Macromolecular Crowding by Polyethylene Glycol Reduces Protein Breathing

Crowding-induced stabilization and destabilization in a single protein

The protein concentration in cells can reach 300 g/L. These crowded conditions affect protein stability. Classic crowding theories predict entropically driven stabilization, which occurs via steric repulsion, but growing evidence shows a role for non-covalent chemical interactions. To aid our understanding of physiologically relevant crowding, we used NMR-detected 1H-2H exchange to examine a simple, semi-reductionist system: protein self-crowding at the residue level using the widely studied model globular protein, GB1 (the B1 domain streptococcal protein G) at concentrations up to its solubility limit, 100 g/L. The surprising result is that self-crowding stabilizes some residues but destabilizes others, contradicting predictions. Two other observations are also contradictory. First, temperature-dependence data show that stabilization can arise enthalpically, not just entropically. Second, concentration-dependence data show destabilization often increases with increasing concentration. These results show a key role for chemical interactions. More specifically, self-crowding increases the free energy required to expose those residues that are only exposed upon complete unfolding, and stabilization of these globally unfolding residues increases with GB1 concentration, a result we attribute to repulsive chemical interactions between GB1 molecules. On the other hand, residues exposed upon local unfolding tend to be destabilized, with destabilization increasing with concentration, a result we attribute to attractive chemical interactions between GB1 molecules.

Biological Condensate Growth: Examining the Impact of Solute Crowder on Size Expansion

Biological condensation refers to the formation of micrometer-sized or smaller condensates by biological macromolecules, a process often influenced by the crowded cellular environment. Poly(ethylene glycol) (PEG) is commonly used to mimic cellular crowding, and its ability to reduce the critical nucleation concentration has been well established. However, its impact on condensate size has been less explored. This study investigates how PEG affects the size of condensates formed between protein TNP1 and DNA. Our experimental findings show that PEG molecules increase condensate size. Notably, at equal mass concentrations of PEG400, PEG3350, and PEG10000, longer PEG molecules have a much greater effect on condensate expansion. Computational simulations further reveal that longer PEG molecules enhance protein-DNA condensation more effectively and contribute to shaping the condensates into regular forms. Overall, our study provides key insights into how crowding factors influence the size and shape of colloidal growth.

Quest for space: Tenacity of DNA, Protein, and Lipid macromolecules in intracellular crowded environment

The biochemical processes in the cellular milieu involving biomacromolecular interaction usually occur in crowded and heterogeneous environments, impacting their structure, stability, and reactivity. The crowded environment in vivo is typically ignored for experimental investigations since the studies get complex due to intracellular biophysical interactions between nucleic acids, proteins, cellular membranes, and various cations/anions present in the cell. Thus, being a ubiquitous property of all cells, studying those biophysical aspects affecting biochemical processes under realistically crowded conditions is of prime importance. Crowders or crowding agents are usually exploited to mimic the in vivo conditions on interacting with such genomic species, revealing structural and functional changes resulting from excluded volume and soft interactions. In the last few years, studies including crowders of varied sizes have gained attention concerning the consequences of crowding agents on biomolecular structural transitions and stability. This review comprehensively summarizes macromolecular crowding, emphasizing the biophysical effects and contribution of soft interactions in the heterogeneous cellular environment.

Pitfalls of Using ANS Dye Under Molecular Crowding Conditions

The 1-anilino-8-naphthalenesulfonate (ANS) fluorescent dye is widely used in protein folding studies due to the significant increase in its fluorescence quantum yield upon binding to protein hydrophobic regions that become accessible during protein unfolding. However, when modeling cellular macromolecular crowding conditions in protein folding experiments in vitro using crowding agents with guanidine hydrochloride (GdnHCl) as the denaturant, the observed changes in ANS spectral characteristics require careful consideration. This study demonstrates that crowding agents can form clusters that interact differently with ANS. Furthermore, GdnHCl can disrupt these clusters and directly affect the ANS spectral characteristics. A model for the interaction between GdnHCl, crowders, and ANS is proposed. Using bovine serum albumin (BSA) as a model protein, the limitations of using ANS for studying conformational transitions induced by GdnHCl in the presence of crowding agents are demonstrated.

Impact of Cellular Crowding on Protein Structural Dynamics Investigated by EPR Spectroscopy

The study of how the intracellular medium influences protein structural dynamics and protein-protein interactions is a captivating area of research for scientists aiming to comprehend biomolecules in their native environment. As the cellular environment can hardly be reproduced in vitro, direct investigation of biomolecules within cells has attracted growing interest in the past two decades. Among magnetic resonances, site-directed spin labeling coupled to electron paramagnetic resonance spectroscopy (SDSL-EPR) has emerged as a powerful tool for studying the structural properties of biomolecules directly in cells. Since the first in-cell EPR experiment was reported in 2010, substantial progress has been made, and this Review provides a detailed overview of the developments and applications of this spectroscopic technique. The strategies available for preparing a cellular sample and the EPR methods that can be applied to cells will be discussed. The array of spin labels available, along with their strengths and weaknesses in cellular contexts, will also be described. Several examples will illustrate how in-cell EPR can be applied to different biological systems and how the cellular environment affects the structural and dynamic properties of different proteins. Lastly, the Review will focus on the future developments expected to expand the capabilities of this promising technique.

Studying protein stability in crowded environments by NMR

Most proteins perform their functions in crowded and complex cellular environments where weak interactions are ubiquitous between biomolecules. These complex environments can modulate the protein folding energy landscape and hence affect protein stability. NMR is a nondestructive and effective method to quantify the kinetics and equilibrium thermodynamic stability of proteins at an atomic level within crowded environments and living cells. Here, we review NMR methods that can be used to measure protein stability, as well as findings of studies on protein stability in crowded environments mimicked by polymer and protein crowders and in living cells. The important effects of chemical interactions on protein stability are highlighted and compared to spatial excluded volume effects.

Molecular Crowding: The History and Development of a Scientific Paradigm

It is now generally accepted that macromolecules do not act in isolation but "live" in a crowded environment, that is, an environment populated by numerous different molecules. The field of molecular crowding has its origins in the far 80s but became accepted only by the end of the 90s. In the present issue, we discuss various aspects that are influenced by crowding and need to consider its effects. This Review is meant as an introduction to the theme and an analysis of the evolution of the crowding concept through time from colloidal and polymer physics to a more biological perspective. We introduce themes that will be more thoroughly treated in other Reviews of the present issue. In our intentions, each Review may stand by itself, but the complete collection has the aspiration to provide different but complementary perspectives to propose a more holistic view of molecular crowding.

Charged Biological Membranes Repel Large Neutral Molecules by Surface Dielectrophoresis and Counterion Pressure

Macromolecular crowding is the usual condition of cells. The implications of the crowded cellular environment for protein stability and folding, protein-protein interactions, and intracellular transport drive a growing interest in quantifying the effects of crowding. While the properties of crowded solutions have been extensively studied, less attention has been paid to the interaction of crowders with the cellular boundaries, i.e., membranes. However, membranes are key components of cells and most subcellular organelles, playing a central role in regulating protein channel and receptor functions by recruiting and binding charged and neutral solutes. While membrane interactions with charged solutes are dominated by electrostatic forces, here we show that significant charge-induced forces also exist between membranes and neutral solutes. Using neutron reflectometry measurements and molecular dynamics simulations of poly(ethylene glycol) (PEG) polymers of different molecular weights near charged and neutral membranes, we demonstrate the roles of surface dielectrophoresis and counterion pressure in repelling PEG from charged membrane surfaces. The resulting depletion zone is expected to have consequences for drug design and delivery, the activity of proteins near membrane surfaces, and the transport of small molecules along the membrane surface.