Model protocells

Fig. 10 Model protocell systems, a An encapsulated polymerase (polynucleotide phos-phorylase) can synthesize RNA from nucleoside diphosphates such as ADP [79,80], b RNA can be synthesized by a template-dependent T7 RNA polymerase [83], c Proteins such as green fluorescent protein (GFP) can be synthesized by an encapsulated translation system [84], If mRNA coding for hemolysin is also present, the hemolysin forms a pore in the lipid bilayer. Amino acids then permeate the bilayer, and protein synthesis can continue for several days [85]...

Zhu TF, Szostak JW (2009) Coupled growth and division of model protocell membranes. J Am Chem Soc 131 5705-5715... 

Chen lA, Roberts RW, Szostak JW. The emergence of competition between model protocells. Science 2004 305 1474-1476. 

Of most note, spherules made from proteinoid and lecithin have been recently shown to possess electrical membrane phenomena. Membrane, action, and oscillatory potentials were next recorded in the microspheres made from proteinoid only. While many of the functions of the model protocells are much weaker than the corresponding functions in modern cells, e.g., the catalytic functions, the electrical behavior is quantitatively comparable. Consequently the units are models of modern excitable cells, as well as of excitable protocells, and are treated mainly as such in this chapter. 

The formation of the encapsulating membranes is discussed by Turk-MacLeod et al. the operative strictures of thermodynamics in these processes and in the functional role of cell membranes are elaborated. The competition between vesicles that encapsulate RNA and those incapable of doing so, considered as model protocells, and its relation to the evolutionary fitness of replicator functions, is considered at length in terms of the driving forces of thermodynamics. It is noted that membrane stabilization is a key objective in this competition but this results also in a reduction of permeability, thus diminishing the ability of the protocell to use nutrients. Further evolution of the membrane and its constituents is necessary to overcome this restriction in function. In this respect it is of interest that model protocell membranes composed of particular mixtures of amphiphiles have superior... 

Mansy, S.S., Schrum, J.P., Krishnamurthy, M., Tobe, S., Treco, D.A., Szostak, J.W. Template-directed synthesis of a genetic polymer in a model protocell. Nature 454, 122-125... 

Mansy, S.S., Szostak, J.W. Reconstructing the emergence of cellular life through the synthesis of model protocells. Cold Spring Harb. Symp. Quant. Biol. 74, 47-54 (2009)... 

Kurihara, K., Okura, Y., Matsuo, M., Toyota, T., Suzuki, K., Sugawara, T. A recursive vesicle-based model protocell with a primitive model cell cycle. Nat. Common. 6, 8352 (2015). doi 10.1038/ncomms9352... 

Mavelli, F., Ruiz-Mirazo, K. Theoretical conditions for the stationary reproduction of model protocells. Integr. Biol. 5, 324—341 (2013)... 

A second historical model for protocells is provided by the microspheres (Fox, 1980 Nakashima, 1987 Lehninger, 1975). These are formed when hot saturated proteinoid solutions are allowed to cool (see Sect. 5.4.2). In recent years, the microspheres were also consigned to the limbo of unimportant scientific models. Perhaps there will come a time when coacervates or microspheres (in their original or in modified forms) find their way back into the scientific discussion. 

After more than 20 years, Walde et al. (1994) returned in a way to coacervate experiments, although using other methods. Walde (from the Luisi group) repeated nucleotide polymerisation of ADP to give polyadenylic acid, catalysed by polynucleotide phosphorylase (PNPase). But instead of Oparin s coacervates, the Zurich group used micelles and self-forming vesicles. They were able to demonstrate that enzyme-catalysed reactions can take place in these molecular structures, which can thus serve as protocell models. Two different supramolecular systems were used ... 

The favourable properties which mark out vesicles as protocell models were confirmed by computer simulation (Pohorill and Wilson, 1995). These researchers studied the molecular dynamics of simple membrane/water boundary layers the bilayer surface fluctuated in time and space. The model membrane consisted of glycerine-1-monooleate defects were present which allowed ion transport to occur, whereby negative ions passed through the bilayer more easily than positive ions. The membrane-water boundary layer should be particularly suited to reactions which are accelerated by heterogeneous catalysis. Thus, the authors believe that these vesicles fulfil almost all the conditions required for the first protocells on earth ... 

The formation of relatively stable vesicles did not require the presence of pure compounds mixtures of components could also have done the job. However, whether the concentrations of the compounds isolated from the Murchison meteorite would have been sufficient for the formation of prebiotic protocells or vesicles is unclear, even if concentration effects are assumed. Sequences in which the technical Fischer-Tropsch synthesis is the role model have been proposed as possible sources of amphiphilic building blocks. 

Attempts have recently been made to link the RNA world with the lipid world. Two groups involved in RNA and ribozyme research joined up with an expert on membrane biophysics (Szostak et al., 2001). They developed a model for the formation of the first protocells which takes into account both the most recent experimental results on replication systems and the self-organisation processes of amphiphilic substances to give supramolecular structures. 

D. W. Deamer and J. P. Dworkin have reported in detail on the contribution of chemistry and physics to the formation of the first primitive membranes during the emergence of precursors to life the authors discussion ranges from sources of amphiphilic compounds, growth processes in protocells, self-organisation mechanisms in mixtures of prebiotic organic compounds (e.g., from extracts of the Murchison meteorite) all the way to model systems for primitive cells (Deamer and Dworkin, 2005). 

From Simple Amphiphiles to Protocell Models Volume Editor Walde, P. 

A critical property of minimum protocells in the prebiotic environment would be their ability to sequester other molecules, including macromolecules. [142] In 1982, Deamer and Barchfeld [143] subjected phospholipid vesides to dehydration-rehydration cycles in the presence of either monomeric 6-carboxyfluorescein molecules or polymeric salmon sperm DNA molecules as extraneous solutes. The experiment modeled a prebiotic tidal pool containing dilute dispersions of phospholipids in the presence of external solutes, with the dehydration-rehydration cydes representing episodic dry and wet eras. They found that the vesides formed after rehydration... 

It is likely that early cells were more permissive, and perhaps an early step in the transition to life is the transition from permeable, simple protocells, to hard and impermeable structures, like our present POPC liposomes. In fact, the common stand of chemists to work with pure compounds may not be the best to model prebi-otic systems. In aprebiofic scenario, most probably, mixtures of several surfactants and CO surfactants were dominating the scene. It is known that the permeability of vesicles increases when co surfactants - like long-chain alcohols - are added. This observation about the importance of mixtures would in principle open the way to a vast area of research (see Sidebox 7.1). 

Proteinoids, as a model of primitive abiotic proteins >, are formed by polymerization from protobiologically plausible micromolecules (amino acids) under presumed protobiological conditions. Proteinoids have enzyme-like activities and metabolic qualities. Proteinoid microspheres are the most suitable model for protocells since they do not consist of macromolecules extracted from contemporary organisms. 

Vesicle and micelles are considered to be useful models for minimum protocells that had emerged in prebiotic times [200]. One of their properties should have been to sequester other molecules, including macromolecules, for self-replication. A central enigma to be addressed is related to various routes by which the enantiopure homochiral biopolymers were formed within such architectures. Polymerization of NCA of natural hydrophobic amino acids in water in the presence of phospholipids by Luisi et al. [201] has demonstrated that the hydrophobic environment enhances their rate of polymerization. 

Fig. 9 One of the vesicle models (as depicted by the SCM). Different templates (labelled by open and closed circles) contribute to the well being of the compartments (protocells) in that they catalyse steps of metabolism, for example. During protocell growth (-- ) templates replicate at differential expected rates, but stochastically. Upon division (- ) there is chance assortment of templates into offspring compartments. Stochastic replication and reassortment generate variation among protocells, on which natural selection at the compartment level can act and oppose to (correct) internal deterioration due to within-cell competition...

The consequences of imperfect replication in vesicle models are somewhat puzzling [85,90]. For small mutation rates an increased level of polyploidy favours the persistence of protocell lineages since the random loss of essential genes after fission is attenuated. However, for large mutation rates the situation is reversed, resulting in that those lineages with low levels of polyploidy... 

We have witnessed spectacular development revolving around the minimal life/protocell idea in the last few years, although the roots of this vision go back to more than thirty years ago. A fairly unusual interplay between theory and experiment seems to unfold theoretical models investigate possible dynamics, and experiments either confirm them or come up with surprising, unexpected novelties, but increasingly often reflecting back on theoretical considerations. 

Vaults, composed of proteins and discussed above, are able to encapsulate mRNA, RNA-binding proteins and some transferases, in short almost everything a primitive self-replicating system would require, so could represent an interesting model for a protocell. It is a particular attractive proposition as it only requires a single protein type that can self-assemble to form the entire structure. 

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