Nanovesicles released by Dictyostelium cells: a potential carrier for drug delivery

Nanovesicles released by Dictyostelium discoideum cells grown in the presence of the DNA-specific dye Hoechst 33342 have been previously shown to mediate the transfer of the dye into the nuclei of Hoechst-resistant cells. The present investigation extends this work by conducting experiments in the presence of hypericin, a fluorescent therapeutic photosensitizer assayed for antitumoral photodynamic therapy. Nanovesicles released by Dictyostelium cells exhibit an averaged diameter between 50 and 150 nm, as measured by transmission cryoelectron microscopy. A proteomic analysis reveals a predominance of actin and actin-related proteins. The detection of a lysosomal membrane protein (LIMP II) indicates that these vesicles are likely generated in the late endosomal compartment. The use of the hypericin-containing nanovesicles as nanodevices for in vitro drug delivery was investigated by fluorescence microscopy. The observed signal was almost exclusively located in the perinuclear area of two human cell lines, skin fibroblasts (HS68) and cervix carcinoma (HeLa) cells. Studies by confocal microscopy with specific markers of cell organelles, provided evidence that hypericin was accumulated in the Golgi apparatus. All these data shed a new light on in vitro drug delivery by using cell-released vesicles as carriers.

Peptide P5 (residues 628-683), comprising the entire membrane proximal region of HIV-1 gp41 and its calcium-binding site, is a potent inhibitor of HIV-1 infection

The membrane proximal region (MPR) of the transmembrane subunit, gp41, of the HIV envelope glycoprotein plays a critical role in HIV-1 infection of CD4⁺ target cells and CD4-independent mucosal entry. It contains continuous epitopes recognized by neutralizing IgG antibodies 2F5, 4E10 and Z13, and is therefore considered to be a promising target for vaccine design. Moreover, some MPR-derived peptides, such as T20 (enfuvirtide), are in clinical use as HIV-1 inhibitors. We have shown that an extended MPR peptide, P5, harbouring the lectin-like domain of gp41 and a calcium-binding site, is implicated in the interaction of HIV with its mucosal receptor. We now investigate the potential antiviral activities of P5 and other such long MPR-derived peptides. Structural studies of gp41 MPR-derived peptides using circular dichroism showed that the peptides P5 (a.a.628–683), P1 (a.a.648–683), P5L (a.a.613–683) and P7 (a.a.613–746) displayed a well-defined α-helical structure. Peptides P5 inhibited HIV-1 envelope mediated cell-cell fusion and infection of peripheral blood mononuclear cells by both X4- and R5-tropic HIV-1 strains, whereas peptides P5 mutated in the calcium binding site or P1 lacked antiviral activity, when P5L blocked cell fusion in contrast to P7. Strikingly, P5 inhibited CD4-dependent infection by T20-resistant R5-tropic HIV-1 variants. Cell-cell fusion studies indicated that the anti-HIV-1 activity of P5, unlike T20, could not be abrogated in the presence of the N-terminal leucine zipper domain (LZ). These results suggested that P5 could serve as a potent fusion inhibitor.

The binding of HIV-1 gp41 membrane proximal domain to its mucosal receptor, galactosyl ceramide, is structure-dependent

The peptide of HIV-1 envelope gp41 (a.a 628-683), referred to herein as P5, contains P1, a conserved galactose-specific lectin domain for binding the mucosal HIV-1-receptor, galactosyl ceramide (GalCer), as shown earlier, and a potential calcium-binding site (a.a 628-648). P1 contains contiguous epitopes recognized by the broadly neutralizing antibodies 2F5, 4E10, Z13. However, similar neutralizing antibodies could not be raised in animal model using immunogens based on these epitopes. We now show that the structure of both P5 and P1 peptides, as measured by circular dichroism, differs according to their environment: aqueous or lipidic, and as a function of calcium concentration. P5, but not P1, binds to calcium with a low binding affinity constant in the order of 2.5x10(4). Calcium binding results in a conformational change of P5, leading in turn to a decrease in affinity for GalCer. Hence, the affinity of the gp41-lectin site for the galactose harbored by the mucosal HIV-1 receptor GalCer is modulated by the peptide secondary and tertiary structure and the local environment. Therefore, definition of the conformation of this novel extended gp41 membrane proximal region, containing the conserved peptide P1 and the Ca(2+) binding site, could help designing an immunogen efficient at inducing neutralizing anti-HIV-1 antibodies.

Humoral immune responses by prime-boost heterologous route immunizations with CTB-MPR649–684, a mucosal subunit HIV/AIDS vaccine candidate

CTB-MPR(649-684) is a translational fusion protein consisting of the cholera toxin B subunit and a 36-residue peptide, MPR(649-684), corresponding to the conserved membrane proximal ectodomain of gp41. CTB-MPR(649-684) was previously shown to induce HIV-1 transcytosis-blocking antibodies in mice. In this report, we describe an effective immunization regimen for this novel anti HIV-1 vaccine-candidate. Bacterially-produced CTB-MPR(649-684) was intranasally and/or intraperitoneally administered to investigate several prime-boost heterologous route immunization regimens. Mucosal priming with the adjuvant cholera toxin elicited significant levels of vaginal IgA and serum IgG specific to MPR(649-684). Systemic boosting after mucosal priming enhanced the levels of serum and mucosal antibodies. Systemic priming induced a strong serum anti-MPR(649-684) IgG response, which was efficiently recalled and augmented by either systemic or mucosal boosting. However, this regimen was less effective in inducing secretory anti-MPR(649-684) IgA. The serum anti-MPR(649-684) IgG subtype profile revealed that both IgG1 and IgG2a were induced in all the immunization regimens, and that mucosal co-administration of cholera toxin shifted the bias to the latter subtype. We concluded that, of the various immunization regimens examined here, mucosal priming with adjuvant followed by systemic boosting exhibited the best response in respect to either systemic or mucosal anti-MPR(649-684) antibodies. Most importantly, mucosal antibodies elicited by this regimen significantly inhibited HIV-1 transcytosis in a human tight epithelium model.

HIV-1-infected blood mononuclear cells form an integrin- and agrin-dependent viral synapse to induce efficient HIV-1 transcytosis across epithelial cell monolayer

The heparan sulfate proteoglycan agrin and adhesion molecules are key players in the formation of neuronal and immune synapses that evolved for efficient communication at the sites of cell-cell contact. Transcytosis of infectious virus across epithelial cells upon contact between HIV-1-infected cells and the mucosal pole of the epithelial cells is one mechanism for HIV-1 entry at mucosal sites. In contrast, transcytosis of cell-free HIV-1 is not efficient. A synapse between HIV-1-infected cells and the mucosal epithelial surface that resembles neuronal and immune synapses is visualized by electron microscopy. We have termed this the "viral synapse." Similarities of the viral synapse also extend to the functional level. HIV-1-infected cell-induced transcytosis depends on RGD-dependent integrins and efficient cell-free virus transcytosis is inducible upon RGD-dependent integrin cross-linking. Agrin appears differentially expressed at the apical epithelial surface and acts as an HIV-1 attachment receptor. Envelope glycoprotein subunit gp41 binds specifically to agrin, reinforcing the interaction of gp41 to its epithelial receptor galactosyl ceramide.

A mucosally targeted subunit vaccine candidate eliciting HIV-1 transcytosis-blocking Abs

A vaccine that would engage the mucosal immune system against a broad range of HIV-1 subtypes and prevent epithelial transmission is highly desirable. Here we report fusing the mucosal targeting B subunit of cholera toxin to the conserved galactosylceramide-binding domain (including the ELDKWA-neutralizing epitope) of the HIV-1 gp41 envelope protein, which mediates the transcytosis of HIV-1 across the mucosal epithelia. Chimeric protein expressed in bacteria or plants assembled into oligomers that were capable of binding galactosyl-ceramide and G(M1) gangliosides. Mucosal (intranasal) administration in mice of the purified chimeric protein followed by an i.p. boost resulted in transcytosis-neutralizing serum IgG and mucosal IgA responses and induced immunological memory. Plant production of mucosally targeted immunogens could be particularly useful for immunization programs in developing countries, where desirable product traits include low cost of manufacture, heat stability, and needle-free delivery.

Environmental Factors in HIV/AIDS Epidemic Development: New Perspectives for Gender Equity and Global Protection against HIV Transmission

The HIV/AIDS epidemic is increasingly regarded as a socioeconomic problem. Among factors causing poverty, cultural aspects, including religion and traditions, appear to play an essential role in the rapid and global development of AIDS epidemic. AIDS is a pathologic syndrome caused by the human immunodeficiency virus (HIV). Scientific knowledge is required to prevent and treat AIDS. Although considerable progress has been made in antiretroviral therapy, neither actual cure of HIV infection, nor an efficient protection method, nor a vaccine are currently globally accessible. Consequently, the funding of scientific research is of utmost importance. On the basis of recent scientific findings, new perspectives for global protection and gender equity against HIV transmission are emerging. Progress is being made in developing microbicides or virucides, anti-infective medication formulated for topical self-administration, to protect against HIV and other sexually transmitted pathogens. Such developments need to be supported by extensive education campaigns geared to women to give them the possibility of protecting themselves and their children from HIV transmission. The level of funding for microbicide and vaccine development needs to be greatly increased. New possibilities have emerged for an efficient vaccine which would engage the mucosal immune system, first involved in the sexual transmission of HIV-1. The idea of vaccine production in edible tissues of transgenic crop plants has also gained momentum. The use of minimally processed, low-cost, orally delivered immunogens is especially valuable when raising mucosal antibodies is the object and when frequent boosting is anticipated, as is the case for mucosal immunity.

Entry of viruses through the epithelial barrier: pathogenic trickery

Mucosal surfaces--such as the lining of the gut or the reproductive tract--are the main point of entry for viruses into the body. As such, almost all viruses interact with epithelial cells, and make use of the normal epithelial signalling and trafficking pathways of the host cell. In addition to protein receptors, carbohydrate chains of proteoglycans and epithelial-membrane glycosphingolipids have emerged as a new class of receptors for viral attachment to the host cell.

HIV-1 gp41 envelope residues 650-685 exposed on native virus act as a lectin to bind epithelial cell galactosyl ceramide

The initial step in the interaction between human immunodeficiency virus (HIV-1) and epithelial cells is the binding of HIV-1 envelope glycoproteins to the epithelial cell galactosyl ceramide (GalCer). Here we show that HIV-1 envelope gp41 residues 650-685 bind GalCer in a galactose-specific manner. The gp41 residues that display this lectin activity are highly conserved among HIV-1 isolates and constitute three regions: residues 650-661, which encompass a charged helix; residues 662-667, referred to as the conserved epitope ELDKWA, the epitope recognized by antibodies that neutralize HIV-1 entry in epithelial and CD4(+)-mononucleated cells; and residues 668-685, a hydrophobic Trp-rich sequence that stabilizes the structure of the galactose binding site. Similar to other galactose-specific lectins, the gp41 lectin site is active only as an oligomer. Finally the orientation of the galactose toward the gp41 lectin site appears to be controlled by the lipid microenvironment of the epithelial membrane. From the experimental data we construct a theoretical model of the interaction between gp41 and GalCer based on thermodynamic considerations. This model integrates the dynamics and the spatial organization of the viral envelope glycoproteins, GalCer organized in raft microdomains in the apical region of the epithelial cell membrane and the interfacial water. Characterization of the minimal sequence and structure of gp41 in direct interaction with GalCer may help unravel the still unknown immunogenic determinant able to elicit antibodies against ELDKWA and target of one of the rare neutralizing antibodies against gp41.

Secretory IgA specific for a conserved epitope on gp41 envelope glycoprotein inhibits epithelial transcytosis of HIV-1

As one of the initial mucosal transmission pathways of HIV (HIV-1), epithelial cells translocate HIV-1 from apical to basolateral surface by nondegradative transcytosis. Transcytosis is initiated when HIV-1 envelope glycoproteins bind to the epithelial cell membrane. Here we show that the transmembrane gp41 subunit of the viral envelope binds to the epithelial glycosphingolipid galactosyl ceramide (Gal Cer), an alternative receptor for HIV-1, at a site involving the conserved ELDKWA epitope. Disrupting the raft organization of the Gal Cer-containing microdomains at the apical surface inhibited HIV-1 transcytosis. Immunological studies confirmed the critical role of the conserved ELDKWA hexapeptide in HIV-1 transcytosis. Mucosal IgA, but not IgG, from seropositive subjects targeted the conserved peptide, neutralized gp41 binding to Gal Cer, and blocked HIV-1 transcytosis. These results underscore the important role of secretory IgA in designing strategies for mucosal protection against HIV-1 infection.

Dictyostelium discoideum cells shed vesicles with associated DNA and vital stain Hoechst 33342

Dictyostelium discoideum cells are highly resistant to xenobiotics. We previously observed that these primitive eukaryotic cells contain a 170-kDa P-glycoprotein, mediating multidrug resistance in mammalian cells, but nonfunctional in Dictyostelium cells. We show here that D. discoideum cells vitally stained with the DNA-specific dye, Hoechst 33342, release fluorescent material in their culture medium. Electron microscopy and lipid analysis demonstrate the vesicular nature of this material. Moreover, nucleic acids associate with these extracellular vesicles independently of Hoechst vital staining. The main vesicular DNA component exhibits a size > 21 kb. Shedding of microvesicles during cell growth is not concomitant with programmed cell death. We propose that these extracellular vesicles are involved in a new cellular resistance mechanism against xenobiotics. Furthermore, since the association of DNA with vesicles occurs in physiological growth conditions and independently of vital staining, the new shedding process might be involved in a more general intercellular mechanism.

In adrenocortical tissue, annexins II and VI are attached to clathrin coated vesicles in a calcium-independent manner

We have previously characterized three populations of clathrin coated vesicles (CCVs) isolated from bovine adrenocortical tissue and designated them as large, medium and small coated vesicles, i.e., LCV, MCV and SCV, respectively. Here, we show that annexins II and VI, two of the annexins involved in membrane traffic, are present in the three populations of CCVs but with different distributions between coat proteins (CP) and lipidic vesicle membrane. Annexin VI is only associated with the membrane, whatever the CCV population. In contrast, annexin II is differently distributed between coat and membrane, depending on the CCV population. Both annexins are bound to membranes in a calcium-independent manner and solubilization studies in Triton X114 (TX114) suggest that they interact poorly with lipids by hydrophobic interactions. Ligand blotting experiments show that both annexins bind to CCV proteins: annexin II to a 200-kDa component in all CCVs and annexin VI to a 100-kDa component in LCV and SCV identified as dynamin, a GTPase essential for endocytic CCV pinching off. Dynamin is tightly associated to annexin VI only in LCVs, the endocytic [transferrin (Tf) positive] vesicles. Our data suggest that annexins II and VI could define specific protein-lipid interaction microdomains that could play a role in the different functions of the CCVs.

Specific lipid protein interactions characterize 3 populations of clathrin coated vesicles involved in the LDL receptor traffic

We have previously isolated 3 different populations of clathrin coated vesicles (CCV) involved in the LDL-receptor traffic in bovine adrenal cortex. We now show that each CCV type contains the transferrin-R and the CI-MPR, therefore, they provide a good model for studying the membrane organization that may govern their targeting in one of the biosynthetic, endocytic and/or recycling pathways. Transferrin--prototype of recylcing ligand--, and alpha adaptin, dynamin and the 110 kDa phosphatidylinositol-3-kinase subunit--of the trafficking machinery--were mainly detected in only 2 of the vesicle populations which could be involved in the endocytic/recycling pathway. The third population which contained larger amounts of gamma adaptin and do not carry transferrin could be involved in the biosynthetic pathway. The vesicle lipid pattern and the saturation of their fatty acyl chains were analyzed and confirmed these results. The nature of the interactions between vesicle components was then determined using several classes of detergents. Only non ionic ones could solubilize the LDL-R in a complex with either alpha or gamma adaptin. In contrast, they dissociated clathrin or beta-beta' adaptins. Taken together these results prompt us to suggest an integrated model for targeting in membrane traffic. Besides specific targeting signals carried by cargo proteins and recognized by proteins from the coat and the cytosolic trafficking machinery, lipids would play a key modulatory role. At each step in the membrane traffic, the proteins which carry multiple targeting signals would interact transiently with a specific set of lipids. This would result in the exposure of the appropriate targeting signals which could now become recognized by the proper targeting machinery.

Annexins from bovine adrenal cortex exhibit specific cytosol/membrane solvation

Cytosol/membrane localization of annexins I to VI was analyzed in tissue extracts from bovine adrenal cortex. Based on their solubility in either aqueous or detergents solutions, they were subfractionated in three groups named cytosolic (C), membrane-bound (MB) and membrane-inserted (MI). Less than 1% of the total annexins present in the tissue were recovered in the C fraction when as much as 76.5 and 22.5% were obtained respectively in the MB and the MI fractions. By immunoblotting after SDS-PAGE, it was shown that the various members of the annexin family were not equally recovered in the different fractions. A-V and A-VI were found present in the three fractions whereas the distribution of A-I, A-II, A-III and A-IV was distinct, suggesting different cellular functions.

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.

Influence of lactoferrin on the entry process of Escherichia coli HB101 (pRI203) in HeLa cells

Lactoferrin (Lf) is an iron-binding protein which plays an important role in the host defense systems of different mucosal surfaces including the intestinal mucosa. In the present research the role of apo-Lf and iron-saturated Lf in the invasion process of enteroinvasive bacteria, grown in iron stress or excess, was investigated. As enteroinvasive bacterium, Escherichia coli HB101 strain harboring a plasmid which contains the chromosomal inv gene from Yersinia pseudotuberculosis was utilized. The product of this gene (invasin) enables this microorganism to invade human epithelial cultured cells (HeLa). The results obtained showed that apo-Lf and iron-saturated Lf added at physiological concentration during the infection exerted a significant inhibition of adhesion (3.2 x 10(5) instead 3.4 x 10(6) adherent bacteria grown in iron excess; 1.6 x 10(3) instead of 2.3 x 10(4) adherent bacteria grown in iron-limited medium) and internalization (4.0 x 10(5) instead of 3.7 x 10(6) internalized bacteria grown in iron excess; 2.1 x 10(3) instead 2.8 x 10(4) internalized bacteria grown in iron-limited medium). It has also been demonstrated that in these experimental conditions Lf binds to HeLa cell membrane as well as to bacterial outer membrane. It is likely that this binding interfere with the early events of interaction between bacteria and eukaryotic cells. This inhibiting effect of Lf on the invasion efficiency of E. coli HB101 (pRI203) could be related to the cationic nature of the molecule, although other mechanisms cannot be ruled out.

Binding and internalization of ricin labelled with fluorescein isothiocyanate

The toxic lectin ricin has been covalently labelled with fluorescein isothiocyanate on the enzymatically active A chain. The fluorescein reacted toxin maintains its biological activity. The lateral diffusion coefficient of cell surface bound ricin, studied in two cell lines by fluorescence photobleaching recovery, is D = 1 - 2 x 10(-10) cm2/s. Fluorescence microscopy provides preliminary evidence for secondary endosomes in the cytoplasm.

Biochemical characterization of algal coated vesicles

Thin sections of tissue preparations from a green alga, Ulva lactuca (Ulvophyceae), and brown alga, Laminaria digitata (Pheophyceae) showed the presence of coated pits and coated vesicles in these 2 species. A discontinuous sucrose gradient after subcellular fractionation of the tissue homogenate resulted in an enriched coated vesicle fraction. Electron microscopy of negatively stained samples revealed the presence of coated vesicles of diameter ranging from 40-125 nm, together with large sheets of polygonal nets of clathrin. Electrophoresis of the CV purified fraction revealed various polypeptide components. Two of them, a 175 kDa and a 70 kDa, exhibited a positive response to bovine brain anticlathrin antibodies raised in goat or in rabbit. A third component of 30-40 kDa also gave a faint positive response. These 3 components corresponded to the clathrin heavy and light chains already described in higher plants. Clathrin was released from the CV algal preparations by treatment with 2M urea in Tris buffer, pH 8.5. Interestingly, in Ulva lactuca, the proportion of clathrin relative to the other proteins from the CV decreased with plant growth. Biochemical analysis of the purified CV revealed the presence of all the major phospholipids characterized in mammalian CV. The ratio of protein over lipid was also in the same range as that calculated for mammalian CV. Carbohydrate analysis demonstrated a high proportion of N-acetylgalactosamine and N-acetylglucosamine in both algal CV whereas these sugars were not detectable in the crude homogenate. These results demonstrate the presence of clathrin and coated vesicles in 2 species of algae.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro insertion of the myelin proteolipid apoprotein into oligodendrocyte plasma membranes

Uncoated vesicles (UCV) loaded with the myelin proteolipid apoprotein covalently tagged with fluorescein (PLPF) were found to interact with isolated oligodendrocytes from bovine brain at 4 degrees C as well as at 37 degrees C. After 1.5 hours of incubation, the labeled protein was localized in the cell membranes. After 2.5 hours the fluorescence intensity associated with the oligodendrocytes decreased and completely disappeared at t = 3.5 hours. Addition of KCl or EDTA in the incubation medium significantly hindered the interaction with cells. In contrast, the elimination of membrane proteins from UCV did not perturb cell labeling. A specific role of PLP was suggested since UCV loaded with a soluble protein (BSAF) led to a weak cell labeling.

Demonstration of cell-ricin interaction by electrophoretic light scattering

Electrophoretic light scattering has been used to investigate the interaction of ricin, a vegetal toxin, with cells. This technique allowed measurements in the presence of free ligand and proved particularly useful for the study of a system with low affinity. The electrophoretic mobility of erythrocytes and oligodendrocytes was found equal to 2.08 x 10(-8) and 2.35 x 10(-8)m2s-1V-1, respectively. Upon ricin binding, these values decreased significantly. This change was related to the saturation of the binding sites. The specificity of the interaction was demonstrated by conducting the experiments in the presence of lactose. This specific inhibitor fully prevented the ricin-cell interaction.

Comparative study of myelin proteolipid apoprotein solvation by multilayer membranes of synthetic DPPC and biological lipid extract from bovine brain. An FT-IR investigation

The interaction between the aqueous form of the myelin proteolipid apoprotein (PLA) and model membranes prepared with either synthetic dipalmitoylphosphatidyl choline (DPPC) or biological lipids extracted from bovine brain (BE) has been investigated by Fourier-Transform IR spectroscopy. IR spectra obtained with lyophilized samples of PLA demonstrated 2 main peaks (amide I and amide II) culminating at 1656 cm-1 and 1545 cm-1, which we assigned to helical conformation. When PLA was solvated in DPPC or BE membranes, both the amide I and amide II features remained located at 1655 cm-1 and 1545 cm-1, although their half-width significantly decreased, demonstrating that the lipid environment favoured alpha helix structures. However differences between both mixtures were detected by measuring the amide I and amide II half-widths as a function of the L:P molar ratio. Moreover, analysis of the 1545/1515 peak intensity ratio brought evidence of different localization and/or molecular arrangement of the protein segments containing tyrosine residues, depending on the lipid composition of the membrane. According to previously published models, these data suggest that recombinants prepared with PLA and BE multilayers better mimic the biological membrane than do DPPC-PLA mixtures.

Biochemical and functional characterization of three types of coated vesicles in bovine adrenocortical cells: implication in the intracellular traffic

Three populations of pure coated vesicles from adrenocortical cells, differing in their density, i.e., 1.125-1.155, 1.155-1.175, and 1.175-1.210 g/cm3, are obtained after separation on two successive sucrose-2H2O gradients. They are involved in LDL internalization and in the receptor cycle as confirmed by the presence, in each population, of the LDL receptor. Electron micrographs confirm the existence of three homogeneous populations exhibiting the typical polygonal structure of the clathrin coat. They differ in their size distribution (small, congruent to 70-nm diameter; medium, congruent to 90-nm diameter; large, congruent to 110-nm diameter) and in the organization of clathrin and of the coat proteins as evidenced on electrophoreses carried out under nondenaturing and denaturing conditions. Activity measurements of marker enzymes, phosphodiesterase and galactosyltransferase, suggest that medium coated vesicles might originate from plasma membranes and small ones from the Golgi complex. Large coated vesicles exhibit phosphokinase enzyme and substrate polypeptides different from those of the two other populations, tubulins being the preferred kinase substrates for the small and medium coated vesicles. These kinases are autophosphorylating enzymes and are revealed, by nondenaturing electrophoreses, as different high molecular mass complexes in the three populations. Clathrin and coat proteins are not part of these complexes.

Separation of ricin A- and B-chains after dithiothreitol reduction

After complete cleavage of ricin interchain disulfide bridge by 0.05 M dithiothreitol in nondenaturing conditions at 37 degrees C during 1 h 30 min, A- and B-chains were separated on a lactosaminyl-aminoethyl Biogel P-150 column at 4 degrees C, in the presence of 0.01 M dithiothreitol and 0.5 M MgCl2. A- and B-chains have been characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunology. Their specific activities have been tested by protein synthesis inhibition in a cell-free assay (rabbit reticulocyte lysate) and on whole cells (Zajdela hepatoma cells) and by hemagglutination. From these tests, the apparent cross contamination of the chains was about 0.1%.

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.

[Homology between antigenic determinants of bovine clathrin and clathrin from the brown alga Laminaria digitata]

Coated vesicles, essential organelles of intracellular membrane traffic, have been extensively studied in animal and higher plant cells. In the algae, cytological studies only have been performed which demonstrate the presence of such coated vesicles with their surrounding clathrin lattice. The present work has been carried out on coated vesicles isolated for the first time from the brown algae Laminaria digitata. For comparison of the antigenic characteristics of clathrin prepared from the Bovine brain or adrenocortical cells and the clathrin prepared from algae, polyclonal antibodies have been raised to a purified Bovine brain clathrin in Goat and to Bovine adrenocortical clathrin in Rabbit. The positive immunological responses of the coated vesicles and the clathrin from Algae to these antibodies, evidence an homology between antigenic determinants of clathrin from animal and vegetal 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.

Lipid solvation of the aqueous form of the myelin proteolipid apoprotein: evidence and characterization of two lipid populations by fluorescence polarization, differential calorimetry, and sucrose gradient centrifugation

The interaction between dipalmitoylphosphatidylcholine (DPPC) and the aqueous form of the myelin proteolipid apoprotein (PLA) has been investigated. Lyophilization was found to be an efficient and nonperturbing method for membrane reconstitution. Mixtures of different lipid/protein ratios were analyzed by means of differential calorimetry, fluorescence polarization, and sucrose gradient centrifugation. The presence of two coexisting lipid populations, termed "bulk" and "interacting" lipids, was demonstrated by these three techniques. By differential calorimetry, 23 DPPC molecules per molecule of protein (30 kDa) were shown to be excluded from the lipid phase transition. By fluorescence polarization, we detected above the phase-transition temperature a large perturbation of the lipid acyl chain dynamics induced by the aqueous form of PLA. Increasing the protein content above 35% by weight within the recombinants caused drastic changes in both delta H values and the fluorescence anisotropy parameter, which could stem from protein aggregation.

Ligand-blotting visualization of the LDL receptor in plasma membranes and in two classes of coated vesicles from adrenocortical cells

Two populations of coated vesicles, different in size, have been isolated from the bovine adrenal cortex. The enrichment of the LDL receptor from the plasma membrane to the large coated vesicles and then to the small ones was evidenced by ligand-blotting ELISA assays. The LDL receptor has been characterized as a 130-kDa proteic component which retains the binding specificity and structural features in plasma membranes as well as in the two classes of coated vesicles.

Structural characterization of labeled clathrin and coated vesicles

Clathrin (8 S) and coated vesicles have been covalently labeled by using the sulfhydryl-labeling fluorescent probe N-(1-anilinonaphthalene)maleimide. A large increase in energy transfer from Trp to anilinonaphthalene (AN) residues was observed in clathrin in the pH range approximately 6.5-6.0, where the rate of clathrin self-association increased rapidly. The change in energy transfer was indicative of a conformational rearrangement, which could be responsible for the initiation of the clathrin self-association reaction to form coat structure. The AN label was found in both the coat and membrane proteins after dissociation of coated vesicles at pH 8.5. The labeled coat and membrane proteins readily recombined to form coated vesicles after reducing the pH to 6.5, indicating that the labeling did not interfere with the ability of clathrin to self-associate and interact with uncoated vesicles to form coat structure. A comparison of the AN fluorescence with the Coomassie blue pattern after electrophoresis in sodium dodecyl sulfate-gels revealed that a 180,000-Da protein (clathrin) was mainly labeled in coated vesicles, while a 110,000-Da protein was also strongly labeled in uncoated vesicles. AN-labeled baskets and coated vesicles have been prepared. Trypsin digestion reduced the sedimentation rate of baskets from 150 S to 120 S and of coated vesicles from 200 S to 150 S. Gel electrophoresis of baskets and coated vesicles showed extensive conversion of clathrin (Mr 180,000) to a product of Mr approximately equal to 110,000, suggesting equivalent structural organization of the coat in coated vesicles as in baskets. In both cases, the peptide(s) released from the vesicles by digestion were essentially free of fluorescent label. In the case of the uncoated vesicles, tryptic digestion released most of the proteins remaining after coat removal.

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

Three characteristics have been demonstrated by the chemical analysis of bovine brain coated vesicles following removal of the coat proteins: a high protein content, a high cholesterol/lipid ratio and a high percentage of phosphatidylethanolamine amongst the phospholipids. The study of lipid bilayer organization and dynamics has been performed using the fluorescent probes pyrene and parinaric acid (cis and trans). This has allowed the study of both lateral mobility and rotational motion in the lipid bilayer of the coated and uncoated vesicles. Lateral mobility in the fluid phase of the lipid is slightly reduced by the presence of the clathrin coat, as indicated by the lower diffusion coefficient of pyrene in coated compared with uncoated vesicles. At all temperatures from 6 degrees to 30 degrees C, solid-phase domains, probed by trans parinaric acid, coexist with fluid-phase domains in the lipid bilayer. The temperature dependence of the parinaric acid lifetimes and of their amplitudes strongly suggests that the solid phase domains decrease in size with temperature, both in coated and uncoated vesicles. However, the difference in the value of the anisotropy at long times (r infinity), between coated and uncoated vesicles (a difference which is more pronounced for cis than for trans parinaric acid), indicates that the presence of the clathrin coat introduces disorder in the surrounding lipids, thus suggesting a possible role of the clathrin in the formation of the pits on the plasma membrane.

Effect of cholesterol and protein content on membrane fluidity and 3 beta-hydroxysteroid dehydrogenase activity in mitochondrial inner membranes of bovine adrenal cortex

The steroid biosynthetic enzymes in the adrenal cortex are localised in endoplasmic reticulum and mitochondrial membranes. For some of the enzymes in endoplasmic reticulum the activity appears to be modulated by lipid fluidity, (21-hydroxysteroid hydroxylase and 3 beta-hydroxysteroid dehydrogenase). A mechanism for the regulation of corticosteroid biosynthesis mediated by the membrane fluidity has been suggested. Therefore a study of the mitochondrial inner membrane of the bovine adrenal cortex has been undertaken in comparison with a previous study of the endoplasmic reticulum. The kinetic parameters of the 3 beta-hydroxysteroid dehydrogenase were studied as a function of pH and temperature. No thermal transition can be observed in the Arrhenius plot for this enzyme in contrast with the results obtained for the microsomal enzyme. Membrane fluidity using, as fluorescent probes, diphenylhexatriene and a set of n-(9-anthroyloxy)fatty acids has been also studied as a function of temperature with or without addition of cholesterol. No thermal transition in the lipid phase can be observed. The addition of cholesterol to total mitochondrial membrane as to a lipid extract of the membrane decreases fluidity to the same extent as it does with microsomes. The presence of a large amount of protein in mitochondria has an effect which is additive to that of the cholesterol.

Reversibility of coated vesicle dissociation

The dissociation of the coated vesicles to clathrin and uncoated vesicles and their reassociation have been studied under various conditions. The extent of reassociation is pH dependent and increases slightly with increasing concentrations of the components. Unlike the self-association of clathrin which is strongly salt dependent, the reassociation of clathrin and uncoated vesicles is practically independent of salt concentration. The coated vesicle gradually loses its coat with increasing pH, and the dissociation process is not an all or none reaction. Ca2+ inhibits dissociation of the coated vesicles and enhances the reassociation of clathrin and uncoated vesicles. Our results show that, although many conditions result in reassociation of protein and lipid vesicle, few conditions result in vesicles of both the same size and composition as native coated vesicles.

Dynamic structure of bovine adrenal cortex microsomal membranes studied by time-resolved fluorescence anisotropy of all-trans-1,6-diphenyl-1,3,5-hexatriene

The motional properties of all-trans-1,6-diphenyl-1,3,5-hexatriene embedded in the microsomal membranes and in extracted lipid liposomes from bovine adrenal cortex are studied by fluorescence anisotropy decay measurements. The results demonstrate the existence of a nonvanishing value of the anisotropy (r infinity) in both systems. The r infinity values are significantly higher in the total membranes than in the protein-depleted liposomes, following the trend of the steady state anisotropy values previously reported (Gallay, J., Vincent, M., de Paillerets, C., Rogard, M., and Alfsen, A. (1981) J. Biol. Chem. 256, 1235-1241). This indicates that the depolarizing rotations of the probe are more restricted in the presence of proteins. No detectable effect of proteins on the reorientational correlation time can be observed. The temperature dependence of r infinity is not similar in both systems. In the complete membranes, a sharp decrease of r infinity value takes place between 27 and 32 degrees C, corresponding to 1/3 of the total curve of r infinity versus temperature is translated to lower temperatures. Thermal dependence of the reorientational correlation time is essentially the same in lipids and in complete membranes with an activation energy of 5 kcal mol-1. The results suggest that proteins play an essential role in organizing the deepest lipid region in the microsomal membranes from bovine adrenal cortex.

Nanosecond fluorescence anisotropy decays of n-(9-anthroyloxy) fatty acids in dipalmitoylphosphatidylcholine vesicles with regard to isotropic solvents

A set of n-(9-anthroyloxy) fatty acids [2-, 7-, 9-, and 12-(9-anthroyloxy)stearic acid (AS) and 16-(9-anthroyloxy)palmitic acid (16-AP)] has been studied by time-resolved and steady-state fluorescence anisotropy measurements in isotropic media (i.e., propylene glycol and a liquid paraffinic oil, Primol 342) and in vesicles of dipalmitoylphosphatidylcholine. The two modes of rotation, "in-plane" and "out-of-plane", of the anthroyl ring can be detected by varying the excitation wavelength. In both isotropic solvents, the value of the in-plane rotational rate is of the same order of magnitude as the out-of-plane rate for each one of the n-(9-anthroyloxy) fatty acids. In propylene glycol, the anthroyl ring motions are similar for all derivatives except for the 16-AP for which the fluorophore rotates at a higher rate. In the liquid paraffinic oil, identical motions of the fluorophore are observed for the 7-, 9-, and 12-AS; the motion for the 16-AP is again faster, while that for the 2-AS is slower. Moreover, the fluorophore motion for each probe is faster in this solvent that in propylene glycol in conditions of identical viscosity. When embedded in phospholipid bilayers, these probes report the microenvironment at a graded series of depths from the surface to the center of the bilayer [Haigh, E. A., Thulborn, K. R., & Sawyer, W. H. (1979) Biochemistry 18, 3525--3532]. Studies in dipalmitoylphosphatidylcholine vesicles have been performed at three temperatures (21, 37, and 47 degrees C) corresponding to different lipid phases. The out-of-plane mode of rotation is unhindered as demonstrated by an anisotropy decay profile asymptotic to zero. Thus, evaluation of a membrane "fluidity" parameter at different depths of the bilayer is possible, even in the steady-state mode of observation. When the in-plane mode of rotation contributes to the anisotropy decay, a hindrance to the motion is observed below the gel to liquid-crystalline transition. Then information about lipid order can be obtained from the plateau value (r infinity) of the fluorescence anisotropy decay. In the pretransition temperature range (37 degrees C), the results evidence the existence of structural lipid changes mainly localized in the hydrophobic core of the bilayer. The main transition leads to a complete disappearance of the hindrances on the in-plane rotation.

Influence of the position of the double bond in steroid substrates on the efficiency of the proton-transfer reaction by Pseudomonas testosteroni 3-oxo-steroid delta 4-delta 5-isomerase

Studies of the proton-transfer reaction by Pseudomonas testosteroni 3-oxo steroid Delta(4)-Delta(5)-isomerase with Delta(5(6))- and Delta(5(10))-steroid substrates demonstrate the importance of the position of the double bond for the efficiency of the isomerization process. Thus 3-oxo-Delta(5(6))-substrates have markedly high k(cat.) values, whereas those of 3-oxo-Delta(5(10))-substrates are very low and their apparent K(m) values approach equilibrium dissociation constants. The first step in the isomerization process is: [Formula: see text] which is governed by the k(-1)/k(+1) ratio and is shown to be very similar for the two classes of substrates (3-oxo-Delta(5(6))- and -Delta(5(10))-steroids). They therefore differ in the steps distal to the initial formation of the Michaelis-Menten complex. The use of the deuterated androst-5(6)-ene-3,17-dione substrate enabled us to calculate individual rate constants k(+1) and k(-1) as well as to determine the apparent rate-limiting step in the isomerization process. With the deuterated oestr-5(10)-ene-3,17-dione substrate, no significant isotope effect was observed suggesting that a different rate-limiting step may be operative in this isomerization process. Data are presented that indicate that under optimal concentrations of the efficient androst-5(6)-ene-3,17-dione substrate, the forward reaction for ES complex formation (as defined by k(+1)) is limited only by diffusion and the apparent K(m) does not approach the equilibrium constant, suggesting that the evolution of this enzyme has proceeded close to ;catalytic perfection'.

Study of fluorescent tryptophyl residues and extrinsic probes for the characterization of molecular domains of Folch-Pi apoprotein

The highly hydrophobic myelin Folch-Pi apoprotein can be solubilized in organic as well as in aqueous media. In order to understand the molecular organization changes consecutive to changes in the solvent medium, the environment of intrinsic probes and extrinsic labels has been studied by fluorescence and accessibility to some reagents. In acqueous solution, only two tryptophan residues per protein molecule of 23,500 molecular weight have been shown to fluoresce, and their fluorescence characterisitics indicate an hydrophobic and/or constrained environment. Two ANS binding sites have also been observed having a high quenching effect on the intrinsic chromophore fluorescence. A large accessibility has been evidenced for the protein sulfhydryl groups in chloroform-methanol 2:1 (v/v), both by kinetic study of the protein reaction with a specific reagent, N-(1-anilino-naphtyl-4) maleimide, and by the fluorescence characteristics of this probe once linked to the protein. The free sulfhydryl groups were still reactive in acqueous solution, but extrinsic fluorescence of the labelled apoprotein transferred from chloroform-methanol 2:1 (v/v) into water gave evidence of constraints on the probe or on its environment. Such constraints may contribute to the solubilization in acqueous solution of this highly hydrophobic protein.

The molecular size and shape of the Folch-Pi apoprotein in aqueous and organic solvents

In 1% acetic acid, sedimentation velocity measurements and equilibrium ultracentrifuge experiments demonstrate that the Folch-Pi apoprotein is not monodisperse. The weight-average molecular weight calculated from ultracentrifuge experiments and combining sedimentation coefficient and viscosity measurements, ranged from 64000 to 80000. The intrinsic viscosity value suggests an asymetric shape for the apoprotein if a low value of hydration is considered. In dioxan/1% acetic acid (2:3, v/v) a smaller sedimentation coefficient was found, the intrinsic viscosity value remaining identical to that in 4% acetic acid. In pure 2-chloroethanol, light-scattering experiments led to a molecular weight of 165000 indicating that even in this solvent the protein is not monomeric. Intrinsic viscosity and light scattering measurements on the one hand, primary sequence on the other hand (six proline residues per monomer of Mr 23500) suggest that the molecule in 2-chloroethanol may consist of rod-like segments with flexible junctions.