The effect of binary hydrophilic polymers on a pair of representative bio‐macromolecules in a living cell has been examined. The results showed that these bio‐macromolecules exhibited specific localization in cell‐sized droplets that were spontaneously formed through water/water microphase segregation under crowding conditions with coexisting polymers. In these experiments, a simple binary polymer system with poly(ethylene glycol) (PEG) and dextran (DEX) was used. Under the conditions of microphase segregation, DNA was entrapped within cell‐sized droplets rich in DEX. Similarly, F‐actin, linearly polymerized actin, was entrapped specifically within microdroplets rich in DEX, whereas G‐actin, a monomeric actin, was distributed evenly inside and outside these droplets. This study has been extended to a system with both F‐actin and DNA, and it was found that DNA molecules were localized separately from aligned F‐actin proteins to create microdomains inside microdroplets, reflecting the self‐emergence of a cellular morphology similar to a stage of cell division.

We report the spontaneous generation of a cell-like morphology in an environment crowded with the polymers dextran and polyethylene glycol (PEG) in the presence of DNA. DNA molecules were selectively located in the interior of dextran-rich micro-droplets, when the composition of an aqueous two-phase system (ATPS) was near the critical condition of phase-segregation. The resulting micro-droplets could be controlled by the use of optical tweezers. As an example of laser manipulation, the dynamic fusion of two droplets is reported, which resembles the process of cell division in time-reverse. A hypothetical scenario for the emergence of a primitive cell with DNA is briefly discussed.

By facilitating a water/water phase separation (w/wPS), crowded biopolymers in cells form droplets that contribute to the spatial localization of biological components and their biochemical reactions. However, their influence on mechanical processes driven by protein motors has not been well studied. Here, we show that the w/wPS droplet spontaneously entraps kinesins as well as microtubules (MTs) and generates a micrometre-scale vortex flow inside the droplet. Active droplets with a size of 10–100 µm are generated through w/wPS of dextran and polyethylene glycol mixed with MTs, molecular-engineered chimeric four-headed kinesins and ATP after mechanical mixing. MTs and kinesin rapidly created contractile network accumulated at the interface of the droplet and gradually generated vortical flow, which can drive translational motion of a droplet. Our work reveals that the interface of w/wPS contributes not only to chemical processes but also produces mechanical motion by assembling species of protein motors in a functioning manner.

The front cover picture shows selective localization of F‐actin (polymerized filament) and double‐stranded DNA within a cell‐sized aqueous/aqueous droplet that was generated spontaneously through simple mixing with binary hydrophilic polymers, poly(ethylene glycol) and dextran (DEX). Interestingly, the manner of localization of these biomacromolecules switches depending on their molecular size. Long double‐stranded DNA molecules, above the size of several tens of kilo base pairs, are located solely inside DEX‐rich droplets. In contrast, short oligomeric single‐stranded DNA exists homogeneously without any apparent concentration difference. For the actins, G‐actins (monomeric) are evenly distributed, but F‐actins are localized inside DEX‐rich droplets. As in the picture, the existence of both F‐actin and DNA creates a specific structure in which DNA molecules are depleted by actin fibers arranged in parallel, something like the morphology of cell mitosis. The self‐emergence of characteristic morphologies in cell‐sized droplets could shed light on the underlying mechanism of the spatiotemporal self‐organization of living cellular systems. More information can be found in the communication by K. Takiguchi, K. Tsumoto, K. Yoshikawa, et al. on page 1370 in Issue 13, 2018 (DOI: 10.1002/cbic.201800066).