Protein self-association in the cell: a mechanism for fine tuning the level of macromolecular crowding?

Biophysical Reviews' "Meet the Editors Series": a profile of Damien Hall

This piece introduces Damien Hall, Chief Editor of the Biophysical Reviews journal since 2019. Currently working as an Assistant Professor at Kanazawa University, the author describes his association with the journal along with some parts of his family history and academic journey.

A fortunate period of overlap with Prof. Haruki Nakamura

The author recounts a period of overlap with Prof. Haruki Nakamura that stretched from 2007 till the present day. Starting as a short-term research fellow in his laboratory, the author has also been a coauthor, academic colleague, and joint journal editorial board member of Prof. Nakamura.

Chaperone-Like Activity of HSPB5: The Effects of Quaternary Structure Dynamics and Crowding

Small heat-shock proteins (sHSPs) are ATP-independent molecular chaperones that interact with partially unfolded proteins, preventing their aberrant aggregation, thereby exhibiting a chaperone-like activity. Dynamics of the quaternary structure plays an important role in the chaperone-like activity of sHSPs. However, relationship between the dynamic structure of sHSPs and their chaperone-like activity remains insufficiently characterized. Many factors (temperature, ions, a target protein, crowding etc.) affect the structure and activity of sHSPs. The least studied is an effect of crowding on sHSPs activity. In this work the chaperone-like activity of HSPB5 was quantitatively characterized by dynamic light scattering using two test systems, namely test systems based on heat-induced aggregation of muscle glycogen phosphorylase b (Phb) at 48 °C and dithiothreitol-induced aggregation of α-lactalbumin at 37 °C. Analytical ultracentrifugation was used to control the oligomeric state of HSPB5 and target proteins. The possible anti-aggregation functioning of suboligomeric forms of HSPB5 is discussed. The effect of crowding on HSPB5 anti-aggregation activity was characterized using Phb as a target protein. The duration of the nucleation stage was shown to decrease with simultaneous increase in the relative rate of aggregation of Phb in the presence of HSPB5 under crowded conditions. Crowding may subtly modulate sHSPs activity.

On the nature of the optimal form of the holdase-type chaperone stress response

The holdase paradigm of chaperone action involves preferential binding by the chaperone to the unfolded protein state, thereby preventing it from either, associating with other unstable proteins (to form large dysfunctional aggregates), or being degraded by the proteolytic machinery of the cell/organism. In this paper, we examine the necessary physical constraints imposed upon the holdase chaperone response in a cell-like environment and use these limitations to comment on the likely nature of the optimal form of chaperone response in vivo.

Ionic liquids and protein folding-old tricks for new solvents

One important aspect of the green chemistry revolution has been the use of ionic liquids as the solvent in liquid-phase enzymatic catalysis. An essential requirement for protein enzyme function is the correct folding of the polypeptide chain into its functional "native" state. Quantitative assessment of protein structure may be carried out either empirically, or by using model-based characterization procedures, in which the parameters are defined in terms of a standard reference state. In this short note, we briefly outline the nature of the parameters associated with different empirical and model-based characterization procedures and point out factors which affect their interpretation when using a base solvent different from water. This review principally describes arguments developed by Wakayama et al., Protein Solubility and Amorphous Aggregation: From Academic Research to Applications in Drug Discovery and Bioindustry, 2019, edited by Y. Kuroda and F. Arisaka; CMC Publishing House. Sections of that work are translated from the original Japanese and republished here with the full permission of CMC Publishing Corporation.

Toward an understanding of biochemical equilibria within living cells

Four types of environmental effects that can affect macromolecular reactions in a living cell are defined: nonspecific intermolecular interactions, side reactions, partitioning between microenvironments, and surface interactions. Methods for investigating these interactions and their influence on target reactions in vitro are reviewed. Methods employed to characterize conformational and association equilibria in vivo are reviewed and difficulties in their interpretation cataloged. It is concluded that, in order to be amenable to unambiguous interpretation, in vivo studies must be complemented by in vitro studies carried out in well-characterized and controllable media designed to contain key elements of selected intracellular microenvironments.

The functional roles of the unstructured N- and C-terminal regions in αB-crystallin and other mammalian small heat-shock proteins

Small heat-shock proteins (sHsps), such as αB-crystallin, are one of the major classes of molecular chaperone proteins. In vivo, under conditions of cellular stress, sHsps are the principal defence proteins that prevent large-scale protein aggregation. Progress in determining the structure of sHsps has been significant recently, particularly in relation to the conserved, central and β-sheet structured α-crystallin domain (ACD). However, an understanding of the structure and functional roles of the N- and C-terminal flanking regions has proved elusive mainly because of their unstructured and dynamic nature. In this paper, we propose functional roles for both flanking regions, based around three properties: (i) they act in a localised crowding manner to regulate interactions with target proteins during chaperone action, (ii) they protect the ACD from deleterious amyloid fibril formation and (iii) the flexibility of these regions, particularly at the extreme C-terminus in mammalian sHsps, provides solubility for sHsps under chaperone and non-chaperone conditions. In the eye lens, these properties are highly relevant as the crystallin proteins, in particular the two sHsps αA- and αB-crystallin, are present at very high concentrations.

Measurement of amyloid formation by turbidity assay-seeing through the cloud

Detection of amyloid growth is commonly carried out by measurement of solution turbidity, a low-cost assay procedure based on the intrinsic light scattering properties of the protein aggregate. Here, we review the biophysical chemistry associated with the turbidimetric assay methodology, exploring the reviewed literature using a series of pedagogical kinetic simulations. In turn, these simulations are used to interrogate the literature concerned with in vitro drug screening and the assessment of amyloid aggregation mechanisms.

One ring (or two) to hold them all – on the structure and function of protein nanotubes

Understanding the structural determinants relevant to the formation of supramolecular assemblies of homo-oligomeric proteins is a traditional and central scope of structural biology. The knowledge thus gained is crucial both to infer their physiological function and to exploit their architecture for bionanomaterials design. Protein nanotubes made by one-dimensional arrays of homo-oligomers can be generated by either a commutative mechanism, yielding an 'open' structure (e.g. actin), or a noncommutative mechanism, whereby the final structure is formed by hierarchical self-assembly of intermediate 'closed' structures. Examples of the latter process are poorly described and the rules by which they assemble have not been unequivocally defined. We have collected and investigated examples of homo-oligomeric circular arrangements that form one-dimensional filaments of stacked rings by the noncommutative mechanism in vivo and in vitro. Based on their quaternary structure, circular arrangements of protein subunits can be subdivided into two groups that we term Rings of Dimers (e.g. peroxiredoxin and stable protein 1) and Dimers of Rings (e.g. thermosome/rosettasome), depending on the sub-structures that can be identified within the assembly (and, in some cases, populated in solution under selected experimental conditions). Structural analysis allowed us to identify the determinants by which ring-like molecular chaperones form filamentous-like assemblies and to formulate a novel hypothesis by which nanotube assembly, molecular chaperone activity and macromolecular crowding may be interconnected.

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.

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.

Macromolecular crowding and its potential impact on nuclear function

It is well established, that biochemical reactions are dependent on pH, ionic strength, temperature and the concentration of reactants. However, the steric repulsion among bulky components of biological systems also affect biochemical behavior: The 'excluded volume effect of macromolecular crowding' drives bulky components into structurally compact organizations, increases their thermodynamic activities and slows down diffusion. The very special composition of the cell nucleus, which is packed with high-molecular chromatin, ribonucleo-particles and associated proteins, suggests that crowding-effects are part of nuclear functionality. Realizing that many nuclear processes, notably gene transcription, hnRNA splicing and DNA replication, use macromolecular machines, and taking into account that macromolecular crowding provides a cooperative momentum for the assembly of macromolecular complexes, we here elaborate why macromolecular crowding may be functionally important in supporting the statistical significance of nuclear activities.

Coarse-grained molecular simulation of diffusion and reaction kinetics in a crowded virtual cytoplasm

We present a general-purpose model for biomolecular simulations at the molecular level that incorporates stochasticity, spatial dependence, and volume exclusion, using diffusing and reacting particles with physical dimensions. To validate the model, we first established the formal relationship between the microscopic model parameters (timestep, move length, and reaction probabilities) and the macroscopic coefficients for diffusion and reaction rate. We then compared simulation results with Smoluchowski theory for diffusion-limited irreversible reactions and the best available approximation for diffusion-influenced reversible reactions. To simulate the volumetric effects of a crowded intracellular environment, we created a virtual cytoplasm composed of a heterogeneous population of particles diffusing at rates appropriate to their size. The particle-size distribution was estimated from the relative abundance, mass, and stoichiometries of protein complexes using an experimentally derived proteome catalog from Escherichia coli K12. Simulated diffusion constants exhibited anomalous behavior as a function of time and crowding. Although significant, the volumetric impact of crowding on diffusion cannot fully account for retarded protein mobility in vivo, suggesting that other biophysical factors are at play. The simulated effect of crowding on barnase-barstar dimerization, an experimentally characterized example of a bimolecular association reaction, reveals a biphasic time course, indicating that crowding exerts different effects over different timescales. These observations illustrate that quantitative realism in biosimulation will depend to some extent on mesoscale phenomena that are not currently well understood.

Mixed Macromolecular Crowding Accelerates the Refolding of Rabbit Muscle Creatine Kinase: Implications for Protein Folding in Physiological Environments

The effects of four single macromolecular crowding agents, Ficoll 70, dextran 70, polyethylene glycol (PEG) 2000, and calf thymus DNA (CT DNA), and three mixed crowding agents containing both CT DNA and polysaccharide (or PEG 2000) on the refolding of guanidine hydrochloride-denatured rabbit muscle creatine kinase (MM-CK) have been examined by activity assay. When the total concentration of the mixed crowding agent is 100 g/l, in which the weight ratio of CT DNA to Ficoll 70 is 1:9, the refolding yield of MM-CK after refolding for 3 h under these conditions increases 23% compared with that in the presence of 10 g/l CT DNA, 18% compared with 100 g/l Ficoll 70, and 19% compared with that in the absence of crowding agents. A remarkable increase in the refolding yield of MM-CK by a mixed crowding agent containing CT DNA and dextran 70 (or PEG 2000) is also observed. Further folding kinetics analyses show that these three mixed crowding agents remarkably accelerate the refolding of MM-CK, compared with single crowding agents. Aggregation of MM-CK in the presence of any of the three mixed crowding agents is less serious than that in the presence of a single crowding agent at the same concentration but more serious than that in the absence of crowding agents. Both the refolding yield and the refolding rate of MM-CK in mixtures of these agents are increased relative to the individual agents by themselves, indicating that mixed macromolecular crowding agents are more favorable to MM-CK folding and can be used to reflect the physiological environment more accurately than single crowding agents.

Accommodating space, time and randomness in network simulation

Interest in the possibility of dynamically simulating complex cellular processes has escalated markedly in recent years. This interest has been fuelled by three factors: the generally accepted value in understanding living processes as integrated systems; the dramatic increase in computational capability; and the availability of new or improved technology for making the quantitative measurements that are needed to drive and validate cellular simulations. Between the extremes of atom-scale and organism-scale simulation is a vast middle-ground requiring simulation strategies that are capable of dealing with a range of spatial, temporal and molecular abundance scales that are crucial for a comprehensive understanding of integrative cell biology. Although at an early stage, methodological improvements and the development of computational platforms provide some hope that simulations will emerge that can bridge the gap between network models and the true operation of the cell as a complex machine.

Expanding to fill the gap: A possible role for inert biopolymers in regulating the extent of the ‘macromolecular crowding’ effect

We discuss the potential for inert biopolymers existing in cells to play a role in regulating the macromolecular crowding effect via their ability to undergo shape changing structural transitions. We have explored this possibility by the use of theory and experiment. The theoretical component utilized Monte-Carlo based simulations to examine the folding of a hypothetical protein in a concentrated environment of hard spheres which are themselves capable of reversible expansion and contraction. The experimental component of the study involved examination of the effect of different sized crowding agents on the thermally induced denaturation of cytochrome c [in phosphate buffered saline solution containing 1.0M guanidinium hydrochloride at pH 7.0]. On the basis of our findings we suggest that in a crowded solution environment the presence of a non-reactive polymer capable of reversible expansion/contraction via folding and unfolding may alter the excluded volume component of the solution. This ability would confer on the non-reactive polymer a novel role in influencing other processes in solution affected by macromolecular crowding.