Phospholipid Membrane Formation Templated by Coacervate Droplets

Abstract

We report the formation of coacervate-supported phospholipid membranes by hydrating a dried lipid film in the presence of coacervate droplets. Coacervate-supported membranes were characterized by fluorescence imaging, polarization, fluorescence recovery after photobleaching of labeled lipids, lipid quenching experiments, and solute uptake experiments. Our findings are consistent with the presence of lipid membranes around the coacervates, with many droplets fully coated by what appear to be continuous lipid bilayers. In contrast to traditional giant lipid vesicles formed by gentle hydration in the absence of coacervates, the coacervate-templated membrane vesicles are more uniform in size, shape, and apparent lamellarity. Due to their fully coacervate model cytoplasm, these simple artificial cells are macromolecularly crowded and can be easily pre-loaded with high concentrations of proteins or nucleic acids. Within the same population, in addition to coacervate droplets having intact lipid membrane coatings, other coacervate droplets are coated with membranes having defects or pores that permit solute entry, and some are coated with multilayered membranes. Membranes surrounding protein-based coacervate droplets provided protection from a protease added to the external solution. The simplicity of producing artificial cells having a coacervate model cytoplasm surrounded by a model membrane is at the same time interesting as a potential mechanism for prebiotic protocell formation and appealing for biotechnology. We anticipate that such structures could serve as a new type of model system for understanding interactions between intracellular phases and cell or organelle membranes, which are implicated in a growing number of processes ranging from neurotransmission to signaling.

Protocells: Milestones and Recent Advances

“…[117,134,143] Coacervates are suggested as precursors to membranous vesicles with various levels of hierarchical complexity. [144][145][146] For example, Tang et al proposed the spontaneous self-assembly of a continuous fatty-acid membrane at the surface of preformed coacervate microdroplets. [147] Martin and Douliez reviewed fatty acid vesicles and coacervates, and emphasized that coacervates can reversibly transform into fatty acid vesicles due to changes in pH, suggesting to consider this phenomenon as a new direction to explore the origins of life.…”

“…Coacervates are suggested as precursors to membranous vesicles with various levels of hierarchical complexity. [ 144–146 ] For example, Tang et al. proposed the spontaneous self‐assembly of a continuous fatty‐acid membrane at the surface of preformed coacervate microdroplets.…”

A material‐based panspermia hypothesis: The potential of polymer gels and membraneless droplets

“…Structural evolution of compartments may have resulted in acquisition of more complex structural components which contributed more complex functions to the compartment system, such as demonstrated via coacervate acquisition of liquid crystal character [ 147,168 ] or lipid membranes. [ 169–171 ] Emergence of such structures in polymeric gel material droplets could have occurred, perhaps through acquisition of the nucleic acids or lipids that formed those structures. In particular, such compartments would have benefited from the ability to actively scaffold the assembly of additional structures (rather than passively acquiring such structures).…”

DNA‐Mediated Protein Shuttling between Coacervate‐Based Artificial Cells

“…[9,18] Coacervates have been increasingly studied as bottomup artificial cell platforms, owing to their crowded nature and their potential to act as framework on which semipermeable membranes can be assembled. [6,19,[20][21][22] Moreover, in nature, analogous liquid-liquid phase separated compartments have been found to localize macromolecules and thus cellular processes and as such, they have the innate ability to sequester proteins. [23,24] Only a few coacervate-based artificial cell examples exist where specific interactions between the protein and coacervate matrix were employed.…”