Molecular Mechanism for Lipid Flip-Flops

Abstract

Transmembrane lipid translocation (flip-flop) processes are involved in a variety of properties and functions of cell membranes, such as membrane asymmetry and programmed cell death. Yet, flip-flops are one of the least understood dynamical processes in membranes. In this work, we elucidate the molecular mechanism of pore-mediated transmembrane lipid translocation (flip-flop) acquired from extensive atomistic molecular dynamics simulations. On the basis of 50 successful flip-flop events resolved in atomic detail, we demonstrate that lipid flip-flops may spontaneously occur in protein-free phospholipid membranes under physiological conditions through transient water pores on a time scale of tens of nanoseconds. While the formation of a water pore is induced here by a transmembrane ion density gradient, the particular way by which the pore is formed is irrelevant for the reported flip-flop mechanism: the appearance of a transient pore (defect) in the membrane inevitably leads to diffusiVe translocation of lipids through the pore, which is driven by thermal fluctuations. Our findings strongly support the idea that the formation of membrane defects in terms of water pores is the rate-limiting step in the process of transmembrane lipid flip-flop, which, on average, requires several hours. The findings are consistent with available experimental and computational data and provide a view to interpret experimental observations. For example, the simulation results provide a molecular-level explanation in terms of pores for the experimentally observed fact that the exposure of lipid membranes to electric field pulses considerably reduces the time required for lipid flip-flops.

Collective Dynamics in Lipid Membranes: From Pore Formation to Flip-Flops

“…To visualize lipid flip-flops across a membrane, it is instructive to follow the time evolution of the positions of centers of mass (CM) of lipid headgroups as well as the lipids' overall orientation measured through the angle between the bilayer normal and the vector directed from the CM of lipid's hydrocarbon chains to the CM of its headgroup. Figure 5.6 presents trajectories for four representative flip-flopped lipids of a DMPC bilayer with NaCl [34]. Remarkably, the translocation of a lipid molecule across a membrane closely correlates with the overall reorientation of the lipid within a membrane: when a lipid molecule is accommodated in its initial membrane leaflet, the lipid's tail-to-head vector makes an average angle of 22 degrees (or 158 degrees depending on the leaflet) with the bilayer normal.…”

“…Remarkably, it turns out that formation of transient water pores is the rate-limiting step in the process of transmembrane lipid flip-flop [33,34]: once a pore has been formed, the subsequent lipid translocation occurs on timescales accessible through state-of-the-art computer simulations. Below, we demonstrate how this takes place.…”

“…To consider this scenario, let us highlight some recent data [34] for 8 double-bilayer systems with preformed pores (4 systems with NaCl as well as KCl) that were simulated over 200 ns, or until the transient water pore closed. In all the considered systems, one finds spontaneous pore-mediated translocation processes of lipid molecules from one leaflet to another.…”

“…More specifically, sodium ions are known to bind strongly to carbonyl oxygens of phosphatidylcholines, leading to the formation of tight complexes between neighboring lipids [11,[43][44][45][46][47], thus weakening the desorption of lipids out of membrane leaflets toward the membrane interior. Overall, the molecular mechanism of lipid flip-flop discussed here consists of two steps [34]. The first step is the formation of a transient water defect in a membrane.…”

“…Therefore, the transmembrane electrostatic potential can hardly play the main role. Indeed, the membrane-normal component of the total electrostatic force exerted on the lipids involved in flip-flops is found to fluctuate around zero [34], without a component that would drive translocation. Second, as already discussed, the electric field across a membrane is spatially inhomogeneous, as it is defined by instantaneous positions of ions.…”