Induction of helical liposomes by Ca²⁺-mediated intermembrane binding

Abstract

Extensive freeze-fracture electron microscopic studies of Ca²⁺-containing mixtures of an acidic phospholipid (cardiolipin) and phosphatidylcholines^1–5, have revealed certain replica features which have been interpreted as regions of membrane–membrane attachment^3–5. Spin-label paramagnetic resonance spectra of mixtures of another acidic phospholipid (phosphatidylserine) and phosphatidylcholines have provided evidence for Ca²⁺-mediated lateral phase separations of the acidic phospholipids^6–8, and their involvement in intermembrane binding⁷. We report here that in the presence of Ca²⁺, binary mixtures of cardiolipin and phosphatidylcholines readily form tightly folded single- and double-helical liposomes; we invoke membrane–membrane attachment to explain this phenomenon.