Membrane regulators such as sterols and hopanoids play a major role in the physiological and physicochemical adaptation of the different plasmic membranes in Eukarya and Bacteria. They are key to the functionalization and the spatialization of the membrane, and therefore indispensable for the cell cycle. No archaeon has been found to be able to synthesize sterols or hopanoids to date. They also lack homologs of the genes responsible for the synthesis of these membrane regulators. Due to their divergent membrane lipid composition, the question whether archaea require membrane regulators, and if so, what is their nature, remains open. In this review, we review evidence for the existence of membrane regulators in Archaea, and propose tentative location and biological functions. It is likely that no membrane regulator is shared by all archaea, but that they may use different polyterpenes, such as carotenoids, polyprenols, quinones and apolar polyisoprenoids, in response to specific stressors or physiological needs.

Archaea synthesize methyl-branched, ether phospholipids, which confer the archaeal membrane exceptional physicochemical properties. A novel membrane organization was proposed recently to explain the thermal and high pressure tolerance of the polyextremophilic archaeon Thermococcus barophilus. According to this theoretical model, apolar molecules could populate the midplane of the bilayer and could alter the physicochemical properties of the membrane, among which is the possibility to form membrane domains. We tested this hypothesis using neutron diffraction on a model archaeal membrane composed of two archaeal diether lipids with phosphocholine and phosphoethanolamine headgroups in the presence of the apolar polyisoprenoid squalane. We show that squalane is inserted in the midplane at a maximal concentration between 5 and 10 mol % and that squalane can modify the lateral organization of the membrane and induces the coexistence of separate phases. The lateral reorganization is temperature-and squalane concentration-dependent and could be due to the release of lipid chain frustration and the induction of a negative curvature in the lipids.

It has been proposed that adaptation to high temperature involved the synthesis of monolayer-forming ether phospholipids. Recently, a novel membrane architecture was proposed to explain the membrane stability in polyextremophiles unable to synthesize such lipids, in which apolar polyisoprenoids populate the bilayer midplane and modify its physico-chemistry, extending its stability domain. Here, we have studied the effect of the apolar polyisoprenoid squalane on a model membrane analogue using neutron diffraction, SAXS and fluorescence spectroscopy. We show that squalane resides inside the bilayer midplane, extends its stability domain, reduces its permeability to protons but increases that of water, and induces a negative curvature in the membrane, allowing the transition to novel non-lamellar phases. This membrane architecture can be transposed to early membranes and could help explain their emergence and temperature tolerance if life originated near hydrothermal vents. Transposed to the archaeal bilayer, this membrane architecture could explain the tolerance to high temperature in hyperthermophiles which grow at temperatures over 100 °C while having a membrane bilayer. The induction of a negative curvature to the membrane could also facilitate crucial cell functions that require high bending membranes.

It is now well established that cell membranes are much more than a barrier that separate the cytoplasm from the outside world. Regarding membrane's lipids and their self-assembling, the system is highly complex, for example, the cell membrane needs to adopt different curvatures to be functional. This is possible thanks to the presence of non-lamellar-forming lipids, which tend to curve the membrane. Here, we present the effect of squalane, an apolar isoprenoid molecule, on an archaea-like lipid membrane. The presence of this molecule provokes negative membrane curvature and forces lipids to self-assemble under inverted cubic and inverted hexagonal phases. Such non-lamellar phases are highly stable under a broad range of external extreme conditions, e.g. temperatures and high hydrostatic pressures, confirming that such apolar lipids could be included in the architecture of membranes arising from cells living under extreme environments.

The presence of the osmolyte mannosylglycerate in alive cells of Thermococcus barophilus, a hyperthermophile and piezophile, limits the structural rearrangements of its proteome under the archaeon optimal growth conditions, i.e. 358 K and 40 MPa.

No abstract

During the last decades, high pressure has been an important physical parameter not only to study biomolecules, but also for its biotechnological applications. High pressure affects organism's ability to survive by altering most of cell's macromolecules. These effects can be used, for example, to inactivate microorganisms, enhance enzymatic reactions or to modulate cell activities. Moreover, some organisms are capable to growth under high pressures thanks to their adaptation at all cellular levels. Such adaptation confers a wide range of potentially interesting macromolecules still to be discovered. In this chapter, we firstly present the different effects of pressure on cells and the diverse strategies used to cope against this harsh environment. Secondly, we explored the pressure biotechnological applications on pressure-sensitive and adaptedpressure organisms.

All cells use organized lipid compartments to facilitate specific biological functions. Membrane‐bound organelles create defined spatial environments that favor unique chemical reactions while isolating incompatible biological processes. Despite the fundamental role of cellular organelles, there is a scarcity of methods for preparing functional artificial lipid‐based compartments. Here, we demonstrate a robust bioconjugation system for sequestering proteins into zwitterionic lipid sponge phase droplets. Incorporation of benzylguanine (BG)‐modified phospholipids that form stable covalent linkages with an O6‐methylguanine DNA methyltransferase (SNAP‐tag) fusion protein enables programmable control of protein capture. We show that this methodology can be used to anchor hydrophilic proteins at the lipid‐aqueous interface, concentrating them within an accessible but protected chemical environment. SNAP‐tag technology enables the integration of proteins that regulate complex biological functions in lipid sponge phase droplets, and should facilitate the development of advanced lipid‐based artificial organelles.