Studies on glucosylceramidase binding to phosphatidylserine liposomes: the role of bilayer curvature

Proteomics study on the protective mechanism of soybean isoflavone against inflammation injury of bovine mammary epithelial cells induced by Streptococcus agalactiae

This study aimed to verify the anti-inflammatory effect of soybean isoflavones (SI) on the inflammatory response induced by Streptococcus agalactiae (S. agalactiae) of bovine mammary epithelial cells (bMECs) and to elucidate its possible mechanism. BMECs were pretreated with SI of different concentrations (20, 40, 60, 80, 100 μg/mL) for 0.5, 3, 6, 9, 12, 15, 18, 24 h. And then, S. agalactiae was used to infect bMECs for 6 h (MOI = 50:1) to establish the inflammation model. Cell viability, growth curves of S. agalactiae, cytotoxicity, and S. agalactiae invasion rate were determined. A proteomics technique was used to further detect differential proteins and enrichment pathways. SI (40 μg/mL) improved the viability of bMECs at 12 h (p < 0.05) and 60 and 80 μg/mL of SI greater (p < 0.01). Moreover, 60 μg/mL of SI protects cells from bacterial damage (p < 0.05). SI could inhibit S. agalactiae growth and internalization into bMECs in a time- and dose-dependent manner. In addition, proteomics results showed that 133 proteins were up-regulated and 89 proteins were down-regulated significantly. The differentially significantly expressed proteins (DSEPs) were mainly related to cell proliferation, differentiation, apoptosis, and migration. GO annotation showed that 222 DSEPs were divided into 23 biological processes (BP) terms, 14 cell components (CC) terms, and 12 molecular functions (MF) terms. DSEPs were significantly enriched in 10 pathways, of which the immune pathway was the main enrichment pathway.

Biophysical parameters of the Sec14 phospholipid exchange cycle - Effect of lipid packing in membranes

Sec14, a yeast phosphatidylinositol/phosphatidylcholine transfer protein, functions at the trans-Golgi membranes. It lacks domains involved in protein-protein or protein-lipid interactions and consists solely of the Sec14 domain; hence, the mechanism underlying Sec14 function at proper sites remains unclear. In this study, we focused on the lipid packing of membranes and evaluated its association with in vitro Sec14 lipid transfer activity. Phospholipid transfer assays using pyrene-labelled phosphatidylcholine suggested that increased membrane curvature as well as the incorporation of phosphatidylethanolamine accelerated the lipid transfer. The quantity of membrane-bound Sec14 significantly increased in these membranes, indicating that "packing defects" of the membranes promote the membrane binding and phospholipid transfer of Sec14. Increased levels of phospholipid unsaturation promoted Sec14-mediated PC transfer, but had little effect on the membrane binding of the protein. Our results demonstrate the possibility that the location and function of Sec14 are regulated by the lipid packing states produced by a translocase activity at the trans-Golgi network.

Chapter 4 - Reconstitution of membrane budding with unilamellar vesicles

Enveloped virus particles select their lipid-protein components and egress by budding from the host cell membranes. The matrix protein of many enveloped viruses has been proposed as a crucial element for viral budding; however, molecular mechanisms behind membrane remodeling by the matrix protein are yet to be unraveled. Here, we describe a set of in vitro functional reconstitution assays that allow quantitative evaluation of both, membrane binding and creation of membrane curvature by the matrix protein isolated from Newcastle Disease Virus. Individual budding events orchestrated by the matrix protein can be resolved in real time. The assays may be applied for direct reconstitution of the on-membrane action of cellular proteins involved in membrane curvature induction upon binding in vivo.

Saposin B mobilizes lipids from cholesterol-poor and bis(monoacylglycero)phosphate-rich membranes at acidic pH

Sphingolipid activator proteins (SAPs), GM2 activator protein (GM2AP) and saposins (Saps) A-D are small, enzymatically inactive glycoproteins of the lysosome. Despite of their sequence homology, these lipid-binding and -transfer proteins show different specificities and varying modes of action. Water-soluble SAPs facilitate the degradation of membrane-bound glycosphingolipids with short oligosaccharide chains by exohydrolases at the membrane-water interface. There is strong evidence that degradation of endocytosed components of the cell membrane takes place at intraendosomal and intralysosomal membranes. The inner membranes of the lysosome differ from the limiting membrane of the organelle in some typical ways: the inner vesicular membranes lack a protecting glycocalix, and they are almost free of cholesterol, but rich in bis(monoacylglycero)phosphate (BMP), the anionic marker lipid of lysosomes. In this study, we prepared glycosylated Sap-B free of other Saps by taking advantage of the Pichia pastoris expression system. We used immobilized liposomes as a model for intralysosomal vesicular membranes to probe their interaction with recombinantly expressed Sap-B. We monitored this interaction using SPR spectroscopy and an independent method based on the release of radioactively labelled lipids from liposomal membranes. We show that, after initial binding, Sap-B disturbs the membrane structure and mobilizes the lipids from it. Lipid mobilization is dependent on an acidic pH and the presence of anionic lipids, whereas cholesterol is able to stabilize the liposomes. We also show for the first time that glycosylation of Sap-B is essential to achieve its full lipid-extraction activity. Removal of the carbohydrate moiety of Sap-B reduces its membrane-destabilizing quality. An unglycosylated Sap-B variant, Asn215His, which causes a fatal sphingolipid storage disease, lost the ability to extract membrane lipids at acidic pH in the presence of BMP.

Saposin A Mobilizes Lipids from Low Cholesterol and High Bis(monoacylglycerol)phosphate-containing Membranes

Saposin A (Sap-A) is one of five known sphingolipid activator proteins required for the lysosomal degradation of sphingolipids and for the loading of lipid antigens onto antigen-presenting molecules of the CD1 type. Sap-A assists in the degradation of galactosylceramide by galactosylceramide-beta-galactosidase in vivo, which takes place at the surface of intraendosomal/intralysosomal vesicles. Sap-A is believed to mediate the interaction between the enzyme and its membrane-bound substrate. Its dysfunction causes a variant form of Krabbe disease. In the present study we prepared glycosylated Sap-A free of other Saps, taking advantage of the Pichia pastoris expression system. Using liposomes and surface plasmon resonance spectroscopy, we tested the binding and lipid mobilization capacity of Sap-A under different conditions. Along the endocytic pathway, the pH value decreases, and the lipid composition of intraendosomal and intralysosomal membranes changes drastically. In the inner membranes the cholesterol concentration decreases, and that of the anionic phospholipid bis(monoacylglycero)phosphate increases. Here, we show that Sap-A is able to bind to liposomes and to mobilize lipids out of them at acidic pH values below pH 4.7. Low cholesterol levels and increasing concentrations of bis(monoacylglycero)phosphate favor lipid extraction significantly. Galactosylceramide as a bilayer component is not essential for lipid mobilization by Sap-A, which requires intact disulfide bridges for activity. We also show for the first time that glycosylation of Sap-A is essential for its lipid extraction activity. Variant Sap-A proteins, which cause storage of galactosylceramide in humans (Krabbe disease, Spiegel, R., Bach, G., Sury, V., Mengistu, G., Meidan, B., Shalev, S., Shneor, Y., Mandel, H., and Zeigler, M. (2005) Mol. Genet. Metab. 84, 160-166) and in mutant mice (Matsuda, J., Vanier, M. T., Saito, Y., Tohyama, J., and Suzuki, K. (2001) Hum. Mol. Genet. 10, 1191-1199) are deficient in lipid extraction capacity.

Interaction(s) of rotavirus non-structural protein 4 (NSP4) C-terminal peptides with model membranes

Rotavirus is the major cause of dehydrating gastroenteritis in children and young animals. NSP4 (non-structural protein 4), a rotaviral non-structural glycoprotein and a peptide NSP4(114-135) (DKLTTREIEQVELLKRIYDKLT), corresponding to NSP4 amino acids 114-135, induce diarrhoeal disease in a neonatal mouse model and interact with model membranes that mimic caveolae. Correlation of the mechanisms of diarrhoea induction and membrane interactions by NSP4 protein and peptide remain unclear. Several additional NSP4 peptides were synthesized and their interactions with membranes studied by (i) CD, (ii) a filtration-binding assay and (iii) a fluorescent molecule leakage assay. Model membranes that varied in lipid compositions and radius of curvature were utilized to determine the compositional and structural requirements for optimal interaction with the peptides of NSP4. Similar to the intact protein and NSP4(114-135), peptides overlapping residues 114-135 had significantly higher affinities to membranes rich in negatively charged lipids, rich in cholesterol and with a high radius of curvature. In the leakage assay, small and large unilamellar vesicles loaded with the fluorophore/quencher pair 8-aminonaphthalene-1,3,6-trisulphonic acid disodium salt/p -xylene-bis-pyridinium bromide were incubated with the NSP4 peptides and monitored for membrane disruption by lipid reorganization or by pore formation. At a peptide concentration of 15 microM, none of the NSP4 peptides caused leakage. These results confirm that NSP4 interacts with caveolae-like membranes and the alpha-helical region of NSP4(114-135) comprises a membrane interaction domain that does not induce membrane disruption at physiological concentrations.

Phospholipid membrane interactions of saposin C: in situ atomic force microscopic study

Saposin C (Sap C) is a small glycoprotein required for hydrolysis of glucosylceramidase in lysosomes. The full activity of glucosylceramidase requires the presence of both Sap C and acidic phospholipids. Interaction between Sap C and acidic phospholipid-containing membranes, a crucial step for enzyme activation, is not fully understood. In this study, the dynamic process of Sap C interaction with acidic phospholipid-containing membranes was investigated in aqueous buffer using atomic force microscopy. Sap C induced two types of membrane restructuring: formation of patch-like structural domains and the occurrence of membrane destabilization. The former caused thickness increase whereas the latter caused thickness reduction in the gel-phase membrane bilayer, possibly as a result of lipid loss or an interdigitating process. Patch-like domain formation was independent of acidic phospholipids, whereas membrane destabilization is dependent on the presence and concentration of acidic phospholipids. Sap C effects on membrane restructuring were further studied using synthetic peptides. Synthetic peptides corresponding to the amphipathic alpha-helical domains 1 (designated "H1 peptide") and 2 (H2 peptide) of Sap C were used. Our results indicated that H2 contributed to domain formation but not to membrane destabilization, whereas H1 induced neither type of membrane restructuring. However, H1 was able to mimic Sap C's destabilization effect in conjunction with H2, but only when H1 was present first and H2 was added afterwards. This study provides an approach to investigate the structure-function aspects of Sap C interaction with phospholipid membranes, with insights into the mechanism(s) of Sap C-membrane interaction.

Stimulation of acid sphingomyelinase activity by lysosomal lipids and sphingolipid activator proteins

Acid sphingomyelinase is a water-soluble, lysosomal glycoprotein that catalyzes the degradation of membrane-bound sphingomyelin into phosphorylcholine and ceramide. Sphingomyelin itself is an important component of the extracellular leaflet of various cellular membranes. The aim of the present investigation was to study sphingomyelin hydrolysis as a membrane-bound process. We analyzed the degradation of sphingomyelin by recombinant, highly purified acid sphingomyelinase in a detergent-free, liposomal assay system. In order to mimic the in vivo intralysosomal conditions as closely as possible a number of negatively charged, lysosomally occuring lipids including bis(monoacylglycero)phosphate and phosphatidylinositol were incorporated into substrate-carrying liposomes. Dolichol and its phosphate ester dolicholphosphate were also included in this study. Bis(monoacylglycero)phosphate and phosphatidylinositol were both effective stimulators of sphingomyelin hydrolysis. Dolichol and dolicholphosphate also significantly increased sphingomyelin hydrolysis. The influence of membrane curvature was investigated by incorporating the substrate into small (SUVs) and large unilamellar vesicles (LUVs) with varying mean diameter. Degradation rates were substantially higher in SUVs than in LUVs. Surface plasmon resonance experiments demonstrated that acid sphingomyelinase binds strongly to lipid bilayers. This interaction is significantly enhanced by anionic lipids such as bis(monoacylglycero)phosphate. Under detergent-free conditions only the sphingolipid activator protein SAP-C had a pronounced influence on sphingomyelin degradation in both neutral and negatively charged liposomes, catalyzed by highly purified acid sphingomyelinase, while SAP-A, -B and -D had no noticeable effect on sphingomyelin degradation.

Further studies on the reconstitution of glucosylceramidase activity by Sap C and anionic phospholipids

The reconstitution of the activity of the lysosomal enzyme glucosylceramidase requires anionic phospholipids and, at least, a protein factor, saposin C (Sap C). We have previously proposed a mechanism for the glucosylceramidase activation [Vaccaro et al. (1993) FEBS Lett. 336, 159-162] which implies that Sap C promotes the association of the enzyme with anionic phospholipid-containing membranes, thus favoring the contact between the enzyme and its lipid substrate, glucosylceramide. We have further investigated the properties of Sap C using a fluorescent hydrophobic probe such as 4, 4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS). The binding between bis-ANS and Sap C was pH-dependent, indicating that protonation leads to increased exposure of hydrophobic surfaces of Sap C. The interaction of Sap C with membranes, triggered by the development of hydrophobic properties at low pH values, was affected by the content of anionic phospholipids, such as phosphatidylserine or phosphatidylinositol, suggesting that anionic phospholipids have the potential to modulate the insertion of Sap C in the hydrophobic environment of lysosomal membranes. We previously showed that Sap C and anionic phospholipids are both required for the binding of glucosylceramidase to large vesicles. We have presently observed that Sap C is able to promote the association of glucosylceramidase with the lipid surface only when anionic phospholipids exceed a concentration of 5-10%. This level can be reached by summing lower amounts of individual anionic phospholipids, since they have additive effects. The present data extend and refine our model of the mechanism of glucosylceramidase activation and stress the key role of pH, Sap C and anionic phospholipids in promoting the interaction of the enzyme with membranes.

Saposins and their interaction with lipids

The lysosomal degradation of several sphingolipids requires the presence of four small glycoproteins called saposins, generated by proteolytic processing of a common precursor, prosaposin. Saposins share several structural properties, including six similarly located cysteines forming three disulfide bridges with the same cysteine pairings. Recently it has been noted that also other proteins have the same polypeptide motif characterized by the similar location of six cysteines. These saposin-like (SAPLIP) proteins are surfactant protein B (SP-B), 'Entamoeba histolytica' pore-forming peptide, NK-lysin, acid sphingomyelinase and acyloxyacyl hydrolase. The structural homology and the conserved disulfide bridges suggest for all SAPLIPs a common fold, called 'saposin fold'. Up to now a precise fold, comprising five alpha-helices, has been established only for NK-lysin. Despite their similar structure each saposin promotes the degradation of specific sphingolipids in lysosomes, e.g. Sap B that of sulfatides and Sap C that of glucosylceramides. The different activities of the saposins must reside within the module of the alpha-helices and/or in additional specific regions of the molecule. It has been reported that saposins bind to lysosomal hydrolases and to several sphingolipids. Their structural and functional properties have been extensively reviewed and hypotheses regarding their molecular mechanisms of action have been proposed. Recent work of our group has evidenced a novel property of saposins: some of them undergo an acid-induced change in hydrophobicity that triggers their binding to phospholipid membranes. In this article we shortly review recent findings on the structure of saposins and on their interactions with lipids, with special attention to interactions with phospholipids. These findings offer a new approach for understanding the physiological role of saposins in lysosomes.

Lysosomal degradation on vesicular membrane surfaces. Enhanced glucosylceramide degradation by lysosomal anionic lipids and activators

According to a recent hypothesis (Sandhoff, K., and Kolter, T. (1996) Trends Cell Biol. 6, 98-103), glycolipids, which originate from the plasma membrane, are exposed to lysosomal degradation on the surface of intralysosomal vesicles. Taking the interaction of membrane-bound lipid substrates and lysosomal hydrolases as an experimental model, we studied the degradation of glucosylceramides with different acyl chain lengths by purified glucocerebrosidase in a detergent-free liposomal assay system. Our investigation focused on the stimulating effect induced by lysosomal components such as sphingolipid activator protein C (SAP-C or saposin C), anionic lysosomal lipids, bis(monoacylglycero)phosphate, and dolichol phosphate, as well as degradation products of lysosomal lipids, e.g. dolichols and free fatty acids. The size of the substrate-containing liposomal vesicles was varied in the study. Enzymatic hydrolysis of glucosylceramide carried by liposomes made of phosphatidylcholine and cholesterol was rather slow and only weakly accelerated by the addition of SAP-C. However, the incorporation of anionic lipids such as bis(monoacylglycero)phosphate, dolichol phosphate, and phosphatidylinositol into the substrate carrying liposomes stimulated glucosylceramide hydrolysis up to 30-fold. Dolichol was less effective. SAP-C activated glucosylceramide hydrolysis under a variety of experimental conditions and was especially effective for the increase of enzyme activity when anionic lipids were inserted into the liposomes. Glucosylceramides with short acyl chains were found to be degraded much faster than the natural substrates. Dilution experiments indicated that the added enzyme molecules associate at least partially with the membranes and act there. Surface plasmon resonance experiments demonstrated binding of SAP-C at concentrations up to 1 microM to liposomes. At higher concentrations (2.5 microM SAP-C), liposomal lipids were released from the liposome coated chip. A model for lysosomal glucosylceramide hydrolysis is discussed.

A UDP glucosyltransferase from Bacillus subtilis successively transfers up to four glucose residues to 1,2-diacylglycerol: expression of ypfP in Escherichia coli and structural analysis of its reaction products

We have isolated the ypfP gene (accession number P54166) from genomic DNA of Bacillus subtilis Marburg strain 60015 (Freese and Fortnagel, 1967) using PCR. After cloning and expression in E. coli, SDS-PAGE showed strong expression of a protein that had the predicted size of 43.6 kDa. Chromatographic analysis of the lipids extracted from the transformed E. coli revealed several new glycolipids. These glycolipids were isolated and their structures determined by nuclear magnetic resonance (NMR) and mass spectrometry. They were identified as 3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl]-1,2-diacylgl ycerol, 3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl-(1-->6)-O-bet a-D-glucopyranosyl]-1,2-diacylglycerol and 3-[O-beta-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl-(1-->6)-O-bet a-D-glucopyranosyl-(1-->6)-O-beta-D-glucopyranosyl]-1,2-diacylglycerol. The enzymatic activity expected to catalyse the synthesis of these compounds was confirmed by in vitro assays with radioactive substrates. In these assays, one additional glycolipid was formed and tentatively identified as 3-[O-beta-D-glucopyranosyl]-1,2-diacylglycerol, which was not detected in the lipid extract of transformed cells. Experiments with some of the above-described glycolipids as 14C-labelled sugar acceptors and unlabelled UDP-glucose as glucose donor suggest that the ypfP gene codes for a new processive UDP-glucose: 1,2-diacylglycerol-3-beta-D-glucosyl transferase. This glucosyltransferase can use diacylglycerol, monoglucosyl-diacylglycerol, diglucosyl diacylglycerol or triglucosyl diacylglycerol as sugar acceptor, which, apart from the first member, are formed by repetitive addition of a glucopyranosyl residue in beta (1-->6) linkage to the product of the preceding reaction.

Cucumber root lipoxygenase can act on acyl groups in phosphatidylcholine

A cDNA encoding cucumber root lipoxygenase was isolated and expressed in E. coli. The enzyme showed highest activity at pH 5.5 when alpha-linolenic acid dispersed with Tween 20 was used as a substrate but showed little activity at above pH 8.0. On the contrary, it showed the highest activity at pH 9.0 with trilinolenin emulsified with gum arabic. When the assay was performed with linolenic acid dispersed with different concentrations of Tween 20, little activity which could be seen up to the reaction solution became turbid as the linolenic acid/Tween 20 ratio increased, while the activity rapidly emerged afterward. The enzyme could also act on phosphatidylcholine, although the activity was strongly modified by freeze-thaw and sonication treatment on the lipid vesicles. Addition of deoxycholic acid to the phospholipid vesicles drastically enhanced the activity. Addition of free fatty acid was also revealed to be effective to enhance the activity. In the latter case, myristic acid exerted highest activity. Oleic acid enhanced the activity more highly than palmitic acid did. These lines of evidence suggested that the lipoxygenase strictly recognized a specific physical state of the phospholipid substrate in the reaction mixture. The enzyme was irreversibly inactivated as the reaction proceeded, however, the rate of the inactivation was much influenced by the additives. Furthermore, stoichiometry between consumed oxygen and formed conjugated diene could not be observed. (c) 1998 Elsevier Science B.V.

Effect of saposins A and C on the enzymatic hydrolysis of liposomal glucosylceramide

The degradation of glucosylceramide in lysosomes is accomplished by glucosylceramidase with the assistance of, at least, another protein, saposin C (Sap C), which is generated from a large precursor together with three other similar proteins, saposins A, B, and D. In the present study, we have examined the effects of saposins on the enzymatic hydrolysis of glucosylceramide inserted in large and small phospholipid liposomes. The glucosylceramide contained in large unilamellar vesicles (LUV) was degraded by glucosylceramidase at a rate 7-8-fold lower than glucosylceramide inserted in small unilamellar vesicles (SUV). The separate addition of either Sap A or Sap C to the LUV system partially stimulated the sphingolipid degradation while saposins B and D had no effect. In the presence of both Sap A and Sap C, the rate of sphingolipid degradation was higher than the sum of the rates with the two saposins individually, indicating synergism in their actions. The stimulatory effect of the two saposins depended on the incorporation of an acidic phospholipid such as phosphatidylserine (PS) into LUV. The characteristics of glucosylceramidase activation by Sap C were different from those of Sap A. Sap C increased the rate of hydrolysis of both the artificial water soluble substrate, 4-methylumbelliferyl-beta-D-glucopyranoside, and the lipid substrate, glucosylceramide, while Sap A only stimulated degradation of the sphingolipid. Also the binding properties of Saps A and C were markedly different. At acidic pH values, Sap C bound to PS-containing LUV and promoted the association of glucosylceramidase with the membrane. In contrast, Sap A had poor affinity for the membrane even in the presence of glucosylceramide; moreover, Sap A did not potentiate the capacity of Sap C to mediate glucosylceramidase binding. In conclusion, our results show that both Sap A and Sap C are required for maximal hydrolysis of glucosylceramide inserted in PS-containing LUV, that their effects are synergistic, and that their mode of action is different. Sap C is responsible for the membrane binding of glucosylceramidase, while Sap A stimulation is possibly related to its effect on the conformation of the enzyme. It can be envisaged that Sap A in conjunction with Sap C might have a physiological role in glucosylceramide degradation.

pH-dependent conformational properties of saposins and their interactions with phospholipid membranes

Saposins A, B, C, and D are small lysosomal glycoproteins released by proteolysis from a single precursor polypeptide, prosaposin. We have presently investigated the conformational states of saposins and their interaction with membranes at acidic pH values similar to those present in lysosomes. With the use of phase partitioning in Triton X-114, experimental evidence was provided that, upon acidification, saposins (Sap) A, C, and D acquire hydrophobic properties, while the hydrophilicity of Sap B is apparently unchanged. The pH-dependent exposure of hydrophobic domains of Sap C and D paralleled their pH-dependent binding to large unilamellar vesicles composed of phosphatidylcholine, phosphatidylserine, and cholesterol. In contrast, the binding of Sap A to the vesicles was very restricted, in spite of its increased hydrophobicity at low pH. A low affinity for the vesicles was also shown by Sap B, a finding consistent with its apparent hydrophilicity both at neutral and acidic pH. At the acidic pH values needed for binding, Sap C and D powerfully destabilized the phospholipid membranes, while Sap A and B minimally affected the bilayer integrity. In the absence of the acidic phospholipid phosphatidylserine, the induced destabilization markedly decreased. Of the four saposins, only Sap C was able to promote the binding of glucosylceramidase to phosphatidylserine-containing membranes. This result is consistent with the notion that Sap C is specifically required by glucosylceramidase to exert its activity. Our finding that an acidic environment induces an increased hydrophobicity in Sap A, C, and D, making the last two saposins able to interact and perturb phospholipid membranes, suggests that this mechanism might be relevant to the mode of action of saposins in lysosomes.

Structural analysis of saposin C and B. Complete localization of disulfide bridges

Saposins A, B, C, and D are a group of homologous glycoproteins derived from a single precursor, prosaposin, and apparently involved in the stimulation of the enzymatic degradation of sphingolipids in lysosomes. All saposins have six cysteine residues at similar positions. In the present study we have investigated the disulfide structure of saposins B and C using advanced mass spectrometric procedures. Electrospray analysis showed that deglycosylated saposins B and C are mainly present as 79- and 80-residue monomeric polypeptides, respectively. Fast atom bombardment mass analysis of peptide mixtures obtained by a combination of chemical and enzymatic cleavages demonstrated that the pairings of the three disulfide bridges present in each saposin are Cys4-Cys77, Cys7-Cys71, Cys36-Cys47 for saposin B and Cys5-Cys78, Cys8-Cys72, Cys36-Cys47 for saposin C. We have recently shown that saposin C interacts with phosphatidylserine-containing vesicles inducing destabilization of the lipid surface (Vaccaro, A. M., Tatti, M., Ciaffoni, F., Salvioli, R., Serafino, A., and Barca, A. (1994) FEBS Lett. 349, 181-186); this perturbation promotes the binding of the lysosomal enzyme glucosylceramidase to the vesicles and the reconstitution of its activity. It was presently found that the effects of saposin C on phosphatidylserine liposomes and on glucosylceramidase activity are markedly reduced when the three disulfide bonds are irreversibly disrupted. These results stress the importance of the disulfide structure for the functional properties of the saposin.

Saposin C induces pH-dependent destabilization and fusion of phosphatidylserine-containing vesicles

We have previously shown that saposin C (Sap C), a glucosylceramidase activator protein, interacts with phosphatidylserine (PS) large unilamellar vesicles (LUV), promoting the glucosylceramidase binding to the bilayer [(1993) FEBS Lett. 336, 159-162]. In the present paper the consequences of the Sap C interaction on the lipid organization of the vesicles are reported. It was found that Sap C perturbs the PS bilayer as shown by the release of an encapsulated fluorescent dye. Three different procedures, resonance energy transfer, gel filtration and electron microscopy, indicated that the activator protein is also able to make PS liposomes fuse. The effects of Sap C on PS vesicles were observed at low but not at neutral pH. The lipid composition of the bilayer also affected the Sap C-induced destabilization; in fact, the presence of PS in mixed LUV was essential for significant leakage to occur. These results demonstrate for the first time that Sap C is a protein capable of destabilizing and fusing acidic phospholipid-containing membranes in a pH-dependent fashion.

Function of saposin C in the reconstitution of glucosylceramidase by phosphatidylserine liposomes

The function of saposin C (Sap C), a glucosylceramidase activator protein, in the enzyme stimulation by phosphatidylserine (PS) liposomes has been investigated. Using gel filtration experiments evidence was obtained for Sap C binding to PS large unilamellar vesicles (LUV) but not to glucosylceramidase. PS LUV, which by themselves are unable to tightly bind and stimulate the enzyme, acquire the capacity to also bind the enzyme after interaction with Sap C, making it express its full activity. Our results indicate that the primary step in the Sap C mode of action resides in its association with PS membranes; in turn, this association promotes the interaction between the membranes and glucosylceramidase.