The interior of a living cell is packed with large macromolecules (e.g., nucleic acids, proteins, polysaccharides,), cytoskeletal filaments, and dissolved salts. In aqueous solutions, these components often do not mix ideally. Differences in their solvation, inter- and intramolecular interactions, and electrostatics render the aqueous cytosolic space inhomogeneous producing a patchwork of co-existing fluids. This microcompartmentalization is thought to arise through thermodynamic liquid-liquid phase separation at equilibrium. But how is it affected by the macromolecular crowding in the cytoplasmic space? Do non-equilibrium processes, such as of molecular additions and removals present in living cells, alter these equilibrium tendencies? How this dynamic micro-compartmentation affects membrane shapes and molecular organization?<br/><br/>Building on the experimental model system, developed previously by Keating (PNAS, 2005), Dimova, and Lipowsky (Soft Mattter, 2012), we reconstitute minimal crowding and conditions for dynamic microcompartmentation inside cell-sized giant vesicles. Specifically, we encapsulate a uniform aqueous phase mixture of two water soluble polymers (namely, dextran and poly(ethylene)glycol), which when subject to osmotic deflation, assumes relative concentrations that produce aqueous two-phase separation (ATPS) in real-time. We find that the onset of vesicular micro-compartmentation in the encapsulated aqueous phase experiences a competition with preferential wetting of one of the forming phases by the bounding membrane arresting phase coarsening. Interestingly, this also drives an active membrane remodeling inducing vesicle morphogenesis. Unlike equilibrium configurations, we find that multiple buds of essentially uniform size decorate the membrane boundary reflecting a competition between phase coarsening dynamics during the spinodal decomposition of the ATPS and preferential wetting of the membrane surface by the dextran-rich phase. Our results suggest the stabilization of a rich variety of membrane morphologies through kinetically determined membrane deformation processes, far exceeding the narrow range of shapes obtained under thermodynamic equilibrium.

Compartments and crowding: Biophysical features of living cells

2019 Spring National Meeting

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