Cells are the elementary units of all living things. They are contained within a membrane in the same way as their internal compartments, the organelles that constitute them. Each membrane is a double layer formed by an assemblage of a large number of molecules known as lipids. There exist thousands of these with a huge variety of physicochemical profiles. Depending on the compartments, they combine differently. This determines the thickness, the degree of fluidity and the molecular identity of the membranes and thus the functioning of the proteins that are anchored to or evolving on the surface. It is therefore critical that this lipid composition remains unique in space and stable over time. For this the synthesis and transport mechanisms guarantee a precise distribution of lipids at all points of the cell. Our team at the Institut de Pharmacologie Moléculaire et Cellulaire (CNRS & Université Côte d’Azur) is decrypting the operating mode of so-called lipid transfer proteins at the heart of these allocation processes. This can be used to understand the various cellular functions, as well as the molecular origins of certain diseases.

The internal surface of the membrane encircling the cell is unique: Negatively charged, it exerts a considerable electrostatic force to attract certain proteins. This is due

to an overabundance of an anionic lipid, phosphatidylserine. This lipid is however produced elsewhere, on the membrane of an organelle known as the endoplasmic reticulum. This situation explains the existence of proteins such as Osh6p that can take the newly synthetized phosphatidylserine, distribute it around the cell and deliver it to the peripheral membrane. In exchange, they seize an endogen lipid called PI4P. They return to their starting point where they deposit this lipid, taking a new phosphatidylserine molecule and starting another cycle. These cycles self-perpetuate and the peripheral membrane accumulates phosphatidylserine.

Molecular dynamic of the bonding of the OSH6P protein to a negatively charged membrane. Here, the open protein bonds to the membrane.

The cover (in red) exerts a repulsion force on the membrane limiting the bonding of the closed form.

There is however an unresolved question: How, during a cycle, do these transporters escape the powerful pull exerted by the cellular periphery to return to the endoplasmic reticulum. When the Osh6p protein catches a lipid in its bonding pocket, this is closed by a molecular cover. Our measurements showed that this mechanism triggers the unhooking of a protein from a negative membrane. Thus the Osh6p protein can then go towards another membrane. A whole series of dynamic molecular simulations, of 500 ns each, performed on systems of 750,000 atoms, provided the answer why. Osh6p in its empty and open form bonds easily to a membrane rich in phosphatidylserine. The closed form however remains most often in solution, dissociated from it. The energy computations reveal that when the cover closes it exerts a repulsion force on the membrane. This previously unknown mechanism gives us a better understanding of how lipids are transported in the cell.