Researchers have made self-assembling multilayer tubules with a structure curiously similar to the outermost components of bacterial cell walls. These "artificial cell walls" form when the researchers mix together two biological materials: negatively charged rods of actin (a protein involved in the contraction and expansion of muscle) and positively charged artificial versions of the membranes that form the protective coverings of cells. The resulting structures have potential applications in drug delivery systems and nanofabrication.
To the surprise of the researchers, these actin-membrane capsules spontaneously self-assembled from their constituents, and are hierarchically ordered, with different kinds of organization at different length scales. On the "mesoscopic" length scales that lie between the microscopic and macroscopic, these tubules have a ribbon-like tubule structure, with average widths of ~0.25 microns. The average lengths can be controlled, from as long as ~100 microns to short nano-capsules for possible drug delivery applications.
Within the tubules, the researchers found a remarkable structure. It consists of composite membranes with no direct analog in amphiphilic systems, structures comprised of both water-loving and water-repelling molecules.
The composite membrane is organized into three layers. A middle "phospholipid bilayer," similar to the plasma membrane which surrounds most cells, is sandwiched between two layers of actin protein rods. The actin rods within these layers are themselves arranged into close-packed, 2-dimensional "rafts." This type of spontaneous protein/membrane organization is reminiscent of multi-layered bacterial cell walls which exist far from equilibrium, and requires the hydrolysis of ATP, an important energy storage molecule in biology.
The composite membrane with laterally locked actin rods forms an anisotropic 2-dimensional crystalline sheet. The so-called "crumpling transition," which has been predicted for such anisotropic 2-dimensional crystalline objects but has never been previously observed, appears to be at least partially responsible for the tubule formation.
(Figures and caption courtesy of Gerard C.L. Wong and colleagues.)
Reported by: Gerard C.L. Wong, Jay X. Tang, Alison Lin, Youli Li, Paul A. Janmey, and Cyrus R. Safinya in Science 288, 2035-2039 (16 June 2000)