Biology Reference
In-Depth Information
glycerol-3-phosphate and acyl-coenzyme As. To this end, it was necessary to synthesize the
two enzymes that catalyze this conversion. The first enzyme (the glycerol-3-phosphate
acyltransferase) is an integral membrane protein that could be functionally synthesized
only by a careful design of the liposome membrane (prepared from 1-palmitoyl-2-
oleoyl-sn-glycero-3-phosphatidylcholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-
phosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol
(POPG), and cardiolipin). The second enzyme (the lysophosphatidic acid acyltransferase) is
a peripheral membrane protein and was synthesized. Both results were functional
(catalytically active), but required different redox conditions for assuming the proper
conformation so that their simultaneous synthesis inside the same lipid vesicle was not
possible. The stepwise activation of the two enzymes (in two different populations of
vesicles), however, brought about the production of phosphatidic acid from the precursors.
Due to the low yield, however, the growth of vesicles was not observed.
The fifth case
25
deals with the production of DNA inside self-reproducing giant vesicles. First,
DNA was amplified inside giant vesicles by entrapping all components of the polymerase
chain reaction. After DNA synthesis, lipid precursors were added externally. They were taken
up by the vesicle and transformed into the membrane-forming compound thanks to a
catalyst dissolved in the membrane. As a consequence of the increased number of membrane
molecules, the giant vesicle could grow and divide into several vesicles, each containing part
of the synthesized DNA. Contrary to the first four examples, in this study lipid vesicles were
not composed from phospholipids, but from an ad hoc designed membrane-forming
compound, that could be easily obtained after a chemical reaction catalyzed by a simple
imidazole-based membrane-soluble compound (without needing an enzyme).
The sixth case
26
focuses on the production of cytoskeletal elements inside giant vesicles. In
this study, MreB (an
E. coli
protein that is homologous of actin) and MreC (an anchoring
protein) have been synthesized inside vesicles by transcription/translation processes. It was
observed that flexible filamentous structures were formed in liposomes (smaller than
15
267
μ
m). This study paves the way to the reconstruction of synthetic divisome in SSMCs.
From this short survey on the state-of-the-art of SSMC studies it is evident that there have
been great efforts for developing synthetic cells capable of performing some nontrivial
functions, such as the control of genetic expression, the opening of a pore on the vesicle
membrane, the replication of genetic material, and lipid synthesis. When compared to a
hypothetical minimal cell based on a minimal genome, the current constructs might appear
too simple, and indeed they are. But they have to be considered as the pioneer steps into a
new technology, i.e. the laboratory construction of synthetic cells, that has just been born.
In a certain sense, this way of progressing fully reflects one facet of SB (perhaps not fully
recognized), namely the possibility of learning about living systems by attempting their
construction, in order to understand what the physicochemical constraints are underlying
the observed biological behavior. The ever-increasing involvement of several new research
groups in this field will certainly impact on the advances in this field, so that more progress
is foreseeable in the next few years (
Table 14.2
).
PHYSICAL ASPECTS OF SSMC CONSTRUCTION: IMPLICATIONS
FOR THE ORIGIN OF LIFE
In the previous section we have discussed in detail one of the two ingredients of the current
experimental approaches: the cell-free system. Here we will discuss further the second
ingredient, namely the lipid vesicles and the physical aspects related to their formation and
encapsulation of the solutes.
From the practical viewpoint, SSMCs (or, more generally, solute-filled lipid vesicles) are
generally obtained by forming lipid vesicles in a solution that contains all solutes to be
