Practical considerations for generation of multi-compartment complex coacervates

Formation of multicompartment structures through aging of protein-RNA condensates

Cells can dynamically organize reactions through the formation of biomolecular condensates. These viscoelastic networks exhibit complex material properties and mesoscale architectures, including the ability to form multiphase assemblies. It was shown previously that condensates with complex architectures may arise at equilibrium in multicomponent systems or in condensates that were driven out of equilibrium by changes in external parameters such as temperature. In this study, we demonstrate that the aging of initially homogeneous protein-RNA condensates can spontaneously lead to the formation of kinetically arrested double-emulsion and core-shell structures without changes in external variables such as temperature or solution conditions. By combining time-resolved fluorescence-based experimental techniques with simulations based on the Cahn-Hilliard theory, we show that, as the protein-RNA condensates age, the decrease of the relative strength of protein-RNA interactions induces the release of RNA molecules from the dense phase. In condensates exceeding a critical size, aging combined with slow diffusion of the macromolecules trigger nucleation of dilute phase inside the condensates, which leads to the formation of double-emulsion structures. These findings illustrate a new mechanism of formation of multicompartment condensates.

Determining the Physico-Chemical Composition of Biomolecular Condensates from Spatially-Resolved NMR

Liquid-Liquid phase separation has emerged as fundamental process underlying the formation of biomolecular condensates. Insights into the composition and structure of biomolecular condensates is, however, complicated by their molecular complexity and dynamics. Here, we introduce an improved spatially-resolved NMR experiment that enables quantitative analysis of the physico-chemical composition of multi-component biomolecular condensates in equilibrium and label-free. Application of spatially-resolved NMR to condensates formed by the Alzheimer's disease-associated protein Tau demonstrates decreased water content, exclusion of the molecular crowding agent dextran, presence of a specific chemical environment of the small molecule DSS, and ≈150-fold increased concentration of Tau inside the condensate. The results suggest that spatially-resolved NMR can have a major impact in understanding the composition and physical chemistry of biomolecular condensates.

Phase-specific RNA accumulation and duplex thermodynamics in multiphase coacervate models for membraneless organelles

Liquid-liquid phase separation has emerged as an important means of intracellular RNA compartmentalization. Some membraneless organelles host two or more compartments serving different putative biochemical roles. The mechanisms for, and functional consequences of, this subcompartmentalization are not yet well understood. Here we show that adjacent phases of decapeptide-based multiphase model membraneless organelles differ markedly in their interactions with RNA. Single- and double-stranded RNAs preferentially accumulate in different phases within the same droplet, and one phase is more destabilizing for RNA duplexes than the other. Single-phase peptide droplets did not capture this behaviour. Phase coexistence introduces new thermodynamic equilibria that alter RNA duplex stability and RNA sorting by hybridization state. These effects require neither biospecific RNA-binding sites nor full-length proteins. As such, they are more general and point to primitive versions of mechanisms operating in extant biology that could aid understanding and enable the design of functional artificial membraneless organelles.

Potassium at the Origins of Life: Did Biology Emerge from Biotite in Micaceous Clay?

Intracellular potassium concentrations, [K+], are high in all types of living cells, but the origins of this K+ are unknown. The simplest hypothesis is that life emerged in an environment that was high in K+. One such environment is the spaces between the sheets of the clay mineral mica. The best mica for life's origins is the black mica, biotite, because it has a high content of Mg++ and because it has iron in various oxidation states. Life also has many of the characteristics of the environment between mica sheets, giving further support for the possibility that mica was the substrate on and within which life emerged. Here, a scenario for life's origins is presented, in which the necessary processes and components for life arise in niches between mica sheets; vesicle membranes encapsulate these processes and components; the resulting vesicles fuse, forming protocells; and eventually, all of the necessary components and processes are encapsulated within individual cells, some of which survive to seed the early Earth with life. This paper presents three new foci for the hypothesis of life's origins between mica sheets: (1) that potassium is essential for life's origins on Earth; (2) that biotite mica has advantages over muscovite mica; and (3) that micaceous clay is a better environment than isolated mica for life's origins.

Connecting primitive phase separation to biotechnology, synthetic biology, and engineering

One aspect of the study of the origins of life focuses on how primitive chemistries assembled into the first cells on Earth and how these primitive cells evolved into modern cells. Membraneless droplets generated from liquid-liquid phase separation (LLPS) are one potential primitive cell-like compartment; current research in origins of life includes study of the structure, function, and evolution of such systems. However, the goal of primitive LLPS research is not simply curiosity or striving to understand one of life's biggest unanswered questions, but also the possibility to discover functions or structures useful for application in the modern day. Many applicational fields, including biotechnology, synthetic biology, and engineering, utilize similar phaseseparated structures to accomplish specific functions afforded by LLPS. Here, we briefly review LLPS applied to primitive compartment research and then present some examples of LLPS applied to biomolecule purification, drug delivery, artificial cell construction, waste and pollution management, and flavor encapsulation. Due to a significant focus on similar functions and structures, there appears to be much for origins of life researchers to learn from those working on LLPS in applicational fields, and vice versa, and we hope that such researchers can start meaningful cross-disciplinary collaborations in the future.