Science 1663

A Lifelike Machine

A unique approach to making nanomachines that mimic living cells is beginning to bear fruit. It could usher in an age of incredibly useful technologies.

Imagine a computer chip that, when damaged, "heals itself" by replicating and making new, undamaged copies of itself. Or a swarm of microbe-sized machines that dissolve rocks as part of their "metabolism" and, in the process, remove planet-warming carbon dioxide from the atmosphere. Or a nanoscale device that extracts plutonium from the environment and evolves to become even more efficient at doing the task. Science fiction? Maybe.

Or maybe not. Although the ability to self-replicate, metabolize, and evolve into new forms is considered the defining characteristic of living cells, fundamentally there are no reasons a human-made "machine" couldn't have those abilities as well. Indeed, the Department of Commerce, the National Science Foundation, and the European Commission already predict that so-called "convergent, or living, technologies" will have a large impact on our technology and economy within the next 25 years and as such will become key to our technological leadership and national security.

The current version of the Los Alamos protocell consists of a "sensitizer" molecule (orange oval) and an information molecule (gold circle) within a membrane-like container. The two molecules work together, along with a consumable resource (black diamond) to convert nutrients (blue box with squiggly tail) into container material.

Now, collaborating teams of scientists from Los Alamos National Laboratory and across Europe are close to developing the "protocell," a system of molecules that exhibits living cell-like behavior. The microscopic protocell would be the first step toward making lifelike devices that perform specific, useful functions.

"Our goal is not to modify existing living cells and turn them into little machines," says Steen Rasmussen, leader of the Los Alamos Protocell Assembly project. "Our goal is to take those features that make living cells so successful and apply them to something new."

"Because we're starting from scratch," adds Rasmussen, "we can design our protocell to do things that living cells cannot. In theory, we can make it so different that it can operate in any environment—toxic, radioactive, or otherwise. Protocells can also be designed so they don't interact directly with the biosphere, which would make them less controversial to use than genetically modified cells."

The Protocell Different

Taking its cues from living cells, the simple protocell has just three components:

• A metabolism (the chemical processes used to obtain energy and create the protocell's building blocks).
• An information system (which instructs the metabolism). Like DNA, the molecules that make up the information system have the ability to copy themselves.
• A container (which keeps everything together).

These components interact strongly with each other—the information system participates in the metabolism, while parts of the metabolism produce copies of the information system and more container material.

Steen Rasmussen (right) and Hans Ziock are part of a team of some 23 researchers in the Los Alamos Protocell Assembly project. Team members are listed at www.lanl.gov/science/1663

The Los Alamos team has already built a pared-down version of a protocell. While not yet able to replicate all of itself, it has nonetheless achieved the team's major milestone: it used its information system to control a metabolic pathway that converted external resources into container material. This machine built its own container!

This early form of protocell is strikingly simple. Its information system (in its most rudimentary form) is actually a single molecule—a modified DNA base. The information molecule works with a second molecule, known as the "sensitizer," to carry out the metabolic pathway (see "The Protocell's Metabolism").

"In a living cell, the DNA creates proteins that control the metabolism's chemical reactions," says Hans Ziock, a Los Alamos physicist and member of the protocell project. "In our protocell, the information molecule participates directly in the metabolic pathway, so no protein synthesis is necessary. It's a great simplification."

Ziock continues, "Our simplistic information molecule nonetheless captures the essential features of an information system. It makes the reaction work and, in that sense, controls or instructs the metabolism. Though it's a modified DNA base, it can still pair with another base and, in principle, copy itself the way DNA does. Lastly, it can join with other bases and become more complex. That might allow for more-efficient metabolic pathways, thereby opening up a way for the protocell to evolve."

Building a Container

But how did the protocell build its own container? It did so by exploiting the wonderful properties of fatty acids. Fatty acid molecules have large-diameter "heads" that are comfortable in water and long, narrow "tails" that have the characteristics of an oil molecule. The tails dislike being in water.

In a watery environment, fatty acids will try to get their tails out of water by spontaneously forming aggregate structures—in this case bi-layered membranes that have curled up into cell-like vesicles, wherein the membrane encloses a pocket of water. The bi-layer structure satisfies the fatty acid's dual preferences: the water-loving heads face toward and touch water, while the oily tails face each other and are shielded from water. (See the illustration on the previous page.) The tail arrangement creates an oil-like environment in the bi-layer's interior.

These spheroidal vesicles—with their (bright) bi-layer membrane enclosing a (dark) pocket of water—were created as a result of the protocell's metabolism. Each vesicle is a potential new protocell.

This bi-layer is the protocell's container. Unlike a living cell, where the vast majority of the metabolic and genetic processes occur in the watery volume enclosed by the cell membrane, the Los Alamos protocell conducts all of its business within the oily part of the bi-layer or at its surface. This makes it possible for the protocell to freely exchange nutrients and wastes with the environment and eliminates the complex process used by a cell to transport resources through its membrane.

To build the container, researchers mix sensitizer molecules and informational molecules together with chemical nutrients in a water-based soup. The mixture is then exposed to a bright light.

The sensitizer absorbs the light energy and, together with the information molecule, catalyzes the breakup of a nutrient into a fatty acid and a waste product. After some time, enough fatty acids have been produced to spontaneously form a bi-layer. At that point, the metabolism has built the protocell's container!

The process continues, however. A part of the sensitizer and information molecule prefers to reside in an oily environment, so when they bump into the container, they tend to "stick" to it. Likewise, the nutrient is designed to be oil-loving, so it too will associate with the container. With a ready supply of resources, the protocell—a sensitizer and information molecule in a container—produces more fatty acids and grows. But when a fatty-acid aggregate grows too large, it splits in two, that is, it replicates.

"On paper, this pared-down protocell has all the features of a complete system" notes Ziock. "However, we still need to demonstrate experimentally that the protocell's information molecule can replicate. This is the critical missing link for completing the first human-made, fully functional protocell."

Multiscale Modeling

Results from a computer simulation showing an information molecule “sticking” to the protocell’s container.

To help understand and build a more complex system, the team has developed a computational version of all key protocell processes, combining different simulation methods to provide different levels of detail.

The computations have already helped answer what had been an open question about the protocell's growth process. If the container, sensitizer, and information molecule are produced at different rates, will one component be produced far in excess of the others? Theoretical and computational work indicates the answer is "no": the protocell's three components automatically regulate each other's duplication rate, thereby achieving coordinated growth. This is one of the cleverer features of the protocell's design.

Still, it has been surprisingly difficult to simulate the protocell's full life cycle, as there are numerous cross interactions between the molecules. The complexity of the system initially made its behavior difficult to predict.

"Those times when our simulations differ from our experiments are nonetheless the times when we learn the most," observes Rasmussen. "We then have to revise our thinking and change our approach."

Vision

The fruit of all its labor is the Los Alamos team's greater understanding of how to make self-replicating materials, which is the first step toward creating living technology that will benefit society in fundamental ways.

"When your computer crashes or when your car breaks down, somebody has to repair it," says Rasmussen. "In contrast, when you scratch your hand, it heals itself. I predict that, in the future, this distinction between nonliving and living systems will slowly disappear as our technology becomes more and more lifelike."