Dynamic Behavior of Complex Coacervates with Internal Lipid Vesicles under Nonequilibrium Conditions

Abstract

During the evolution of life on earth, the emergence of lipid membrane-bounded compartments is one of the most enigmatic events. Endosymbiosis has been hypothesized as one of the solutions. In this work, using a coacervate droplet formed by single-stranded oligonucleotides (ss-oligo) and poly(L-lysine) (PLL) as the protocell model, we monitored the uptake of liposomes of different types and studied the dynamic behavior of the resulting composite droplet under the electric field. The coacervate droplet exhibits affinity for the liposomes of varying charges. However, the permeation of liposome is also controlled by electrostatic interactions. Dominated by electrostatic attraction, the positively charged liposome is retained inside the droplet as growing fibrous structures, while the negatively charged liposome is mainly coated on the droplet surface. Permeation and even distribution occur when the liposome and the droplet carry the same charges, or at least one of them is neutral. As an electric field is applied to trigger repetitive cycles of vacuolization in the ss-oligo/ PLL droplet, the fibrous structure formed by the positively charged liposome is basically intact, while a new phase is generated together with uneven mass transport as the negatively charged liposome is internalized. Interestingly, the release of daughter droplets with similar components occurs on the droplet containing neutral liposomes. Our work not only provides a step toward creating protocells with hierarchical structures and biofunctions using a biogenetic material via simple mixing but also sheds light on the possible origin of the lipid structure inside a living organism.

Interactions between Phase-Separated Liquids and Membrane Surfaces

“…The simplicity and biological relevance of such droplets have made them systems of interest as models for protocells and biomolecular condensates[ 77 , 86 ]. The charge ratio of the coacervate (that is, the ratio of polycation to polyanion) impacts its physical properties: Coacervates with a high ratio (more polycation) have a more positive zeta potential[ 87 , 88 ]. Vesicles have assembled at the surface of complex coacervates, much the same as in some ATPSs ( Fig.…”

“…Vesicle localization in relation to complex coacervates is dependent on several factors. A recent study indicates that vesicles have a general tendency to diffuse into a complex coacervate, as their component molecules generally contain positive, negative, and hydrophobic sections[ 88 ]. That study found that negatively-charged vesicles primarily remained at coacervate surfaces with a net positive charge, but the same did not hold for positively-charged vesicles and negatively-charged coacervate surfaces ( Fig.…”

“…That study found that negatively-charged vesicles primarily remained at coacervate surfaces with a net positive charge, but the same did not hold for positively-charged vesicles and negatively-charged coacervate surfaces ( Fig. 4 a – b )[ 88 ]. This finding is consistent with previous research which used negatively-charged vesicles to generate a vesicle coating on a coacervate[ 77 , 87 ].…”

“…This finding is consistent with previous research which used negatively-charged vesicles to generate a vesicle coating on a coacervate[ 77 , 87 ]. Vesicles with membranes in the gel phase rather than a liquid phase also formed a coating at the phase boundary independent of coacervate charge, presumably because they were less able to deform and so may not be able to enter the coacervate[ 88 ].…”

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

“…Certain LLPS systems can also interact with, scaffold, and/or segregate lipid vesicles, perhaps foreshadowing a transition from membraneless compartments to membrane-bound compartments at some point at the origins of life (Tang et al 2014;Jia et al 2014Jia et al , 2019Jing et al 2019;Pir Cakmak et al 2019;Lin et al 2020). Other prebiotically relevant functions observed in phase-separated droplets include division (Yin et al 2016;Zwicker et al 2016), fusion (Jing et al 2020), and protection of encapsulated molecules from degradation (Okihana and Ponnamperuma 1982; Zhao and Zacharia 2018), all of which could assist in evolution of a primitive compartment. While functions in primitive LLPS systems appear to be relatively simple compared to modern engineering applications, there are cases where functions or properties of primitive LLPS can be used in applied technologies.…”