Structure and assembly of macromolecular lipid complexes composed of globular micelles

Abstract

Negatively-stained preparations of various mixtures of lecithin, cholesterol and saponin have been investigated by electron microscopy. Lamellar, tubular, hexagonal and helical structures were observed and it has been concluded that many of these structures can be satisfactorily interpreted only in terms of small globular lipid micelles. It is postulated that two types of micelle are present : one containing primarily lecithin and cholesterol, and the other containing mainly saponin and cholesterol, with measurable diameters of about 40 A and about 35 A respectively. These micelles appear to aggregate spontaneously in aqueous dispersions to form the observed complex structures, and it is thought that hydrogen bonds and electrostatic interactions between micelles are responsible for the stability of the complete assemblies. Helical structures prepared using a short-chain synthetic lecithin contain subunits of correspondingly reduced dimensions. The tubular structures present in mixtures of lecithin and cholesterol are similar in appearance and dimensions to bacterial flagella, while helices in mixtures of ovolecithin, cholesterol and saponin resemble tobacco mosaic viruses. Some of the implications of these observations are discussed, and it is suggested that small globular micelles, as well as bimolecular leaflets, may function as building blocks in the formation of biological structures containing lipids.