The development of vesicles able to bind specifically took considerable more time than expected. During this time we developed experimental alternatives to explore self-assembling mechanisms. Instead of vesicles we used different types of beads with different sizes (250 nm, 1 μm, 100 μm) coated either with biotin or streptavidin or with ssDNA. These beads were used to test self-assembly concepts as well as a different type of linking molecules for vesicles in case the specific DNA-tagging would fail. One of the difficulties is to control the unspecific adhesion of membrane-coated vesicles to surfaces. DNA-coated beads as well as vesicles bind to surfaces and it is hard to find coating which inhibit this unspecific binding, but does affect neither the vesicles themselves nor the process of bead-bead- and/or bead-vesicle-assembly. One of the promising approaches was to use giant unilamellar vesicles (GUVs) as a self-assembly chamber, because neither beads nor vesicles bind spontaneously to them. The vesicular production methods developed during this project make it easy to produce vesicles with substances differing from the surrounding medium, beads, and/or smaller vesicles in their lumen as illustrated above.
Giant vesicles as self-assembly chambers for smaller units such as vesicles and beads. Here the examples shows two different beads and a smaller vesicles inside two different giant vesicle.
Example of a self-assembled structure of beads based on specific DNA-DNA-hybridization. Green:1 μ m beads coated with 15 mer sense ssDNA. Red: 250 nm bead coated with 15 mer antisense ssDNA.
The experimental results achieved in the PACE-consortium up to the present state give us the opportunity to interlace, anchor and explore theoretical IT-concepts directly with and in experimental work. Thereby, the IT-potential of multi-vesicular compounds can be exploited over a smooth range of increasing complexity. The information processing results from the interplay of only a few types of basic processes:
a) self-assembly
b) fusion and/or material exchange between adjacent vesicles
c) chemical reactions in vesicles and
d) material uptake.
The basic idea is to implement these processes in a manner susceptible to external conditions and/or signals. Applications start at vesicles of different types assembling in patterns governed by external conditions. An intermediate level of sophistication is reached by self-assembled compounds of vesicles of different types and contents, where the assembly is controlled by specific surface interactions between the cells. A next step could be reached by controlled fusion of smaller, but specific vesicles with larger ones. By preparing vesicles with different surface properties as well as different contents, the controlled fusion allows to produce functionally differentiated vesicles. Furthermore, all components, which the current state of the artificial cells is not yet able to reproduce, could be replenished by controlling the fusion externally. This technique would allow exploring the potential of the artificial cell without having a fully self-contained one and would allow testing the IT potential experimentally. A concrete example of a working scenario would be the creation of a "chemical robot" in which functionally differentiated vesicles are seen as the robot's building blocks specialized for structuring, sensing and moving. At the far end of this route stand fully metabolizing systems, whereby the metabolism lead to replication.