Next Life: Synthetic Cells
Theoretical concepts like the RNA World and Dyson’s garbage-bag world have inspired experimental approaches in which systems of molecules enclosed by membranes are sufficiently complex to have some of the properties of life. The ultimate goal is to assemble a cellular system that can use energy to grow through a process of catalyzed polymerization, replication of genetic information, and evolution. Several laboratories have initiated such studies, and there is reason to believe that the goal of artificial life may be achieved in the next decade. I will now recount a brief history of research on fabricating artificial cells.
Perhaps the first thing to understand is that assembling a system of molecules capable of reproducing is old news. More than 50 years ago, Heinz Fraenkel-Conrat and Robley Williams at Berkeley discovered that the tobacco mosaic virus could be separated into its coat protein and RNA. If the two components were mixed together, they reassembled into the infectious agent. More recently, in a remarkable display of modern molecular-biology methods, Jeronimo Cello, Aniko Paul, and Eckard Wimmer at the State University of New York at Stony Brook fabricated a functional polio-virus genome by stitching together hundreds of smaller fragments that were synthesized using chemical techniques. And two years ago Hamilton Smith and his colleagues at the J. Craig Venter Institute in Rockville, MD, managed to synthesize a complete genome of a small bacterial species called Mycoplasma genitalium. The uproar this caused is an indication of what will face the first claims that a living cell has been reassembled from its parts.
The synthesis of viral and bacterial genomes suggests that even more-challenging fabrications may be possible. We have known for years that spontaneous self-assembly processes can produce surprisingly complex systems of functional molecules. Efraim Racker, working at Cornell University, pioneered the effort to dissect and reconstitute mitochondrial membranes in the 1970s. Mitochondria are subcellular organelles that are present in most cells, and embedded in their membranes are enzymes that remove electrons from metabolic products derived from nutrients such as glucose. The process is called electron transport, because the electrons then pass through a chain of enzymes in the mitochondrial membrane and are delivered to oxygen. The electron transport is tightly coupled to a second transport process, in which positively charged protons derived from water are pumped outward, producing an electrical potential of approximately 0.2 volts across the membrane. This voltage provides the energy source for the synthesis of adenosine triphosphate (ATP), which transports chemical energy within cells and therefore drives most life processes. The universal mechanism by which ATP is synthesized, now referred to as chemiosmosis, was proposed in 1961 by Peter Mitchell, a remarkable British scientist who later carried out research in his home in Bodmin, Cornwall.
Racker and his students dissolved mitochondrial membranes with a detergent called deoxycholic acid. One of his first discoveries was that the membranes contained an enzyme that coupled ATP synthesis to electron transport. He referred to this as a coupling factor, but it is now called an ATP synthase. Racker also found that the detergent could be removed by dialysis–simply by placing the clear solution in a bag composed of a material resembling cellophane and letting it sit overnight in a dilute salt solution. The small detergent molecules leaked out of the bag, but larger molecules could not get through the porous material. The next day the solution was turbid, because membranous vesicles containing the original protein components had reassembled. The vesicles were fully capable of electron transport reactions and ATP synthesis. It was the first reconstitution of a very complex biological function.
At about the same time, Walther Stoeckenius at the University of California, San Francisco, became curious about the pigmented membranes of a bacterial species called Halobacterium halobium, which lives in extremely salty water. Stoeckenius and Dieter Oesterhelt were able to isolate the purple pigment–bacteriorhodopsin–and found that its function was to absorb light energy and use the energy to transport protons across the bacterial membrane. The energy of the proton gradient was then used to synthesize ATP. Racker and Stoeckenius, both members of the National Academy of Sciences, then initiated a rare collaboration between two senior scientists. They used Racker’s dialysis method to reconstitute a system of membranous vesicles containing only the proton pump of purple membranes and the ATP synthase of mitochondria. In 1974, they reported that the hybrid vesicles could use light as an energy source to synthesize ATP. Their paper added to the weight of evidence that finally confirmed chemiosmotic synthesis of ATP, for which Peter Mitchell was awarded the Nobel Prize in 1978.