Container

The following functions and properties of container are required for a functional protocell

  1. The maintenance of the integrity of the protocell assembly.
  2. The promotion of the metabolic processes.
  3. The ability to grow and divide to accommodate the newly synthesized building blocks and the self-replication of the protocell as a single entity.

Two approaches to the compartmentalization can be envisioned i) the interfacial model [Fig. (4A)] where a surface, understood here in the broad sense that also encompasses interfaces between hydrophilic and hydrophobic phases represented by amphiphile bilayers, constitutes the protocell container and ii) the enclosed model [Fig. (4B)] as exemplified by amphiphile vesicles and liposomes, where the internal aqueous compartment is the actual container. The latter model clearly depicts a chemical system mimicking cellular organisation.

Figure 4: Two approaches to the protocell container: (A)the Interfacial model and (B) the Enclosed model. (A) The interfacial system self-assembles due to the properties of all its components (catalytic system components, some with a hydrophobic moiety, substrates and the amphiphiles). The compartmentalization occurs within the amphiphile structure itself, here the bilayer of a vesicle or micellar structures such as spheres, discs or rod-like systems. Substrate molecules insert into the bilayers or interact with them and upon reaction are transformed in-situ into product and waste molecules. On the bottom, ruthenium trisbypyridine interacts electrostatically with the fatty acid bilayers and can clearly be concentrated on them. (B) The catalytic system is encapsulated in the internal aqueous volume of a liposome or vesicle. Substrates (S) are added to the external medium. They must cross the membrane using, e.g., transient membrane packing defects (substrates, dotted arrow), after which they are incorporated in metabolic pathways (black arrow). In both systems, the product and waste molecules can either remain within the conatiner or be released into the external medium (dash arrows). Again, in the enclosed model, the release can only occur upon crossing of the compartment boundaries. On the bottom picture, DNA visualized with acridine orange has been partially encapsulated (white circles). Some vesicles however did not encapsulate any solute (black circle). Bar = 20 μm.

For our protocellular model, we have decided to follow the interfacial approach which has the following advantages:

  1. A true self-assembly of all components (information, metabolism and container) can be achieved provided the building blocks share common properties (see here and here). No encapsulation is required, thus it should permit to increase the number of non-functional system (see bottom picture 4B).
  2. The addition of precursor molecules ("feeding") is simplified as precursor molecules with similar properties would directly insert within the container and thus be available for the production of building blocks.
  3. The system will be more easily divided. That is, during the fragmentation of the membrane, which can be expected during division, the complete protocell machinery should remain together whereas risks exist that a machinery or part of it could be released if only encapsulated in the aqueous internal volume of a container.

There is however a drawback to this approach: the size of the container is very small and question of stability of the chemical system may arise during the metabolic cycles.

In our design, we chose fatty acid amphiphiles as container building blocks because they can be synthesized in a single reaction step (i.e., simple metabolism).