the lifetime of a cell, our finding may provide an explanation for age being an important risk factor for protein aggregation-related diseases such as Alzheimer's disease and Parkinson's disease.

Modulation of calmodulin plasticity by the effect of macromolecular crowding

In vitro biochemical reactions are most often studied in dilute solution, a poor mimic of the intracellular space of eukaryotic cells, which are crowded with mobile and immobile macromolecules. Such crowded conditions exert volume exclusion and other entropic forces that have the potential to impact chemical equilibria and reaction rates. In this article, we used the well-characterized and ubiquitous molecule calmodulin (CaM) and a combination of theoretical and experimental approaches to address how crowding impacts CaM's conformational plasticity. CaM is a dumbbell-shaped molecule that contains four EF hands (two in the N-lobe and two in the C-lobe) that each could bind Ca(2+), leading to stabilization of certain substates that favor interactions with other target proteins. Using coarse-grained molecular simulations, we explored the distribution of CaM conformations in the presence of crowding agents. These predictions, in which crowding effects enhance the population of compact structures, were then confirmed in experimental measurements using fluorescence resonance energy transfer techniques of donor- and acceptor-labeled CaM under normal and crowded conditions. Using protein reconstruction methods, we further explored the folding-energy landscape and examined the structural characteristics of CaM at free-energy basins. We discovered that crowding stabilizes several different compact conformations, which reflects the inherent plasticity in CaM's structure. From these results, we suggest that the EF hands in the C-lobe are flexible and can be thought of as a switch, while those in the N-lobe are stiff, analogous to a rheostat. New combinatorial signaling properties may arise from the product of the differential plasticity of the two distinct lobes of CaM in the presence of crowding. We discuss the implications of these results for modulating CaM's ability to bind Ca(2+) and target proteins.

Effect of macromolecular crowding on protein folding dynamics at the secondary structure level

Macromolecular crowding is one of the key characteristics of the cellular environment and is therefore intimately coupled to the process of protein folding in vivo. While previous studies have provided invaluable insight into the effect of crowding on the stability and folding rate of protein tertiary structures, very little is known about how crowding affects protein folding dynamics at the secondary structure level. In this study, we examined the thermal stability and folding-unfolding kinetics of three small folding motifs (i.e., a 34-residue alpha-helix, a 34-residue cross-linked helix-turn-helix, and a 16-residue beta-hairpin) in the presence of two commonly used crowding agents, Dextran 70 (200 g/L) and Ficoll 70 (200 g/L). We found that these polymers do not induce any appreciable changes in the folding kinetics of the two helical peptides, which is somewhat surprising as the helix-coil transition kinetics have been shown to depend on viscosity. Also to our surprise and in contrast to what has been observed for larger proteins, we found that crowding leads to an appreciable decrease in the folding rate of the shortest beta-hairpin peptide, indicating that besides the excluded volume effect, other factors also need to be considered when evaluating the net effect of crowding on protein folding kinetics. A model considering both the static and the dynamic effects arising from the presence of the crowding agent is proposed to rationalize these results.

Entropic flows, crowding effects, and stability of asymmetric proteins

The study of the excluded-volume effects on protein stability and reactions or the stability of colloidal suspensions is an active area of research. Using hard-disk collisional dynamics we investigate whether the presence of a crowding agent can induce a shape change from a nonspherical molecule to a spherical one. We show the averaged density profiles and velocity field of hard-disk crowders with an interior noncircular convex shape as a boundary condition. The density profile is not axially symmetric, consistent with other hard-potential experiments with asymmetry. However, more interestingly, the averaged velocity field was found to have a nonzero curl, implying a region of vorticity without a thermal gradient, advective field, or other motivating potential. To explain the occurrence of the vortices, a theoretical model is provided based on angular momentum of hard disks at contact. All these results, as well as difference in pressure along the axes, support the fact that as the packing fraction of the crowder rises, increasing force is exerted on an asymmetric molecule toward a symmetric one.

Crowding effects on the structural transitions in a flexible helical homopolymer

We elucidate the structural transitions in a helical off-lattice homopolymer induced by crowding agents, as a function of the number of monomers (N) and volume fraction (varphi c) of crowding particles. At varphic=0, the homopolymer undergoes transitions from a random coil to a helix, helical hairpin HH, and helix bundle HB structures depending on N, and temperature. Crowding induces chain compaction that can promote HH or HB formation depending on varphic. Typically, the helical content decreases which is reflected in the decrease in the transition temperatures that depend on varphic, N, and the size of the crowding particles.

Compression of random coils due to macromolecular crowding

The conformation of a linear polymer chain is studied as a function of the concentration of a macromolecular crowding agent by neutron scattering. Excluded volume to random coil due to macromolecular crowding in cells is predicted to exert a compressive force that will tend to reduce its size. It is shown that when reducing free volume due to macromolecular crowding, we observe a compression of the polymer chain with a reduction in its radius of gyration of up to approximately 30% and that the effective chain-chain interactions are strongly modified.

Macromolecular crowding modulates folding mechanism of alpha/beta protein apoflavodoxin

Protein dynamics in cells may be different from those in dilute solutions in vitro, because the environment in cells is highly concentrated with other macromolecules. This volume exclusion because of macromolecular crowding is predicted to affect both equilibrium and kinetic processes involving protein conformational changes. To quantify macromolecular crowding effects on protein folding mechanisms, we investigated the folding energy landscape of an alpha/beta protein, apoflavodoxin, in the presence of inert macromolecular crowding agents, using in silico and in vitro approaches. By means of coarse-grained molecular simulations and topology-based potential interactions, we probed the effects of increased volume fractions of crowding agents (phi(c)) as well as of crowding agent geometry (sphere or spherocylinder) at high phi(c). Parallel kinetic folding experiments with purified Desulfovibro desulfuricans apoflavodoxin in vitro were performed in the presence of Ficoll (sphere) and Dextran (spherocylinder) synthetic crowding agents. In conclusion, we identified the in silico crowding conditions that best enhance protein stability, and discovered that upon manipulation of the crowding conditions, folding routes experiencing topological frustrations can be either enhanced or relieved. Our test-tube experiments confirmed that apoflavodoxin's time-resolved folding path is modulated by crowding agent geometry. Macromolecular crowding effects may be a tool for the manipulation of protein-folding and function in living cells.

Folding, stability and shape of proteins in crowded environments: experimental and computational approaches

How the crowded environment inside cells affects folding, stability and structures of proteins is a vital question, since most proteins are made and function inside cells. Here we describe how crowded conditions can be created in vitro and in silico and how we have used this to probe effects on protein properties. We have found that folded forms of proteins become more compact in the presence of macromolecular crowding agents; if the protein is aspherical, the shape also changes (extent dictated by native-state stability and chemical conditions). It was also discovered that the shape of the macromolecular crowding agent modulates the folding mechanism of a protein; in addition, the extent of asphericity of the protein itself is an important factor in defining its folding speed.

Protein nuclear magnetic resonance under physiological 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/mL. Although reductionism-based approaches have served protein science well for more than a century, biochemists now have the tools to study proteins under these more physiologically relevant conditions. We review a part of this burgeoning postreductionist landscape by focusing on high-resolution protein nuclear magnetic resonance (NMR) spectroscopy, the only method that provides atomic-level information over an entire protein under the crowded conditions found in cells.

Crowding effects on the mechanical stability and unfolding pathways of ubiquitin

The interiors of cells are crowded, thus making it important to assess the effects of macromolecules on the folding of proteins. Using the self-organized polymer (SOP) model, which is a coarse-grained representation of polypeptide chains, we probe the mechanical stability of ubiquitin (Ub) monomers and trimers ((Ub)(3)) in the presence of monodisperse spherical crowding agents. Crowding increases the volume fraction (Phi(c))-dependent average force (f(u)(Phi(c))), relative to the value at Phi(c) = 0, needed to unfold Ub and the polyprotein. For a given Phi(c), the values of f(u)(Phi(c)) increase as the diameter (sigma(c)) of the crowding particles decreases. The average unfolding force f(u)(Phi(c)) depends on the ratio D/R(g), where D approximately sigma(c)(pi/6Phi(c))(1/3), with R(g) being the radius of gyration of Ub (or (Ub)(3)) in the unfolded state. Examination of the unfolding pathways shows that, relative to Phi(c) = 0, crowding promotes reassociation of ruptured secondary structural elements. Both the nature of the unfolding pathways and f(u)(Phi(c)) for (Ub)(3) are altered in the presence of crowding particles, with the effect being most dramatic for the subunit that unfolds last. We predict, based on SOP simulations and theoretical arguments, that f(u)(Phi(c)) approximately Phi(c)(1/3nu), where nu is the Flory exponent that describes the unfolded (random coil) state of the protein.

Macromolecular crowding improves polymer encapsulation within giant lipid vesicles

We report the effect of macromolecular crowding on encapsulation efficiency of fluorescently labeled poly(ethylene glycol) (PEG) and dextran polymers within individual giant lipid vesicles (GVs). Low concentrations of the fluorescently labeled polymers (82 nM to 186 pM) were mixed with varying concentrations of nonfluorescent polymers that served as crowding agents during vesicle formation by gentle hydration. Encapsulation efficiency of the fluorescently labeled polymers in individual GVs (EEind) was determined via confocal fluorescence microscopy. EEind for high molecular weight polymers (e.g., fluorescein isothiocyanate (FITC)-dextran 500 and 2000 kDa) increased substantially in the presence of several weight percent unlabeled PEG or dextran. For example, when 0.24 microM FITC dextran 500 kDa was encapsulated, addition of 3% PEG 8 kDa improved the mean concentration in the GVs from 0.14 microM (+/-50%) to 0.24 microM (+/-12%). Light scattering data indicate reduced hydrodynamic radii for polymers as a function of increasing polymer concentration, suggesting that the improvements in EEind result from polymer condensation due to macromolecular crowding. Polymeric cosolutes did not significantly impact EEind for lower molecular weight polymers (e.g., Alexa Fluor 488-PEG 20 kDa), which already encapsulated efficiently (EEind to approximately 1). However, for both the higher and lower molecular weight labeled polymers, cosolutes led to improved uniformity in EEind for vesicles within a batch. Methods for improving the value and homogeneity of EEind for polymeric solutes in lipid vesicles are important in a variety of applications, including the use of vesicles as microreactors and as vehicles for drug delivery.

The effects of macromolecular crowding on the mechanical stability of protein molecules

Macromolecular crowding, a common phenomenon in the cellular environments, can significantly affect the thermodynamic and kinetic properties of proteins. A single-molecule method based on atomic force microscopy (AFM) was used to investigate the effects of macromolecular crowding on the forces required to unfold individual protein molecules. It was found that the mechanical stability of ubiquitin molecules was enhanced by macromolecular crowding from added dextran molecules. The average unfolding force increased from 210 pN in the absence of dextran to 234 pN in the presence of 300 g/L dextran at a pulling speed of 0.25 microm/sec. A theoretical model, accounting for the effects of macromolecular crowding on the native and transition states of the protein molecule by applying the scaled-particle theory, was used to quantitatively explain the crowding-induced increase in the unfolding force. The experimental results and interpretation presented could have wide implications for the many proteins that experience mechanical stresses and perform mechanical functions in the crowded environment of the cell.

Mimicking the plant cell interior under water stress by macromolecular crowding: disordered dehydrin proteins are highly resistant to structural collapse

The dehydrins are a class of drought-induced proteins in plants that lack a fixed three-dimensional structure. Their specific molecular action, as well as the reason for their disordered character, is as yet poorly understood. It has been speculated, however, that the dehydrins are tuned to acquire a biologically active structure only under the conditions in which they normally function (i.e. upon dehydration). To test this hypothesis, we here investigate the effect of reduced water content and macromolecular crowding on three dehydrins from Arabidopsis (Arabidopsis thaliana). As a simplistic model for mimicking cellular dehydration, we used polyethylene glycol, glycerol, and sugars that plants naturally employ as compatible solutes (i.e. sucrose and glucose). Macromolecular crowding was induced by the large polysaccharides Ficoll and dextran. The results show that the dehydrins are remarkably stable in their disordered state and are only modestly affected by the solvent alterations. A notable exception is the dehydrin Cor47, which shows a small, intrinsic increase in helical structure at high concentrations of osmolytes. We also examined the effect of phosphorylation but found no evidence that such posttranslational modifications of the dehydrin sequences modulate their structural response to osmolytes and crowding agents. These results suggest that the dehydrins are highly specialized proteins that have evolved to maintain their disordered character under conditions in which unfolded states of several globular proteins would tend to collapse.

Crowded, cell-like environment induces shape changes in aspherical protein

How the crowded environment inside cells affects the structures of proteins with aspherical shapes is a vital question because many proteins and protein-protein complexes in vivo adopt anisotropic shapes. Here we address this question by combining computational and experimental studies of a football-shaped protein (i.e., Borrelia burgdorferi VlsE) in crowded, cell-like conditions. The results show that macromolecular crowding affects protein-folding dynamics as well as overall protein shape. In crowded milieus, distinct conformational changes in VlsE are accompanied by secondary structure alterations that lead to exposure of a hidden antigenic region. Our work demonstrates the malleability of "native" proteins and implies that crowding-induced shape changes may be important for protein function and malfunction in vivo.

Polymorphism of human telomeric quadruplex structures

Human telomeric DNA consists of tandem repeats of the sequence d(TTAGGG). Compounds that can stabilize the intramolecular DNA G-quadruplexes formed in the human telomeric sequence have been shown to inhibit the activity of telomerase and telomere maintenance, thus the telomeric DNA G-quadruplex has been considered as an attractive target for cancer therapeutic intervention. Knowledge of intramolecular human telomeric G-quadruplex structure(s) formed under physiological conditions is important for structure-based rational drug design and thus has been the subject of intense investigation. This review will give an overview of recent progress on the intramolecular human telomeric G-quadruplex structures formed in K+ solution. It will also give insight into the structure polymorphism of human telomeric sequences and its implications for drug targeting.

Mechanism of low density lipoprotein (LDL) release in the endosome: implications of the stability and Ca2+ affinity of the fifth binding module of the LDL receptor

Uptake of low density lipoproteins (LDL) by their receptor, LDLR, is the primary mechanism by which cells incorporate cholesterol from plasma. Mutations in LDLR lead to familial hypercholesterolemia, a common disease affecting 1 in 500 of the human population. LDLR is a modular protein that uses several small repeats to bind LDL. The repeats contain around 40 residues, including three disulfide bonds and a calcium ion. Repeat 5 (LR5) is critical for LDL and beta-migrating very low density lipoprotein binding. Based on the crystal structure of LDLR at endosomal pH (but close to extracellular calcium concentration), LR5 has been proposed to bind to the epidermal growth factor (EGF) precursor domain of LDLR in the endosome, thus releasing the LDL particles previously bound in extracellular conditions. We report here the conformational stability of LR5 as a function of temperature and calcium concentration under both extracellular and endosomal pH conditions. The repeat was very stable when it bore a bound calcium ion but was severely destabilized in the absence of calcium and even further destabilized at acidic versus neutral pH. The temperature and calcium concentration dependence of LR5 stability clearly indicate that under endosomal conditions the unfolded conformation of the repeat is largely dominant. We thus propose a new mechanism for LDL release in the endosome in which calcium depletion and decreased stability at acidic pH drives LR5 unfolding, which triggers LDL release from the receptor. Subsequent binding of LR5 to the EGF precursor domain, if it takes place at low calcium concentrations, would contribute to a further shifting of the equilibrium toward dissociation.

Residue-level interrogation of macromolecular crowding effects on protein stability

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.

Effects of trimethylamine N-oxide (TMAO) and crowding agents on the stability of RNA hairpins

We study the effect of the osmolyte, Trimethylamine N-Oxide (TMAO), which accumulates in cells in response to osmotic stress, on the stability of RNA hairpins. All atom molecular dynamics (MD) simulations of a nucleotide and the 22-nucleotide RNA hairpin P5GA in an aqueous TMAO solution show that TMAO preferentially interacts with the base through the formation of a single hydrogen bond. To circumvent the difficulties of adequately sampling the conformational space of polynucleotides, we used coarse-grained models (including one that is inspired by the results of all-atom MD simulations of a single nucleotide) to probe the effects of osmoyltes on the stability of P5GA. If, as revealed by our MD simulations, the cosolute specifically interacts with only one base at a time, then we find practically no change in hairpin stability as measured by Delta T m = T m(Phi) - T m, where T m(Phi) and T m are the melting temperatures at volume fraction Phi of the osmolyte and Phi = 0, respectively. This finding is in qualitative agreement with recent experiments. If the interactions between the RNA and osmolytes are repulsive, which is appropriate for mimicking the effects of crowding, Delta T m can vary from 5 to 15 K depending on the size of the osmolyte and the nature of RNA-osmolyte interactions. Cosolutes that interact favorably with multiple bases simultaneously can stabilize the hairpin more than a crowding agent of the same size. The implications of our predictions for experiments are briefly outlined.

Free energy of sickle hemoglobin polymerization: a scaled-particle treatment for use with dextran as a crowding agent

Fundamental to the analysis of protein polymerization is the free energy of association, typically determined from solubility. It has been previously shown that concentrated 70 kDa dextran lowers the solubility of sickle hemoglobin, due to molecular crowding, and provides a useful ranking tool for the effects of inhibitors and molecular modifications. Because hemoglobin occupies a substantial volume as well, crowding effects of both hemoglobin and dextran contribute to the nonideality of the solution. We show how scaled-particle theory can be used to account for both types of crowding, thus allowing the determination of solubility in the absence of dextran, given data measured in its presence. The approach adopted approximates dextran as a sphere with a volume that decreases as the concentration of dextran increases. We use an asymptotic relation to describe the volume, which decreases nearly linearly by a factor of two over the range studied, from 60 to 230 mg/ml. This compression is similar to previously observed compression of sephadex beads and ficoll solutions. In the limit of low hemoglobin concentrations, the theory reduces to the previously-used approach of Ogston. Our method therefore provides a means of measuring the free energy of association of molecules that occupy significant volume fractions, even when assisted by the crowding of dextran and we present a tabulation of all known free energies of polymerization of sickle hemoglobin measured in the presence of dextran.

Macromolecular crowding and confinement: biochemical, biophysical, and potential physiological consequences

Expected and observed effects of volume exclusion on the free energy of rigid and flexible macromolecules in crowded and confined systems, and consequent effects of crowding and confinement on macromolecular reaction rates and equilibria are summarized. Findings from relevant theoretical/simulation and experimental literature published from 2004 onward are reviewed. Additional complexity arising from the heterogeneity of local environments in biological media, and the presence of nonspecific interactions between macromolecules over and above steric repulsion, are discussed. Theoretical and experimental approaches to the characterization of crowding- and confinement-induced effects in systems approaching the complexity of living organisms are suggested.

Protein folding in confined and crowded environments

Confinement and crowding are two major factors that can potentially impact protein folding in cellular environments. Theories based on considerations of excluded volumes predict disparate effects on protein folding stability for confinement and crowding: confinement can stabilize proteins by over 10k(B)T but crowding has a very modest effect on stability. On the other hand, confinement and crowding are both predicted to favor conformations of the unfolded state which are compact, and consequently may increase the folding rate. These predictions are largely borne out by experimental studies of protein folding under confined and crowded conditions in the test tube. Protein folding in cellular environments is further complicated by interactions with surrounding surfaces and other factors. Concerted theoretical modeling and test-tube and in vivo experiments promise to elucidate the complexity of protein folding in cellular environments.