Spontaneous formation of nanometer scale tubular vesicles in aqueous mixtures of lipid and block copolymer amphiphiles

Many common amphiphiles self-assemble in water to produce heterogeneous populations of discrete and symmetric but polydisperse and multilamellar vesicles isolating the encapsulated aqueous core from the surrounding bulk. But when mixtures of amphiphiles of vastly different elastic properties co-assemble, their non-uniform molecular organization can stabilize lower symmetries and produce novel shapes. Here, using high resolution electron cryomicroscopy and tomography, we identify the spontaneous formation of a membrane morphology consisting of unilamellar tubular vesicles in dilute aqueous solutions of binary mixtures of two different amphiphiles of vastly different origins. Our results show that aqueous phase mixtures of a fluid-phase phospholipid and an amphiphilic block copolymer spontaneously assume a bimodal polymorphic character in a composition dependent manner: over a broad range of compositions (15-85 mol% polymer component), a tubular morphology co-exists with spherical vesicles. Strikingly, in the vicinity of equimolar compositions, an exclusively tubular morphology (L_t; diameter, ∼15 nm; length, >1 μm; core, ∼2.0 nm; wall, ∼5-6 nm) emerges in an apparent steady state. Theory suggests that the spontaneous stabilization of cylindrical vesicles, unaided by extraneous forces, requires a significant spontaneous bilayer curvature, which in turn necessitates a strongly asymmetric membrane composition. We confirm that such dramatic compositional asymmetry is indeed produced spontaneously in aqueous mixtures of a lipid and polymer through two independent biochemical assays-(1) reduction in the quenching of fluorophore-labeled lipids and (2) inhibition in the activity of externally added lipid-hydrolyzing phospholipase A2, resulting in a significant enrichment of the polymer component in the outer leaflet. Taken together, these results illustrate the coupling of the membrane shape with local composition through spontaneous curvature generation under conditions of asymmetric distribution of mixtures of disparate amphiphiles.