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Protocells: at the interface of life and non-life.

Ma W, Feng Y - Life (Basel) (2015)

Bottom Line: Then, the ability to synthesize membrane components (amphiphiles) may have emerged under selective pressure, leading to "true-protocells".Such "unitary-protocells", containing a central genetic molecule, may have appeared as a milestone-in principle, since then life could evolve endlessly, "gaining" more and more functions by introducing new genes.To synthesize in laboratory these different types of protocells, which stand at the interface between life and non-life, would greatly enhance our understanding on the essence of life.

View Article: PubMed Central - PubMed

Affiliation: College of Life Sciences, Wuhan University, Wuhan 430072, China. mwt@whu.edu.cn.

ABSTRACT
The cellular form, manifesting as a membrane-bounded system (comprising various functional molecules), is essential to life. The ultimate reason for this is that, typically, one functional molecule can only adopt one "correct" structure to perform one special function (e.g., an enzyme), and thus molecular cooperation is inevitable. While this is particularly true for advanced life with complex functions, it should have already been true for life at its outset with only limited functions, which entailed some sort of primitive cellular form-"protocells". At the very beginning, the protocells may have even been unable to intervene in the growth of their own membrane, which can be called "pseudo-protocells". Then, the ability to synthesize membrane components (amphiphiles) may have emerged under selective pressure, leading to "true-protocells". The emergence of a "chromosome" (with genes linked together)-thus avoiding "gene-loss" during the protocell division, was another key event in the evolution of protocells. Such "unitary-protocells", containing a central genetic molecule, may have appeared as a milestone-in principle, since then life could evolve endlessly, "gaining" more and more functions by introducing new genes. To synthesize in laboratory these different types of protocells, which stand at the interface between life and non-life, would greatly enhance our understanding on the essence of life.

No MeSH data available.


A scheme to exemplify the different stages of the RNA-based protocells. (a) Pseudo-protocell, the protocell lacking the capability to synthesize its own membrane; (b) True-protocell, the protocell processing a membrane synthesis function; (c) Unitary-protocell, the protocell holding a primordial RNA chromosome as the central genetic molecule and thus acting integrally as a unit for natural selection. Rm = raw material; Nt = nucleotide; Am = amphiphile; Rep = RNA replicase; Nsr = nucleotide synthetase ribozyme; Npsr = nucleotide precursor synthetase ribozyme; Asr = amphiphile synthetase ribozyme; Chr = chromosome. The arrow means that the amphiphiles may join the membrane of the protocell. Note: Nsr and Npsr represent different ribozymes that participate in different nucleotide synthesis steps.
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life-05-00447-f001: A scheme to exemplify the different stages of the RNA-based protocells. (a) Pseudo-protocell, the protocell lacking the capability to synthesize its own membrane; (b) True-protocell, the protocell processing a membrane synthesis function; (c) Unitary-protocell, the protocell holding a primordial RNA chromosome as the central genetic molecule and thus acting integrally as a unit for natural selection. Rm = raw material; Nt = nucleotide; Am = amphiphile; Rep = RNA replicase; Nsr = nucleotide synthetase ribozyme; Npsr = nucleotide precursor synthetase ribozyme; Asr = amphiphile synthetase ribozyme; Chr = chromosome. The arrow means that the amphiphiles may join the membrane of the protocell. Note: Nsr and Npsr represent different ribozymes that participate in different nucleotide synthesis steps.

Mentions: The pseudo-protocells might have developed to a more complex level than containing only an RNA replicase, e.g., incorporating a ribozyme catalyzing the synthesis of nucleotides (the nucleotide synthetase ribozyme) [53] (Figure 1A). However, the emergence of a membrane synthesis function should already have been “in urgent need” by then because if only abiotic pathways were relied on, the shortage of membrane components would seriously limit the proliferation and spread of the protocells. Could natural selection have worked to bring about such a function? Indeed, a relevant mechanism has been suggested [54]. An amphiphile synthetase ribozyme could favor membrane growth (note that the membrane would grow by naturally incorporating new amphiphilic components), and thus bring about an increase in cellular space under the membrane tension caused by the osmotic pressure mentioned above [52]. Raw materials, e.g., nucleotide precursors, in the protocell would then be diluted, leading to a further influx of the raw materials (owing to the concentration equilibrium flanking the membrane) and thus favoring the synthesis of RNA inside the protocell. Instead of being triggered by core-growth as in the pseudo-protocells, in this type of core-membrane coupling, the membrane growth brings about core growth. That is, some sort of amphiphile synthetase ribozyme could have arisen under Darwinian selection—the protocells possessing the ribozyme were superior to those without it on account of the availability of those raw materials. This hypothesis is supported by subsequent computer simulations—an amphiphile synthetase ribozyme, along with the protocells containing this ribozyme, could proliferate and spread in the model system [53,54]. In contrast to “pseudo-protocells”, we call these protocells, already possessing a membrane synthesis function, “true-protocells” (Figure 1B) [53].


Protocells: at the interface of life and non-life.

Ma W, Feng Y - Life (Basel) (2015)

A scheme to exemplify the different stages of the RNA-based protocells. (a) Pseudo-protocell, the protocell lacking the capability to synthesize its own membrane; (b) True-protocell, the protocell processing a membrane synthesis function; (c) Unitary-protocell, the protocell holding a primordial RNA chromosome as the central genetic molecule and thus acting integrally as a unit for natural selection. Rm = raw material; Nt = nucleotide; Am = amphiphile; Rep = RNA replicase; Nsr = nucleotide synthetase ribozyme; Npsr = nucleotide precursor synthetase ribozyme; Asr = amphiphile synthetase ribozyme; Chr = chromosome. The arrow means that the amphiphiles may join the membrane of the protocell. Note: Nsr and Npsr represent different ribozymes that participate in different nucleotide synthesis steps.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4390862&req=5

life-05-00447-f001: A scheme to exemplify the different stages of the RNA-based protocells. (a) Pseudo-protocell, the protocell lacking the capability to synthesize its own membrane; (b) True-protocell, the protocell processing a membrane synthesis function; (c) Unitary-protocell, the protocell holding a primordial RNA chromosome as the central genetic molecule and thus acting integrally as a unit for natural selection. Rm = raw material; Nt = nucleotide; Am = amphiphile; Rep = RNA replicase; Nsr = nucleotide synthetase ribozyme; Npsr = nucleotide precursor synthetase ribozyme; Asr = amphiphile synthetase ribozyme; Chr = chromosome. The arrow means that the amphiphiles may join the membrane of the protocell. Note: Nsr and Npsr represent different ribozymes that participate in different nucleotide synthesis steps.
Mentions: The pseudo-protocells might have developed to a more complex level than containing only an RNA replicase, e.g., incorporating a ribozyme catalyzing the synthesis of nucleotides (the nucleotide synthetase ribozyme) [53] (Figure 1A). However, the emergence of a membrane synthesis function should already have been “in urgent need” by then because if only abiotic pathways were relied on, the shortage of membrane components would seriously limit the proliferation and spread of the protocells. Could natural selection have worked to bring about such a function? Indeed, a relevant mechanism has been suggested [54]. An amphiphile synthetase ribozyme could favor membrane growth (note that the membrane would grow by naturally incorporating new amphiphilic components), and thus bring about an increase in cellular space under the membrane tension caused by the osmotic pressure mentioned above [52]. Raw materials, e.g., nucleotide precursors, in the protocell would then be diluted, leading to a further influx of the raw materials (owing to the concentration equilibrium flanking the membrane) and thus favoring the synthesis of RNA inside the protocell. Instead of being triggered by core-growth as in the pseudo-protocells, in this type of core-membrane coupling, the membrane growth brings about core growth. That is, some sort of amphiphile synthetase ribozyme could have arisen under Darwinian selection—the protocells possessing the ribozyme were superior to those without it on account of the availability of those raw materials. This hypothesis is supported by subsequent computer simulations—an amphiphile synthetase ribozyme, along with the protocells containing this ribozyme, could proliferate and spread in the model system [53,54]. In contrast to “pseudo-protocells”, we call these protocells, already possessing a membrane synthesis function, “true-protocells” (Figure 1B) [53].

Bottom Line: Then, the ability to synthesize membrane components (amphiphiles) may have emerged under selective pressure, leading to "true-protocells".Such "unitary-protocells", containing a central genetic molecule, may have appeared as a milestone-in principle, since then life could evolve endlessly, "gaining" more and more functions by introducing new genes.To synthesize in laboratory these different types of protocells, which stand at the interface between life and non-life, would greatly enhance our understanding on the essence of life.

View Article: PubMed Central - PubMed

Affiliation: College of Life Sciences, Wuhan University, Wuhan 430072, China. mwt@whu.edu.cn.

ABSTRACT
The cellular form, manifesting as a membrane-bounded system (comprising various functional molecules), is essential to life. The ultimate reason for this is that, typically, one functional molecule can only adopt one "correct" structure to perform one special function (e.g., an enzyme), and thus molecular cooperation is inevitable. While this is particularly true for advanced life with complex functions, it should have already been true for life at its outset with only limited functions, which entailed some sort of primitive cellular form-"protocells". At the very beginning, the protocells may have even been unable to intervene in the growth of their own membrane, which can be called "pseudo-protocells". Then, the ability to synthesize membrane components (amphiphiles) may have emerged under selective pressure, leading to "true-protocells". The emergence of a "chromosome" (with genes linked together)-thus avoiding "gene-loss" during the protocell division, was another key event in the evolution of protocells. Such "unitary-protocells", containing a central genetic molecule, may have appeared as a milestone-in principle, since then life could evolve endlessly, "gaining" more and more functions by introducing new genes. To synthesize in laboratory these different types of protocells, which stand at the interface between life and non-life, would greatly enhance our understanding on the essence of life.

No MeSH data available.