Vesicles: Growth, Division, Permeability
I. Environmentally Driven Growth
The first protocell membranes may have assembled from fatty acids and related single-chain lipids available in the prebiotic environment. At different concentrations, fatty acids can partition between several different phases, including soluble monomers, micelles, and lamellar vesicles, with higher concentrations favoring larger vesicle aggregates. We have exploited this property to show experimentally that evaporation can cause vesicles to grow by increasing the total fatty acid concentration. Rainfall could then cause turbulence and dilution, leading to the division of some vesicles and the dissolution of others. Thus, alternation of evaporation and rain could potentially lead to an environmentally controlled cycle of growth and division.
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Budin, I., Debnath, A., Szostak, J.W., 2012. Concentration-Driven Growth of Model Protocell Membranes. J. Am. Chem. Soc. 134, 20812–20819. PDF
Zhu, T.F., Szostak, J.W., 2009. Coupled Growth and Division of Model Protocell Membranes. J. Am. Chem. Soc. 131, 5705–5713. PDF
Zhu, T.F., Adamala, K., Zhang, N., Szostak, J.W., 2012. Photochemically driven redox chemistry induces protocell membrane pearling and division. Proc. Natl. Acad. Sci. U.S.A. 109, 9828–9832. PDF
II. Phospholipid Driven Growth
Vesicle growth can also occur as a result of competition between vesicles for limiting fatty acids. Recently we observed that vesicles that contain some phospholipid grow at the expense of vesicles that contain less phospholipid, suggesting the potential for an evolutionary arms race leading to the synthesis of ever increasing levels of phospholipid. Since this would eventually lead to altered membrane properties including decreased permeability, it is possible that the evolution of phospholipid synthesis set the stage for such downstream events as the evolution of metabolism and membrane transport machinery.
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Zhang, S.J., Lowe, L.A., Anees, P., Krishnan, Y., Fai, T.G., Szostak, J.W., Wang, A.*, 2023. Passive endocytosis in model protocells. Proc. Natl. Acad. Sci. U.S.A. 120, e2221064120. PDF
Fan, Z., Deckel, Y., Lowe, L.A., Loo, D.W.K., Yomo, T., Szostak, J.W., Nisler, C.*, Wang, A.*, 2023. Lipid Exchange Promotes Fusion of Model Protocells. Small Methods 7, 2300126. PDF
Budin, I., Szostak, J.W., 2011. Physical effects underlying the transition from primitive to modern cell membranes. Proc. Natl. Acad. Sci. U.S.A. 108, 5249–5254. PDF
III. Integration with RNA Copying
For many years there appeared to be a fundamental incompatibility between RNA replication, which requires high levels of Mg2+, and fatty acid membranes, which are destroyed by moderate levels of Mg2+. However, we recently found that when Mg2+ ions are chelated by citrate, membranes are protected but RNA copying can still proceed. This allowed us to carry out RNA-copying reactions within fatty acid vesicles by adding activated nucleotides to the outside of vesicles that contained encapsulated primer-template complexes. In addition to being a major step toward the synthesis of a complete protocell, this experiment is significant because it shows that early protocells could have been heterotrophs that grew by taking up nutrients such as nucleotides that were synthesized in the external environment.
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O’Flaherty, D.K., Kamat, N.P.*, Mirza, F.N., Li, L., Prywes, N., Szostak, J.W.*, 2018. Copying of Mixed-Sequence RNA Templates inside Model Protocells. J. Am. Chem. Soc. 140, 5171–5178. PDF
Adamala, K., Szostak, J.W., 2013. Nonenzymatic Template-Directed RNA Synthesis Inside Model Protocells. Science 342, 1098–1100. PDF
IV. Vesicle Dynamics
Images showing vesicle dynamics. A) Upon addition of phosphatidic acid (PA) vesicles to oleic acids (OA), there is fusion as shown by the colocalized signals in the green and blue channel. The white arrows point out colocalization, B) Oleic acid vesicles can undergo endocytosis. The model shows vesicles with multiple internal compartments. Scale bar represents 5 µm.
Figures from Fan et al. (Small Methods, 2023) and Zhang et al. (PNAS, 2023).
Membrane-bound vesicles are a model to study the origins of cellular life. One major goal of the lab is to generate self-replicating protocells containing self-replicating RNA. Recent work in the lab has identified several mechanisms linked to the dynamics of protocells. Stable protocells can be spontaneously assembled from micelles in solution. Protocells can grow, divide, fuse, and undergo endocytosis. Elucidating the biochemical and biophysical changes during these membrane dynamics is one area of research. Compatibility of RNA enzymatic chemistry with protocell chemistry such as the permeability of components required for non-enzymatic RNA copying across membranes is another area of research. Chemical modifications to lipids and diverse membrane compositions are also being tested. In addition to membrane-bound protocells, the lab is also investigating membrane-less models of protocells, coacervates, as an alternative route.
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Joyce, G.F.*, Szostak, J.W.*, 2018. Protocells and RNA Self-Replication. Cold Spring Harb Perspect Biol 10, a034801. PDF
Kindt, J.T., Szostak, J.W*., Wang, A.*, 2020. Bulk Self-Assembly of Giant, Unilamellar Vesicles. ACS Nano 14, 14627–14634. PDF
Fan, Z., Deckel, Y., Lowe, L.A., Loo, D.W.K., Yomo, T., Szostak, J.W., Nisler, C.*, Wang, A.*, 2023. Lipid Exchange Promotes Fusion of Model Protocells. Small Methods 7, 2300126. PDF
Zhang, S.J., Lowe, L.A., Anees, P., Krishnan, Y., Fai, T.G., Szostak, J.W., Wang, A.*, 2023. Passive endocytosis in model protocells. Proc. Natl. Acad. Sci. U.S.A. 120, e2221064120. PDF
Agrawal, A., Radakovic, A., Vonteddu, A., Rizvi, S., Huynh, V.N., Douglas, J.F., Tirrell, M.V.*, Karim, A.*, Szostak, J.W.*, 2024. Did the exposure of coacervate droplets to rain make them the first stable protocells? Sci. Adv. 10, eadn9657. PDF