Direct evidence of charge-induced lipid domain structure in model membranes.

Abstract

The phenomenological membrane properties are expected to be primarily dependent on the lateral and transversal organization of the membrane building units (lipids and proteins). In view of this question lipids with a net charge at the polar headgroups are of outstanding interest. Spin-label studies of model membranes containing phosphatidic acid and labelled lecithin showed that external charges (poly-lysine or Ca’+) may trigger a phase separation leading to a lateral domain structure [ 1,2] . Each monolayer exhibits a mosaic like pattern of domains of polylysine (or Ca2+) bound charged lipid embedded in mixed lecithin-phosphatidic acid regions. In the present paper we provide direct electron microscopic evidence for this charge-induced domain formation in lipid alloys containing dioleyl phosphatidic acid. Charged poly-peptides (poly-lysine) or two valent cations (Ca23 are used as domain inducing external charges. Phosphatidic acid may be considered as representative of charged natural lipids (e.g., phosphatidylserine in erythrocytes, cardiolipin in mitochondria).