Glycine rich segments adopt polyproline II helices: Implications for biomolecular condensate formation

The mechanobiology of biomolecular condensates

The central goal of mechanobiology is to understand how the mechanical forces and material properties of organelles, cells, and tissues influence biological processes and functions. Since the first description of biomolecular condensates, it was hypothesized that they obtain material properties that are tuned to their functions inside cells. Thus, they represent an intriguing playground for mechanobiology. The idea that biomolecular condensates exhibit diverse and adaptive material properties highlights the need to understand how different material states respond to external forces and whether these responses are linked to their physiological roles within the cell. For example, liquids buffer and dissipate, while solids store and transmit mechanical stress, and the relaxation time of a viscoelastic material can act as a mechanical frequency filter. Hence, a liquid-solid transition of a condensate in the force transmission pathway can determine how mechanical signals are transduced within and in-between cells, affecting differentiation, neuronal network dynamics, and behavior to external stimuli. Here, we first review our current understanding of the molecular drivers and how rigidity phase transitions are set forth in the complex cellular environment. We will then summarize the technical advancements that were necessary to obtain insights into the rich and fascinating mechanobiology of condensates, and finally, we will highlight recent examples of physiological liquid-solid transitions and their connection to specific cellular functions. Our goal is to provide a comprehensive summary of the field on how cells harness and regulate condensate mechanics to achieve specific functions.

Hydrogen bonding patterns and cooperativity in polyproline II helical bundles

Hydrogen bond cooperativity (HBC) plays an important role in stabilizing protein assemblies built by α-helices and β-sheets, the most common secondary structures. However, whether HBC exists in other types of protein secondary structures such as polyproline II (PPII) helices remains unexplored. This is intriguing, since PPII systems as assembling blocks are continuously emerging across multiple fields. Here, using a combination of computational chemistry tools and molecular modeling corroborated by experimental observables, we characterize the distinct H-bonding patterns present in PPII helical bundles and establish that HBC stabilizes intermolecular PPII helices as seen in other protein assemblies such as amyloid fibrils. In addition to cooperative interactions in canonical CO···HN H-bonds, we show that analogous interactions in non-canonical CO···HαCα H-bonds are relevant in Gly-rich PPII bundles, thus compensating for the inability of glycine residues to create hydrophobic cores. Our results provide a mechanistic explanation for the assembly of these bundles.

Mechanistic insights into condensate formation of human liver-type phosphofructokinase by stochastic modeling approaches

Human liver-type phosphofructokinase 1 (PFKL) has been shown to regulate glucose flux as a scaffolder arranging glycolytic and gluconeogenic enzymes into a multienzyme metabolic condensate, the glucosome. However, it has remained elusive of how phase separation of PFKL is governed and initiates glucosome formation in living cells, thus hampering to understand a mechanism of glucosome formation and its functional contribution to human cells. In this work, we developed a stochastic model in silico using the principle of Langevin dynamics to investigate how biological properties of PFKL contribute to the condensate formation. The significance of an intermolecular interaction between PFKLs, an effective concentration of PFKL at a region of interest, and its own self-assembled filaments in formation of PFKL condensates and control of their sizes were demonstrated by molecular dynamics simulation using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). Such biological properties that define intracellular dynamics of PFKL appear to be essential for phase separation of PFKL, which may represent an initiation step for the formation of glucosome condensates. Collectively, our computational study provides mechanistic insights of glucosome formation, particularly an initial stage through the formation of PFKL condensates in living human cells.

Microfluidics for multiscale studies of biomolecular condensates

Membraneless organelles formed through condensation of biomolecules in living cells have become the focus of sustained efforts to elucidate their mechanisms of formation and function. These condensates perform a range of vital functions in cells and are closely connected to key processes in functional and aberrant biology. Since these systems occupy a size scale intermediate between single proteins and conventional protein complexes on the one hand, and cellular length scales on the other hand, they have proved challenging to probe using conventional approaches from either protein science or cell biology. Additionally, condensate can form, solidify and perform functions on various time-scales. From a physical point of view, biomolecular condensates are colloidal soft matter systems, and microfluidic approaches, which originated in soft condensed matter research, have successfully been used to study biomolecular condensates. This review explores how microfluidics have aided condensate research into the thermodynamics, kinetics and other properties of condensates, by offering high-throughput and novel experimental setups.

Phase separation of the mammalian prion protein: physiological and pathological perspectives

Abnormal phase transitions have been implicated in the occurrence of proteinopathies. Disordered proteins with nucleic acid binding ability drive the formation of reversible micron-sized condensates capable of controlling nucleic acid processing/transport. This mechanism, achieved via liquid-liquid phase separation (LLPS), underlies the formation of long-studied membraneless organelles (e.g., nucleolus) and various transient condensates formed by driver proteins. The prion protein (PrP) is not a classical nucleic acid-binding protein. However, it binds nucleic acids with high affinity, undergoes nucleocytoplasmic shuttling, contains a long intrinsically disordered region rich in glycines and evenly spaced aromatic residues, among other biochemical/biophysical properties of bona fide drivers of phase transitions. Because of this, our group and others have characterized LLPS of recombinant PrP. In vitro phase separation of PrP is modulated by nucleic acid aptamers, and, depending on the aptamer conformation, the liquid droplets evolve to solid-like species. Herein we discuss recent studies and previous evidence supporting PrP phase transitions. We focus on the central role of LLPS related to PrP physiology and pathology, with a special emphasis on the interaction of PrP with different ligands, such as proteins and nucleic acids, which can play a role in prion disease pathogenesis. Finally, we comment on therapeutic strategies directed at the nonfunctional phase separation that could potentially tackle prion diseases or other protein misfolding disorders.

Glycine-Rich Peptides from FUS Have an Intrinsic Ability to Self-Assemble into Fibers and Networked Fibrils

Glycine-rich regions feature prominently in intrinsically disordered regions (IDRs) of proteins that drive phase separation and the regulated formation of membraneless biomolecular condensates. Interestingly, the Gly-rich IDRs seldom feature poly-Gly tracts. The protein fused in sarcoma (FUS) is an exception. This protein includes two 10-residue poly-Gly tracts within the prion-like domain (PLD) and at the interface between the PLD and the RNA binding domain. Poly-Gly tracts are known to be highly insoluble, being potent drivers of self-assembly into solid-like fibrils. Given that the internal concentrations of FUS and FUS-like molecules cross the high micromolar and even millimolar range within condensates, we reasoned that the intrinsic insolubility of poly-Gly tracts might be germane to emergent fluid-to-solid transitions within condensates. To assess this possibility, we characterized the concentration-dependent self-assembly for three non-overlapping 25-residue Gly-rich peptides derived from FUS. Two of the three peptides feature 10-residue poly-Gly tracts. These peptides form either long fibrils based on twisted ribbon-like structures or self-supporting gels based on physical cross-links of fibrils. Conversely, the peptide with similar Gly contents but lacking a poly-Gly tract does not form fibrils or gels. Instead, it remains soluble across a wide range of concentrations. Our findings highlight the ability of poly-Gly tracts within IDRs that drive phase separation to undergo self-assembly. We propose that these tracts are likely to contribute to nucleation of fibrillar solids within dense condensates formed by FUS.