The plasma membrane

Kepa Ruiz-Mirazo,†,∥ Carlos Briones,‡,∥ and Andreś de la Escosura* †Biophysics Unit (CSIC-UPV/EHU), Leioa, and Department of Logic and Philosophy of Science, University of the Basque Country, Avenida de Tolosa 70, 20080 Donostia−San Sebastiań, Spain ‡Department of Molecular Evolution, Centro de Astrobiología (CSIC−INTA, associated to the NASA Astrobiology Institute), Carretera de Ajalvir, Km 4, 28850 Torrejoń de Ardoz, Madrid, Spain Organic Chemistry Department, Universidad Autońoma de Madrid, Cantoblanco, 28049 Madrid, Spain

Prebiotic systems chemistry: new perspectives for the origins of life.

Liposomes are structurally and functionally some of the most versatile supramolecular assemblies in existence. Since the beginning of active research on lipid vesicles in 1965, the field has progressed enormously and applications are well established in several areas, such as drug and gene delivery. In the analytical sciences, liposomes serve a dual purpose: Either they are analytes, typically in quality-assessment procedures of liposome preparations, or they are functional components in a variety of new analytical systems. Liposome immunoassays, for example, benefit greatly from the amplification provided by encapsulated markers, and nanotube-interconnected liposome networks have emerged as ultrasmall-scale analytical devices. This review provides information about new developments in some of the most actively researched liposome-related topics.

Liposomes: technologies and analytical applications.

Preface 1. The conceptual framework of the research on the origin of life on Earth 2. Approaches to the definitions of life 3. Selection in prebiotic chemistry - why this ... and not that? 4. The bottle neck - macromolecular sequences 5. Self-organization 6. Emergence and emergent properties 7. Self-replication and self-reproduction 8. Autopoiesis - the logic of cellular life 9. Compartments 10. Reactivity and transformation of vesicles 11. Approaches to the minimal cell Outlook Bibliography.

The Emergence of Life: From Chemical Origins to Synthetic Biology

The generation of synthetic forms of cellular life requires solutions to the problem of how biological processes such as cyclic growth and division could emerge from purely physical and chemical systems. Small unilamellar fatty acid vesicles grow when fed with fatty acid micelles and can be forced to divide by extrusion, but this artificial division process results in significant loss of protocell contents during each division cycle. Here we describe a simple and efficient pathway for model protocell membrane growth and division. The growth of large multilamellar fatty acid vesicles fed with fatty acid micelles, in a solution where solute permeation across the membranes is slow, results in the transformation of initially spherical vesicles into long thread-like vesicles, a process driven by the transient imbalance between surface area and volume growth. Modest shear forces are then sufficient to cause the thread-like vesicles to divide into multiple daughter vesicles without loss of internal contents. In an environment of gentle shear, protocell growth and division are thus coupled processes. We show that model protocells can proceed through multiple cycles of reproduction. Encapsulated RNA molecules, representing a primitive genome, are distributed to the daughter vesicles. Our observations bring us closer to the laboratory synthesis of a complete protocell consisting of a self-replicating genome and a self-replicating membrane compartment. In addition, the robustness and simplicity of this pathway suggests that similar processes might have occurred under the prebiotic conditions of the early Earth.

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Coupled Growth and Division of Model Protocell Membranes

Vesicle deformation by an axial load: from elongated shapes to tethered vesicles.

Self-reproduction and the ability to regulate their composition are two essential properties of terrestrial biotic systems. The identification of non-living systems that possess these properties can therefore contribute not only to our understanding of their functioning but also hint at possible prebiotic processes that led to the emergence of life. Growing lipid vesicles have been previously established as having the capacity to self-reproduce. Here it is demonstrated that vesicle self-reproduction can occur only at selected values of vesicle properties. We treat as an example a simple vesicle with membrane elastic properties defined by a membrane bending modulus κ and spontaneous curvature C
0, whose volume variation depends on the membrane hydraulic permeability L
p and whose membrane area doubles in time T
d. Vesicle self-reproduction is described as a process in which a growing vesicle first transforms its shape from a sphere into a budded shape of two spheres connected by a narrow neck, and then splits into two spherical daughter vesicles. We show that budded vesicle shapes can be reached only under the condition that T
d
L
pκC
0
4≥1.85. Thus, in a growing vesicle population containing vesicles of different composition, only the vesicles for which this condition is fulfilled can increase their number in a self-reproducing manner. The obtained results also suggest that at times much longer than T
d the number of vesicles with their properties near the “edge” in the system parameter space defined by the minimum value of the product T
d
L
pκC
0
4, will greatly exceed the number of any other vesicles.

A relationship between membrane properties forms the basis of a selectivity mechanism for vesicle self-reproduction

Membrane inclusions such as membrane-embedded peptides or proteins exhibit a curvature-dependent interaction with the surrounding lipid matrix due to the mismatch between their intrinsic curvature and the local membrane curvature. This interaction causes an inhomogeneous lateral distribution of the inclusions and a corresponding adjustment of the vesicle shape. We have studied theoretically the axisymmetric equilibrium shapes of lipid vesicles with mobile inclusions, taking into account that the membrane free energy includes the elastic energy of the lipid bilayer and a contribution due to an inclusion-membrane interaction. Equations describing the shape are derived by minimizing the total free energy at fixed membrane area, enclosed volume, and number of inclusions and are then solved numerically. It is shown that vesicle shape may assume a symmetry that differs from that of the vesicle with no inclusions. If the inclusion-membrane interaction exceeds a certain value, there is no axisymmetric solution of the equations with a continuous and derivable lateral density of inclusions over the whole area of the vesicle. When approaching the critical vesicle shape, the shapes obtained differ qualitatively from those described by the area difference elasticity model of the elastic properties of lipid membranes. In general, vesicle shapes adjust to the presence of inclusions by increasing regions with favorable curvature and decreasing regions of unfavorable curvature in a way such that the lateral distribution of inclusions becomes inhomogeneous.

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Bojan Božič

Papers

Vesicle deformation by an axial load: from elongated shapes to tethered vesicles.

A relationship between membrane properties forms the basis of a selectivity mechanism for vesicle self-reproduction

Coupling between vesicle shape and lateral distribution of mobile membrane inclusions.

Stability analysis of micropipette aspiration of neutrophils.

A Model of Piezo1-Based Regulation of Red Blood Cell Volume.