Influenza virus-model membrane interaction. A morphological approach using modern cryotechniques

Effects of temperature on viral glycoprotein mobility and a possible role of internal "viroskeleton" proteins in Sendai virus fusion

The effect of temperature on fusion of Sendai virus with target membranes and mobility of the viral glycoproteins was studied with fluorescence methods. When intact virus was used, the fusion threshold temperature (20-22 degrees C) was not altered regardless of the different types of target membranes. Viral glycoprotein mobility in the intact virus increased with temperature, particularly sharply at the fusion threshold temperature. This effect was suppressed by the presence of erythrocyte ghosts and/or dextran sulfate in the virus suspension. In these cases also, no change in the fusion threshold temperature was observed. On the other hand, reconstituted viral envelopes (virosomes) bearing viral glycoproteins but lacking matrix proteins were capable of fusing with erythrocyte ghosts even at temperatures lower than the fusion threshold temperature and no fusion threshold temperature was observed over the range of 10-40 degrees C. The mobility of viral glycoproteins on virosomes was much greater and virtually temperature-independent. The intact virus treated with an actin-affector, jasplakinolide, reduced the extent of fusion with erythrocyte ghosts and the mobility of viral glycoproteins, while the treatment of virosomes with the same drug did not affect the extent of fusion of virosomes with erythrocyte ghosts and the mobility of the glycoproteins. These results suggest that viral matrix proteins including actins affect viral glycoprotein mobility and may be responsible for the temperature threshold phenomenon observed in Sendai virus fusion.

Structure-activity relationships in the fusion of small unilamellar phosphatidylcholine vesicles induced by a model peptide

Limited proteolysis of fatty acid-free bovine serum albumin by pepsin yields several well characterized peptides, one of which (P9, M(r) 9,000), induces fusion of small unilamellar vesicles (SUV) of phosphatidylcholine at pH 3.6. Circular dichroism (CD) of P9 solutions confirmed that the peptide undergoes a reversible transition between pH 7 and pH 3.6. The spectral changes observed with CD suggest that in the low pH conformation there is a decrease in the alpha-helical contents and an exposure of hydrophobic residues. CD and differential ultraviolet spectroscopy demonstrated that P9 binds to micelles of hexadecylphosphorylcholine and the binding produces changes in the tertiary structure of the peptide. Reduction and carboxymethylation of the two disulfide bridges of P9 produced loss of the ability to induce fusion of SUV, although the reduced peptide binds to vesicles, induces loss of entrapped marker and produces vesicle disruption. In the active form P9 exposes hydrophobic groups, one amphiphilic alpha-helix and requires the integrity of the disulfide bridge-stabilized tertiary structure.

Structural features of membrane fusion between influenza virus and liposome as revealed by quick-freezing electron microscopy

The structure of membrane fusion intermediates between the A/PR/8(H1N1) strain of influenza virus and a liposome composed of egg phosphatidylcholine, cholesterol, and glycophorin was studied using quick-freezing electron microscopy. Fusion by viral hemagglutinin protein was induced at pH 5.0 and 23 degrees C. After a 19-s incubation under these conditions, small protrusions with a diameter of 10-20 nm were found on the fractured convex faces of the liposomal membranes, and small pits complementary to the protrusions were found on the concave faces. The protrusions and pits corresponded to fractured parts of outward bendings of the lipid bilayer or "microprotrusions of the lipid bilayer." At the loci of the protrusions and pits, liposomal membranes had local contacts with viral membranes. In many cases both the protrusions and the pits were aligned in regular polygonal arrangements, which were thought to reflect the array of hemagglutinin spikes on the viral surface. These structures were induced only when the medium was acidic with the virus present. Based on these observations, it was concluded that the microprotrusions of the lipid bilayer are induced by hemagglutinin protein. Furthermore, morphological evidence for the formation of the "initial fusion pore" at the microprotrusion was obtained. The protrusion on the convex face sometimes had a tiny hole with a diameter of <4 nm in the center. The pits transformed into narrow membrane connections <10 nm in width, bridging viruses and liposomes. The structures of the fusion pore and fusion neck with larger sizes were also observed, indicating growth of the protrusions and pits to distinct fusion sites. We propose that the microprotrusion of the lipid bilayer is a fusion intermediate induced by hemagglutinin protein, and suggest that the extraordinarily high curvature of this membrane structure is a clue to the onset of fusion. The possible architecture of the fusion intermediate is discussed with regard to the localization of intramembrane particles at the microprotrusion.

Cholesterol and phosphoinositides increase affinity of the epidermal growth factor receptor

The epidermal growth factor receptor (EGF-R) has been purified from human epidermoid carcinoma A431 cells by affinity chromatography in a single step using a monoclonal antibody (528) which competes with EGF for receptor binding. The purified EGF-R exhibits EGF inducible tyrosine kinase and autophosphorylation activity. Steady-state binding of EGF to the purified receptor revealed the presence of one class of binding sites exhibiting an apparent dissociation constant (Kd) of approx. 2 nM. When Angiotensin II was used as a receptor tyrosine kinase substrate the specific activity of the EGF induced kinase was 87 nmol/min per mg and the Km of the reaction was about 2 mM. Reconstitution of the EGF receptors into lipid vesicles was achieved by octylglucoside dialysis. Reconstitution of the receptor into pure dioleoylphosphatidylcholine (DOPC) vesicles had no effect on the EGF-binding properties in comparison to receptors in Triton X-100 micelles. Binding of EGF to the reconstituted receptor with ATP and Angiotensin II incorporated into the vesicles resulted in a five fold stimulation of the receptor kinase activity. The introduction of cholesterol, ranging from 10% to 50% (w/w), into DOPC vesicles resulted in an increase of the affinity of the receptor for its ligand. The Kd for EGF decreased from 1.8 nM in pure DOPC vesicles to 0.3 mM in DOPC/cholesterol (1:1 (w/w)) vesicles. With the introduction of small amounts (2% (w/w)) of phosphatidylinositol lipids into DOPC vesicles the Kd changed from 1.8 nM to 0.2 nM with phosphatidylinositol (PtdIns) and phosphatidylinositol 4,5-biphosphate (PtdIns4,5-P2) and to 0.1 nM in the case of phosphatidylinositol 4 phosphate (PtdIns4-P). No change in affinity was found when equal amounts of phosphatidylserine (PS) or phosphatidic acid (PA) were used.

Human immunodeficiency virus type 1 (HIV-1) fusion with model membranes: kinetic analysis and the role of lipid composition, pH and divalent cations

The kinetics and extent of HIV-1 fusion with model membranes was studied. HIV-1 was labeled with octadecyl rhodamine B chloride, and fusion was monitored continuously as the dilution of the probe into target membranes. The results were analyzed by a mass action model which yielded good simulations and predictions for the kinetics and final extents of fluorescence increase. The model determined the percent of virions capable of fusing and rate constants of fusion, aggregation and dissociation. Ultrastructural analysis of the virus and reaction products by electron microscopy also provided evidence of HIV-1 fusion with membranes lacking CD4. HIV-1 fusion activity depends on the target membrane lipid composition according to the sequence: cardiolipin (CL) > > phosphatidylinositol > CL/dioleoylphosphatidylcholine (DOPC) (3:7), phosphatidic acid > phosphatidylserine (PS), PS/cholesterol (2:1) > PS/PC (1:1), PS/phosphatidylethanolamine (1:1) > DOPC, erythrocyte ghosts. Reduction of pH from 7.5 generally enhances the rate and extent of HIV-1 fusion. Physiologically relevant concentrations of calcium stimulate HIV-1 fusion with several liposome compositions and with erythrocyte ghost membranes. The fusion products of HIV-1 with liposomes consist of a single virus and several liposomes. The mass action analysis revealed that, compared to intact virions, the fusion products show a striking reduction in the fusion rate constant. Like influenza and Sendai viruses, HIV-1 fusion with membranes containing its own envelope glycoprotein(s) is strongly inhibited. Unlike these viruses, HIV-1 fusion is promoted by physiological levels of calcium. HIV-1 fusion with liposomes is qualitatively similar to simian immunodeficiency virus fusion.

Membrane fusion

Common themes are emerging from the study of viral, cell-cell, intracellular, and liposome fusion. Viral and cellular membrane fusion events are mediated by fusion proteins or fusion machines. Viral fusion proteins share important characteristics, notably a fusion peptide within a transmembrane-anchored polypeptide chain. At least one protein involved in a cell-cell fusion reaction resembles viral fusion proteins. Components of intracellular fusion machines are utilized in multiple membrane trafficking events and are conserved through evolution. Fusion pores develop during and intracellular fusion events suggesting similar mechanisms for many, if not all, fusion events.

Membrane vesicles of A431 cells contain one class of epidermal growth factor binding sites

Epidermoid carcinoma A431 cells exhibit two classes of epidermal growth factor (EGF) receptors as deduced from Scatchard analysis. Steady-state binding of EGF to isolated A431 membranes indicated, however, the presence of only one class of EGF binding sites. The apparent dissociation constant (Kd) of these sites was approx. 0.45 nM which is similar to that of the high-affinity receptor of intact A431 cells. These results suggest that the vesicle receptor population consists only of high-affinity receptors. However, further studies indicated that the binding sites were similar to the low-affinity class, since binding of EGF could be blocked entirely by 2E9, a monoclonal anti-EGF receptor antibody which is able to inhibit specifically EGF binding to low-affinity receptors in A431 cells. The difference in affinity of the receptors in membrane vesicles as compared to intact cells may be explained by differences in biophysical parameters such as diffusion-limited EGF binding and receptor distribution. Based upon these considerations, it is concluded that membrane vesicles of A431 cells contain one class of EGF receptors which are apparently identical to the low-affinity receptors of intact cells.

Lipid polymorphism of model and cellular membranes as revealed by electron microscopy

This review surveys the current state of knowledge relating to lipid polymorphism within both model lipid membrane and cellular membrane systems. Emphasis is placed upon the contribution of data obtained by transmission electron microscopy of freeze-fractured specimens. Some consideration is also given to the other important methods for the study of lipid polymorphism, namely X-ray diffraction and NMR spectroscopy. A detailed account of the different phases found in lipid mixtures within model membranes (bilayer, cubic or tetragonal, tubular or hexagonal) provides the background to the understanding of the factors involved in polymorphic phase transitions. The sequential steps involved in lipid polymorphism are defined from electron microscopical data and are related to the structural changes that can be detected within biological membranes. It is proposed that the fine structural changes detected at the initial stages of polymorphic transition in vivo may be highly relevant in relation to membrane fusion events, to the formation of tight junctions, and even to physiological transport processes. Since the later stages of polymorphic transition generally destroy the permeability barrier of model and cellular membranes, extensive rather than localized phase transition of the lipid bilayer is not at the moment considered to be compatible with cellular viability.

Fusion of liposomes with the plasma membrane of epithelial cells: fate of incorporated lipids as followed by freeze fracture and autoradiography of plastic sections

The fusion of liposomes with the plasma membrane of influenza virus-infected monolayers of an epithelial cell line, Madin-Darby canine kidney cells (van Meer et al., 1985. Biochemistry. 24:3593-3602), has been analyzed by morphological techniques. The distribution of liposomal lipids over the apical and basolateral plasma membrane domains after fusion was assessed by autoradiography of liposomal [3H]dipalmitoylphosphatidylcholine after rapid freezing or chemical fixation and further processing by freeze substitution and low temperature embedding. Before fusion, radioactivity was solely detected on the apical cell surface, indicating the absence of redistribution artifacts and demonstrating the reliability of lipid autoradiography on both a light and electron microscopical level. After induction of fusion by a low pH treatment, the basolateral plasma membrane domain became progressively labeled, indicative of rapid lateral diffusion of [3H]dipalmitoylphosphatidylcholine in the plasma membrane. Analysis of individual fusion events by freeze fracture after rapid freezing confirmed the rapid diffusion of the liposomal lipids into the plasma membrane, as intramembrane particle-free lipid patches were never observed. After the induction of liposome-cell fusion, well-defined intramembrane particles were present on the otherwise smooth liposomal fracture faces and on the fracture faces of the plasma membrane. Morphological evidence thus was obtained in favor of a local point fusion mechanism with an intramembrane particle as a specific structural fusion intermediate.