Organization and dynamics of lipids in bovine brain coated and uncoated vesicles

Measuring the elasticity of clathrin-coated vesicles via atomic force microscopy

Using a new scheme based on atomic force microscopy (AFM), we investigate mechanical properties of clathrin-coated vesicles (CCVs). CCVs are multicomponent protein and lipid complexes of approximately 100 nm diameter that are implicated in many essential cell-trafficking processes. Our AFM imaging resolves clathrin lattice polygons and provides height deformation in quantitative response to AFM-substrate compression force. We model CCVs as multilayered elastic spherical shells and, from AFM measurements, estimate their bending rigidity to be 285 +/- 30 k(B)T, i.e., approximately 20 times that of either the outer clathrin cage or inner vesicle membrane. Further analysis reveals a flexible coupling between the clathrin coat and the membrane, a structural property whose modulation may affect vesicle biogenesis and cellular function.

Influence of the clathrin coat on the membrane lipidic organization of endocytic vesicles: a fluorescence study

Endocytic coated vesicles (CV) were purified from bovine brain, and uncoated vesicles (UV) were obtained from the latter by dialysis against 1 M Tris. Membrane dynamics were explored in both vesicle populations using two complementary fluorescence approaches: diphenylhexatriene fluorescence anisotropy to account for rotational lipid movements, and pyrene excimerization with a phosphatidylcholine pyrene derivative for translational motion. It was concluded that membrane fluidity was considerably higher in UV than in CV, and that adding bulk coat proteins (adaptors+clathrin) to uncoated vesicles re-established the low fluidity found in coated vesicles. However, adding coat protein constituents separately had no effect.

Spectroscopical characteristics of galactosylceramide-pyrene and ceramide-pyrene incorporated in model and in clathrin coated vesicles

In order to control the ability of two pyrene-sphingolipids (ceramide-pyrene (Cpyr) and galactosylceramide-pyrene (GCpyr)) to monitor the changes in the lipid bilayer dynamics of cellular membranes, their incorporation in three populations of clathrin coated vesicles which differ in their structural characteristics (Bomsel et al. (1988) Biochemistry 27, 6808-6812) was studied by both absorbance and fluorescence spectroscopy. The method of injection of an ethanolic solution of probe was used. The analysis of the spectra recorded after injection into a free-membrane buffer allowed to discriminate two dispersion states (micellar or aggregated) of the probes. The micellar state was identified as the one suitable for the incorporation within the bilayer. Rising the temperature up to 18 degrees C for a membrane labeling with GCpyr and to 37 degrees C for a membrane labeling with Cpyr was found to be necessary because it allowed to slow down the aggregation process which inhibited the incorporation within the lipid bilayer. The excimer/monomer (E/M) fluorescence intensities ratio of GCpyr was found to be characteristic of each population of coated vesicles. Cpyr could not be used as a diffusion probe because it partly aggregated during the cooling step necessary to establish the E/M versus temperature plot in the heating mode. An important point which arises from these data is that the use of absorbance spectroscopy can avoid misinterpretation of the pyrene derivatives fluorescence spectra in terms of diffusion.

Incorporation of spin-labeled fatty acids into bovine brain clathrin coated vesicles

Stearic acids with a nitroxide radical at selected positions have been incorporated in the phospholipid bilayers of clathrin coated vesicles, uncoated vesicles and sonicated liposomes made from the lipids extracted from the uncoated vesicles. The extent of incorporation was found minimum for stearic acids labeled on C-12 and for bilayers of uncoated vesicles. The ESR spectra of the spin-labeled fatty acids incorporated in the bilayers showed a pronounced temperature dependence (without discontinuity) and a decrease in the hyperfine splitting as the nitroxide group was inserted deeper in the hydrophobic core of the membranes. An abrupt phospholipid phase transition or a phase separation could be excluded. The presence of the external proteins (the clathrin coat) on the membranes was not found to noticeably influence the gradient of flexibility of the fatty acid chains of the phospholipids. The influence of the internal proteins embedded in the bilayers was evidenced by a detailed analysis of the ESR spectra of (7,8)SA in terms of two components: one component arising from the labels surrounded exclusively by phospholipids, the other component arising from labels of reduced mobility perturbed by the vicinity of the proteins. These results support the persistence of lipidic domains in the endocytic vesicles despite the accumulation of receptors which follows their formation.

Interaction of clathrin coat proteins with unilamellar and multilamellar vesicles of phosphatidylcholine

The binding of clathrin and accessory coat proteins to small unilamellar vesicles and to liposomes of uncharged phospholipids has been followed by chromatography, 31P-NMR, ESR and fluorescence anisotropy. At pH 6.5 and at an ionic strength value (0.1 M Mes) close to that used during the purification of clathrin-coated vesicles, the proteins do not restore the characteristic network found around the natural vesicles. Instead, a limited fusion leads to enlarged structures in which the perturbation of the dynamics of the phospholipids decreases gradually with the depth in the membrane. While the rate of motion of the outer polar heads is lowered, the order parameter of doxyl groups located either under or in the vicinity of the glycerol backbone is not affected by the proteins. In the inner core of the membrane, the main thermotropic transition of the hydrocarbon chains is unchanged. All the effects are the results of interactions limited to the membrane surface. The electrostatic nature of these interactions is evidenced when the embedded spin labels have a charge protruding at the membrane surface. An 'anchoring' effect appears which is due to the charged groups of the proteins. The lateral diffusion of the probes is reduced and, at low ionic strength, a cationic derivative no longer detects the thermotropic transition of the hydrocarbon chains. These results indicate that, although it is known that clathrin and accessory proteins bind to membranes by a series of protein-protein interactions, this system is not devoid of lipid-protein interactions, at least when it is not organized as in the natural system.

Instability of bovine brain clathrin-coated vesicles on Sephacryl S-1000 gel chromatography

Clathrin-coated vesicles have been isolated from bovine brain. To allow their further use in biophysical studies, the homogeneity of the preparations has been fully characterized after chromatography on Sephacryl S-1000, which is employed in many studies. It is demonstrated here that clathrin-coated vesicles are not stable on the gels and that their instability is increased in preparations using gels that are not presaturated with phospholipids. In addition, some fractionation occurs during chromatography. It is proposed that the slower migrating fractions contain mainly empty clathrin coats. Changes that occurred during the chromatography step are the result of reversible and irreversible events and are probably related to the assembly/disassembly cycle of clathrin observed in vitro.

Clustering in coated vesicles of polyunsaturated phospholipids segregated from plasma and Golgi membranes of adrenocortical cells

In bovine adrenocortical cells, the fatty acyl chains of the phospholipids have been identified in the membranes of the different cell compartments: plasma membranes, Golgi complex and coated vesicle membranes. An increase in the total number of unsaturation in the fatty acid is demonstrated in the coated vesicle membranes as compared with the plasma and Golgi membranes. Furthermore, it appears that phosphatidylcholine and phosphatidylethanolamine are both enriched in polyunsaturated fatty acyl chains, namely arachidonic and adrenic acids in both types of coated vesicles. Only two of the fatty acids are characteristic of Golgi complex and small coated vesicles, 22:5 (n-6) in PC and 22:6 (n-3) in PE, suggesting that the SCV could originate from the Golgi stacks. A high value of the ratio 22:5 (n-3)/22:6 (n-3) is observed which is, as far as we know, characteristic of adrenal cells.

Lipid bilayer dynamics in plasma and coated vesicle membranes from bovine adrenal cortex. Evidence of two types of coated vesicle involved in the LDL receptor traffic

Pure coated vesicles have been prepared from the bovine adrenal cortex and two homogeneous populations have been separated, one of large diameter (100 nm) and one of small diameter (70 nm). The chemical composition in lipids and proteins of coated vesicles has been compared with that of partially purified plasma membranes and evidences a higher protein/lipid ratio and a higher concentration in phosphatidylethanolamine and unsaturated fatty acids. Evaluation of the lateral diffusion of pyrene in the lipid bilayer of coated vesicles as compared to uncoated vesicles evidences a slowing-down effect of clathrin. Measurements of lipids' rotational diffusion by time-resolved fluorescence indicate a decrease in the order parameter of the lipids in the coated vesicles due to clathrin. A hypothesis is proposed for a possible role of the clathrin coat in the concerted motion of lipids and proteins toward coated pits and in the mechanism of formation of coated vesicles. Separation of the large from the small coated vesicles made it possible to reveal different protein components in the two types of vesicle by electrophoresis and autoradiograms of the [gamma-32P]adenosine triphosphate- (ATP-) treated vesicles. Visualisation of the low-density lipoprotein receptor by ligand blotting and enzyme-linked immunosorbent assay (ELISA) techniques indicates an increased low-density lipoprotein receptor binding capacity in small coated vesicles as compared to large ones and plasma membranes.

Phospholipid, cholesterol, polypeptide and glycoprotein composition of hepatic endosome subfractions

The composition of hepatic endosome subfractions was compared directly with that of plasma membranes and Golgi-apparatus fractions. The neutral lipid and phospholipid composition of two endosome subfractions separated on Nycodenz gradients from a parent endosome fraction was similar to that of plasma membranes. The phospholipid/cholesterol ratios and the sphingomyelin contents were high, as in plasma membranes. However, the phosphatidylserine content was low. Endosomal subfractions contained a simpler polypeptide profile than did plasma membranes. However, a large number of glycoproteins were common to both fractions. Two endosome-specific glycoproteins, of Mr 59 000 and 38 000, were identified. Sialic acid was present at concentrations higher than in plasma membranes. The results indicate that endosomal membranes have a similar composition to plasma membranes that probably reflects their functional interaction during endocytosis and receptor recycling.