The Final Step(s)?

In a 2005 paper in Trends in Biotechnology, David Deamer presented 12 requirements to creating a protocell capitulates life. Then have ben accomplished in different labs and in different ways. Because researchers have used RNA as both a macromolecular catalyst and as an information store, some of the boundaries between objectives become blurred. That's one of the reasons that Deamer says once the 11th requirement is met and macromolecular catalysts are reproduced in the protocell during growth, the final hurdle, in which duplicating genetic information is passed between generations, will already have been achieved. Credit: Illustration by Scott Bodell"> Credit: Illustration by Scott Bodell 1. MEMBRANE ENCLOSURE In 1965 Alec Bangham and colleagues at the Agricultural Research Council Institute of Animal Physiology in Cambridge, England, describe research showing that amphiphilic molecules can self-assemble into microscopic sacs sharing some properties of cell membranes.1 2. ENERGY CAPTURE BY MEMBRANE Efraim Racker and Walther Stoeckenius of Cornell University in Ithica, NY, incorporated bacteriorhodopsins and an ATPase into a liposome membrane to generate ATP from light.2 3. ION CONCENTRATIONS MAINTAINED ACROSS MEMBRANES Several previous attempts had demonstrated liposomes capable of maintaining proton gradients. In 2004 Irene Chen and Jack Szostak of Massachusetts General Hospital found membrane growth could generate gradients lasting several hours.3 4. MACROMOLECULES ENCAPSULATED IN THE COMPARTMENTS In 1985, David Deamer and Roberta Shew of the University of California, Davis, report that macromolecules can easily be captured in lipid vesicles by mixing the vesicles with the molecules, then drying and rehydrating the mixture.4 5. THE MACROMOLECULES GROW BY POLYMERIZATION In 1994, Deamer and colleagues synthesize RNA in lipid vesicles by using a an RNA polymerase to string together molecules of adenosine diphosphate (ADP) that had been encapsulated within the compartments.5 6. MACROMOLECULAR CATALYSTS EVOLVE THAT SPEED THE GROWTH PROCESS Though it hasn't been demonstrated inside a liposome, several groups have built in vitro evolution systems in which successive generations of RNA molecules evolve to more efficiently catalyze reactions such as ligating other RNAs.6 7. INFORMATION IS CAPTURED IN THE SEQUENCE OF MONOMERS IN ONE SET OF POLYMERS Several groups have captured DNA molecules within liposomes to direct the production of proteins, including Green Fluorescent Protein, channel proteins, and polymerases, through transcription and translation.7 8. THE INFORMATION IS USED TO DIRECT THE GROWTH OF CATALYTIC POLYMERS In 2004, researchers at Osaka University, Japan, create a membrane-bound system containing a two-stage genetic cascade. DNA directs production of an RNA polymerase, which is required to produce Green Fluorescent Protein.8 9. THE MEMBRANE-BOUND SYSTEM CAN DIVIDE INTO SMALLER STRUCTURES WHICH CONTINUE TO GROW In 2003, researchers at Massachusetts General Hospital report that fatty acid vesicles grow by taking up additional available fatty acids, and divide when extruded through pores.9 10. OCCASIONAL MISTAKES (MUTATIONS) ARE MADE DURING REPLICATION OR TRANSMISSION OF INFORMATION SO THE SYSTEM CAN EVOLVE In 2001, David Bartel of the Massachusetts Institute of Technology and collagues report developing an RNA enzyme, or ribozyme, that catalyzes the replication of an RNA template molecule, making rare errors along the way.10 REFERENCES 1. A.D. Bangham et al, "Diffusion of univalent ions across the lamellae of swollen phosopholipids," J Mol Biol, 13:238-52, 1965. 2. E Racker, W Stoeckenius "Reconstitution of purple membrane vesicles catalyzing light-driven proton uptake and adenosine triphosphate formation," J Biol Chem, 249:662-3, 1974. 3. I.A. Chen, J.W. Szostak, Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles, Proc Natl Acad Sci, 101:7965-70, 2004. 4. R.L. Shew, D.W. Deamer "A novel method for encapsulation of macromolecules in liposomes," Biochim Biophys Acta, 816:1-8, 1985. 5. A.C. Chakrabarti et al., "Production of RNA by a polymerase protein encapsulated within phospholipid vesicles," J Mol Evol, 39:555-9, 1994. 6. M.C. Wright, G.F. Joyce "Continuous in vitro evolution of catalytic function," Science, 276:614-7, 1997. 7. V. Noireaux, A. Libchaber "A vesicle bioreactor as a step toward an artificial cell assembly," Proc Natl Acad Sci, 101:17669-74, 2004. 8. K. Ishikawa et al., "Expression of a cascading genetic network within liposomes," FEBS Lett, 576:387-90, 2004. 9. M.M. Hanczyc et al., "Experimental models of primitive cellular compartments: encapsulation, growth and division," Science, 302:618-22, 2003. 10. W.K. Johnston et al., "RNA-catalyzed RNA polymerization: accurate and general RNA-templated primer extension," Science, 292:1319-25, 2001.

The Final Step(s)?

The Scientist Staff

1. MEMBRANE ENCLOSURE

2. ENERGY CAPTURE BY MEMBRANE

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