Probably the best "nano-machines" currently known are cells and their components. Therefore, the fastest route to success in many problems of nanotechnology may be inspired by biology. However, cells are very complex structures and our ability to modulate their functions is quite limited. At the other extreme, membrane-bound, cell-like structures (liposomes) are simple and easy to construct but, so far, have been shown to perform only a few isolated functions.
A different approach has been taken by a team from NASA-Ames, Harvard Medical School and University of California. The aim of this approach is to develop a general strategy for building simple, cell-like systems capable of performing functions that may or may not exist in a cell.
The centerpiece of this strategy is a method for the in vitro evolution of protein enzymes toward arbitrary catalytic targets. A similar approach has already been developed for nucleic acids: First, a very large population of candidate molecules is generated using a random synthetic approach. Next, the small number of molecules that can accomplish the desired task are selected. These molecules are next vastly multiplied using the polymerase chain reaction. A mutagenic approach, in which the sequences of selected molecules are randomly altered, can yield further improvements in performance or alterations of specificities. Unfortunately, the catalytic potential of nucleic acids is rather limited. Proteins are more catalytically capable but cannot be directly amplified. In the new technique, this problem is circumvented by covalently linking each protein of the initial, diverse, pool to the RNA sequence that codes for it. Then, selection is performed on the proteins, but the nucleic acids are replicated.
To date, we have obtained "a proof of concept" by evolving simple, novel proteins capable of selectively binding adenosine tri- phosphate (ATP). Our next goal is to create an enzyme that can phosphorylate amino acids and another to catalyze the formation of peptide bonds in the absence of nucleic acid templates. This latter reaction does not take place in contemporary cells.
Once developed, these enzymes will be encapsulated in liposomes so that they will function in a simulated cellular environment. To provide a continuous energy supply, usually needed to activate the substrates, an energy transduction complex which generates ATP from adenosine diphosphate, inorganic phosphate and light will be used. This system, consisting of two modern proteins, ATP synthase and bacteriorhodopsin, has already been built and shown to work efficiently. By coupling chemical synthesis to such a system, it will be possible to drive chemical reactions by light if only the substrates for these reactions are supplied.
We are further actively working towards simplifying the bio- energetics systems. At present, our efforts are concentrated on designing a simple, reversible, photo-induced proton pump using extensive, atomic-level computer simulations. Three different mechanisms of proton pumping have been proposed. One system that is being examined in detail is a proton channel formed by the transmembrane portion of the M2 protein of influenza virus. This system appears to be an excellent candidate for re-engineering into a proton pump.