Self-Assembled Nano-Capsules Resemble Bacterial Cell Walls

"The 'artificial bacteria' consist of the biopolymer actin attached to charged lipid membranes," said Gerard Wong, a professor of materials science and of physics at the University of Illinois. Wong was the lead author of a paper published in the June 16 issue of the journal Science that described the assembly process. "The resulting actin-membrane capsules are fairly rigid and quite stable," he said.

Actin is a protein that provides the structural framework in eukaryotic cells and forms the molecular basis for biological functions as diverse as hearing, muscle contraction and cell division. When mixed with special liposomes, the researchers found, actin immediately undergoes a hierarchical self-assembly process that results in the formation of nano-scale tubular capsules.

"The composite membrane is organized into three layers," Wong said. "A middle lipid layer, similar to the plasma membrane that surrounds most cells, is sandwiched between two layers of actin filaments. The filaments first arrange themselves into two-dimensional parallel arrays on both sides of the lipid layer, and then they spontaneously wrap into tubules."

The spaces between the layers are filled with water and could be used for a variety of applications, including the packaging of certain drugs that would be slowly released to the body. The nano-capsules also could be used as miniature templates in novel, inorganic nanofabrication techniques.

While the structures have no real analog in nature, this type of spontaneous organization is reminiscent of multi-layered bacterial cell walls, Wong said. The protein-membrane interactions in living cell walls, however, exist far from equilibrium and require the hydrolysis of ATP (adenosine triphosphate) -- the biological energy storage molecule ­ to maintain their shape and structure.

"And that's a crucial difference," Wong said. "Our actin-membrane capsules are in equilibrium and do not require energy to remain stable."

The work was performed while Wong was a postdoctoral associate at the University of California at Santa Barbara. Wong's collaborators in the study were materials science professor Cyrus Safinya, staff scientist Youli Li and graduate student Alison Lin at the University of California at Santa Barbara, and physiology professor Paul Janmey (now at the University of Pennsylvania) and postdoctoral researcher Jay Tang at Harvard University.

Funding was provided by the National Institutes of Health, the National Science Foundation and the University of California Biotechnology Research and Education Program.