The evidence presented in this review strongly suggests that, when present as a minor component in liquid crystalline phospholipid bilayers, neutral glycosphingolipids are segregated into compositional domains of small size dispersed in the matrix phospholipid. In many instances the glycosphingolipid in the dispersed domains is in the gel state. Because these domains are in the gel state, the individual molecules escape only very slowly from the surface of the bilayer, much more slowly than do the phospholipid components. There is as yet no direct evidence that this slow escape rate is a property of neutral glycolipids in biological membrane bilayers. If it is, however, then these molecules are well suited for their putative role as cell surface markers, a role that involves them in many important biological functions. There is evidence to suggest that molecules of this type are also present in a dispersed microdomain structure on the external surface of at least some mammalian cell plasma membranes. These small domains of glycolipids with their sugar residues projecting outward from the cell surface are much like a large membrane glycoprotein when viewed from the ambient medium near the cell surface. Thus, whether the sugar residues be of glycoprotein or glycolipid origin, they are localized in groups or patches on the external surface of the cell. One important consequence of this patch structure may be in the obvious effect on the free energy of binding a ligand to a patch. Whether the ligand is mono- or polyvalent, the roughly 2 M concentration of sugar in the surface patch will cause the apparent ligand binding free energy to be substantially larger than it would be for a single isolated sugar residue on the surface. In contrast to the neutral glycosphingolipids (and sulfatides, perhaps) the available information suggests that gangliosides are not localized in small domains in model systems and most probably not in biological membranes. Capping of this type of glycosphingolipid does appear to occur under certain circumstances. However, it is almost certain that capping is not an intrinsic property of ganglioside phospholipid-bilayer systems. Although at 37 degrees C gangliosides rapidly transfer from micelles to phospholipid vesicles and to cell membranes, nothing is known about the rates at which this class of molecules leave a phospholipid bilayer. Their known biological functions on the cell surface appear to require that they leave very slowly, if at all, as do the neutral glycosphingolipids. The glycosphingolipids are, by virtue of their polysaccharide moeity, a unique class of lipids and cell surface components.(ABSTRACT TRUNCATED AT 400 WORDS)

The major glycoprotein of the human erythrocyte membrane has been isolated by treatment with lithium di-iodosalicylate and found to be a single polypeptide chain with a molecular weight of about 50,000. This molecule, which is 60% carbohydrate and 40% protein, carries multiple blood-group antigens, the receptors for influenza viruses, and various plant agglutinins. Four unique carbohydrate-containing peptides (a-i, a-2, a-3, and jB) are produced by tryptic digestion of the isolated glycoprotein; their order in the molecule has been determined by sequential tryptic digestion of intact erythrocyte membranes and partially digested glycoprotein fragments. Cleavage of the native protein with cyanogen bromide produces five fragments; two of these (C-5 and C-i) contain most of the carbohydrate in the molecule and are derived from the N-terminal half of the polypeptide chain. The nonpolar amino acids of this glycoprotein are located predominantly in the C-terminal fragment (C-2).Phytohemagglutinin conjugated to ferritin has been used to map the distribution of glycoprotein receptors over the surfaces of intact erythrocytes by freezeetching and electron microscopy. This label localizes to sites on the membrane that overlie the intramembranous particles. These findings suggest that the glycoprotein is oriented at the cell surface with its oligosaccharide-rich N-terminal end exposed to the exterior, while its C-terminal segment interacts with other components in the interior of the membrane to form intramembranous particles.Glycoproteins comprise about 10% of the total protein of the human erythrocyte membrane (1). Their carbohydrate moieties are antigenic determinants (2, 3) and receptors for viruses and plant agglutinins (4), and their sialic acid residues are responsible for most of the negative charge at the cell surface (5).Glycoproteins have been isolated from erythrocyte membranes with various solvents, such as phenol (6), butanol (7), pyridine (8), sodium dodecyl sulfate (SDS) (9), or formic acid (10 This report summarizes studies on the properties of the major glycoprotein of the human erythrocyte membrane extracted from membranes by a new procedure with lithium di-iodosalicylate as a dissociating agent. A water-soluble glycoprotein has been isolated in high yield that has the chemical properties and biological activities characteristic of the native membrane-bound molecules. This glycoprotein has been partially characterized after tryptic digestion and cyanogen bromide cleavage, and its location in the membrane has been determined by electron microscopy. MATERIALS AND METHODSHuman blood was obtained fresh from donors, and the erythrocytes were washed three times in phosphate-buffered saline (pH 7.4) before preparation of ghost membranes by the procedure of Dodge et al. (11). Glycoprotein was extracted from freeze-dried ghost membranes with 0.3 M lithium diiodosalicylate and purified by phosphocellulose chromatography (12). Samples were electrophoresed in acrylamide gels containing Tris-glycine (pH 8.4), a...

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Small unilamellar dipalmitoylphosphatidylcholine vesicles formed by sonication are shown to fuse spontaneously below the phase transition temperature. The ultimate fusion products are unilamellar vesicles about 700 A in diameter, which are stable and provide an intact ionic permeation barrier either above or below the phase transition. The fused vesicles have been characterized by gel chromatography, trapped volume, 31P nuclear magnetic resonance, and negative stain and freeze-fracture electron microscopy.

Small sonicated dipalmitoylphosphatidylcholine vesicles when incubated at 4 degrees C and high concentrations are shown to fuse completely to vesicles about 700-A diameter in 7 days, and these further fuse to about 950 A diameter vesicles after 3-4 weeks. The 950 A diameter vesicles are spherical, homogeneous, mostly unilamellar, have an internal aqueous space about 10 times that of small vesicles, and are stable for at least 6 months. The 950-A vesicles are characterized by agarose gel chromatography, freeze-fracturing electron microscopy, trapped volume measurements, differential scanning calorimetry, and diphenylhexatriene fluorescence polarization.