ArchaeoMembrane main results

ArchaeoMembranes (ANR-17-CE11-0012)

How can a membrane bilayer be stable at temperature above the boiling point of water ?

Two major structural adaptations have been linked with the adaptation of the membrane to extreme pH and temperature environments: the synthesis of membrane-spanning, bipolar lipids and the binding of the glycerol moiety and the hydrocarbon chains by an ether bound. Bipolar lipids can form lipid monolayers, in which each polar headgroup points out on one side of the membrane. Monolayers are more rigid, less permeable and thermally more resistant than lipid bilayers. The presence of ether lipids also increases thermal stability of the lipids, allows a tighter packing, and consequently, a more impermeable membrane. Conversely, the lack of bipolar ether lipids is proposed to explain the limited temperature growth range of bacteria. However, several hyperthermophilic Archaea are known not to produce bipolar ether lipids, although growing optimally at up to 105°C, implicating that bilayers could also be stable above the boiling point of water. We have proposed a thermally bilayer membrane architecture to explain the stability the hyperthermophilic archaeal membrane. This novel membrane architecture predicts the presence of apolar lipids in the mid-plane of the bilayer, the presence of which would limit charge transfer between the two sides, decrease proton and water permeability, and increase membrane rigidity.

The aim of the project is to demonstrate experimentally the validity of this novel membrane architecture and to explain the adaptation to hydrothermal stress in hyperthermophilic Archaea. To achieve this goal, we will compare the physico-chemical parameters of natural vs. reconstructed synthetic membranes, in presence or absence of apolar lipids, mimicking those of Archaea in order to identify the specific contribution of each lipid type, and each lipid moiety, on membrane stability. We will perform the total synthesis of di- and tetraether lipids. Working with synthetic lipids allows for the control of membrane composition and easier interpretation of molecular dynamics data, while working with natural lipids permits to test yet undetermined effects of polar headgroups/core lipids on membrane stability. Physical parameters will be determined from a combination of X-ray/neutron diffraction and diffusion, SAXS, Fourier-transform infrared (FTIR), fluorescence spectroscopies, liquid and solid state NMR and confocal fluorescence or electronic microscopies. The results will enable us to characterize the order parameters, size, shape and domain formation as well as permeability and viscosity of the lipid membranes. Due to the precise control of lipid compositions, variations in these parameter values can be readily attributed to specific lipid moieties. For the first time, it will allow the construction of a comprehensive model of the archaeal membrane and principles governing its stability, which will include a contribution for polar headgroups and apolar lipids.

The ArchaeoMembranes project addresses a fundamental question of general and philosophical interest concerning life under extreme environmental conditions, and in fine, about the origin of life on Earth. In this project, we propose the groundwork for a novel membrane architecture, which demonstration would constitute a major breakthrough in membrane science, and on how we understand the cellular membrane. Our preliminary results clearly show that such a membrane architecture can exist in vitro. Obtaining definitive proof that it leads to the expected physical and physiological behavior will have a major scientific echo in the community since it will shed a new light on membrane adaptation to extreme conditions. If this novel ultrastructure can be proven, it implies that lipid rafts, e.g. functionally distinct domains, may coexist in the membranes of Archaea, which implications on cell physiology and functioning are numerous. Last, redefining the phase diagram of the membrane will also have concrete biotechnological applications.