Curvature Sorting Of Lipids And Proteins In The Strong Segregation Limit: Curvature Mediated Domain Nucleation And Steady State Transport In Tubular Membranes With Phase Separation

Intracellular sorting centers, including the trans-Golgi network, the endoplasmic reticulum, and the endocytic recycling compartment, all contain membrane tube elements with cylindrical curvatures. The question of how curvature and sorting are coupled is central to the understanding of the function of organelle homeostasis, membrane trafficking, and intracellular sorting. Of particular interest, but underexplored, are non-equilibrium phenomena fundamentally linked to membrane transport. Here we present a straightforward and well controlled model system that allows us to characterize lipid transport at steady state: ternary lipid mixture tubular membranes pulled from phase-separated giant unilamellar vesicles (GUVs) by means of optical tweezers. The tubule composition, when initially formed from the liquid-ordered (Lo) phase region of the vesicle, is dependent on the velocity at which it is pulled: fast extraction velocities create tethers of the Lo phase, while slow extraction velocities can generate tethers that are liquid-disordered (Ld) phase. Thus, the speed with which highly curved tubules are formed may possibly serve as a control variable in living cells to adjust tubule composition. Furthermore, in tubules which are initially Lo phase, we find curvature-induced nucleation of Ld domains at the neck between tubule and vesicle. These Ld domains display characteristic, curvature dependent parabolic growth behavior that can be understood via a straightforward analytical mass transfer model that we derived from linear irreversible thermodynamics. We have also developed numerical schemes that capture shape transitions of tubular membranes on the basis of measured biophysical parameters in support of our experimental findings.