The carbohydrate moiety of the activator protein for glucosylceramide beta-glucosidase

Efficient one-step chromatographic purification and functional characterization of recombinant human Saposin C

Saposin (Sap) C is a small lysosomal disulfide bridge-containing glycoprotein required for glucosylceramide (GC) hydrolysis by glucosylceramidase (GCase). Sap C deficiency causes a variant form of Gaucher disease (GD), a rare genetic disorder characterized by GC accumulation in lysosomes of monocyte/macrophage lineage. Efforts to develop fast and efficient methodologies to express and purify Sap C have been made in the last years. Here, human Sap C was expressed in a bacterial strain that greatly enhances disulfide bond formation, and the recombinant protein was purified in a single chromatographic step using an affinity tag-based protein purification system. Mass spectrometry analysis demonstrated that disulfide bridges required for Sap C stability and functionality were retained. Consistently, the recombinant protein was shown to interact with anionic phospholipids-containing vesicles, and reconstitute GCase activity in vitro. Recombinant Sap C was efficiently endocytosed by Sap C-deficient fibroblasts, and targeted to lysosomes. These findings document that the bacterially purified Sap C exerts biological properties functionally equivalent to those observed for the native protein, indicating its potential use in the development of therapeutic intervention.

Differential membrane interactions of saposins A and C: implications for the functional specificity

Saposins are small, heat-stable glycoprotein activators of lysosomal glycosphingolipid hydrolases that derive from a single precursor, prosaposin, by proteolytic cleavage. Three of these saposins (B, C, and D) share common structural features including a lack of tryptophan, a single glycosylation sequence, the presence of three conserved disulfide bonds, and a common multiamphipathic helical bundle motif. Saposin A contains an additional glycosylation site and a single tryptophan. The oligosaccharides on saposins are not required for in vitro activation functions. Saposins A and C were produced in Escherichia coli to contain single tryptophans at various locations to serve as intrinsic fluorescence reporters, i.e. as topological probes, for interaction with phospholipid membranes. Maximum emission shifts, aqueous and solid quenching, and resonance energy transfer were quantified by fluorescence spectroscopy. Amphipathic helices at the amino- and carboxyl termini of saposins A and C were shown to insert into the lipid bilayer to about five carbon bond lengths. In comparison, the middle region of saposins A or C were either embedded in the bilayer or solvent-exposed, respectively. Conformational changes of saposin C induced by phosphatidylserine interaction suggested the reorientation of functional helical domains. Differential interaction models are proposed for the membrane-bound saposins A and C. By site-directed mutagenesis of saposin A and C, their membrane topological structures were correlated with their activation effects on acid beta-glucosidase. These findings show that proper orientation of the middle segment of saposin C to the outside of the membrane surface is critical for its specific and multivalent interaction with acid beta-glucosidase. Such membrane interactions and orientations of the saposins determine the proximity of their activation and/or binding sites to lysosomal hydrolases or lipoid substrates.

Structural and membrane-binding properties of saposin D

Saposin D is generated together with three similar proteins, saposins A, B and C, from a common precursor, called prosaposin, in acidic organelles such as late endosomes and lysosomes. Although saposin D has been reported to stimulate the enzymatic hydrolysis of sphingomyelin and ceramide, its physiological role has not yet been clearly established. In the present study we examined structural and membrane-binding properties of saposin D. At acidic pH, saposin D showed a great affinity for phospholipid membranes containing an anionic phospholipid such as phosphatidylserine or phosphatidic acid. The binding of saposin D caused destabilization of the lipid surface and, conversely, the association with the membrane markedly affected the fluorescence properties of saposin D. The presence of phosphatidylserine-containing vesicles greatly enhanced the intrinsic tyrosine fluorescence of saposin D, which contains tyrosines but not tryptophan residues. The structural properties of saposin D were investigated in detail using advanced MS analysis. It was found that the main form of saposin D consists of 80 amino acid residues and that the six cysteine residues are linked in the following order: Cys5-Cys78, Cys8-Cys72 and Cys36-Cys47. The disulfide pattern of saposin D is identical with that previously established for two other saposins, B and C, which also exhibit a strong affinity for lipids. The common disulfide structure probably has an important role in the interaction of these proteins with membranes. The analysis of the sugar moiety of saposin D revealed that the single N-glycosylation site present in the molecule is mainly modified by high-mannose-type structures varying from two to six hexose residues. Deglycosylation had no effect on the interaction of saposin D with phospholipid membranes, indicating that the glycosylation site is not related to the lipid-binding site. The association of saposin D with membranes was highly dependent on the composition of the bilayer. Neither ceramide nor sphingomyelin, sphingolipids whose hydrolysis is favoured by saposin D, promoted its binding, while the presence of an acidic phospholipid such as phosphatidylserine or phosphatidic acid greatly favoured the interaction of saposin D with vesicles at low pH. These results suggest that, in the acidic organelles where saposins are localized, anionic phospholipids may be determinants of the saposin D topology and, conversely, saposin D may affect the lipid organization of anionic phospholipid-containing membranes.

Synthesis and characterization of a bioactive 82-residue sphingolipid activator protein, saposin C

The sphingolipid activator protein, saposin C (also termed SAP 2), was chemically synthesized, purified, and characterized. The fully protected 82-residue protein was synthesized by automated solid-phase methods, with multiple recoupling steps resulting in a high average coupling efficiency of 98.8%. The overall yield was estimated to be approx 40%. Deprotection and cleavage of the peptide from the resin was followed by folding in the absence of chaotropic agents at pH 8.5. The protein was purified by reversed-phase high pressure liquid chromatography (HPLC) and its purity determined by capillary electrophoresis and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The composition of the synthetic saposin C was determined by amino acid analysis. Its sequence was verified by Edman sequence analysis of overlapping peptide fragments generated by chymotryptic and Staphylococcus aureus V8 digestions. The sequence at the C-terminus was determined by digestion with carboxypeptidase P, followed by phenylthiohydantoin (PTH) derivitization and HPLC analysis of the released amino acid residues. Deglycosylated native saposin C appeared as a lower molecular-weight species than synthetic saposin C on SDS-PAGE. This has been explained by amino acid and C-terminal analysis showing native saposin C to be two amino acids shorter at the C terminus than a deduced sequence (from cDNA) previously published. Synthetic saposin C displayed 85% of full biological activity as determined by its ability to stimulate glucocerebrosidase activity in vitro: Synthetic and native saposin C increased glucocerebrosidase catalyzed hydrolysis of 4-methylumbelliferyl beta-D-glucoside by factors of 6.0 and 7.1, respectively. Furthermore, synthetic and native saposin C share similar K(act) values (0.5 and 1.5 microM respectively) indicating that they bind to glucocerebrosidase with similar affinities.

Activator proteins and topology of lysosomal sphingolipid catabolism

The lysosomal degradation of several sphingolipids by acid hydrolases is dependent on small non-enzymic cofactors, called sphingolipid activator proteins some of which have been identified as sphingolipid binding proteins. This review summarizes the information available on the structure, function, biosynthesis, gene organization and pathobiochemistry of the known sphingolipid activator proteins. It also offers models for their mode of action and for the topology of lysosomal digestion of glycolipids.

Saposin-C from bovine spleen; complete amino acid sequence and relation between the structure and its biological activity

Saposin-C, a small acidic glycoprotein that can activate glucosylceramide-beta-glucosidase, has been isolated from bovine spleen. The complete amino acid sequence of bovine saposin-C was determined by Edman degradation of the purified protein and its fragmented peptides. It contains 80 amino acids, one carbohydrate chain attached to a single asparagine residue and six cysteine residues in oxidized form. The sequence of bovine saposin-C is 76 and 65% identical with the sequences of saposin-C from human spleen and guinea pig liver, respectively. Hydropathy profiles of the sequence of saposin-C from three species were similar despite the significant residue substitutions. Bovine saposin-C had a stronger effect in stimulating bovine beta-glucosidase compared to human saposin-C. However, the effect of human saposin-C in stimulating human enzyme was stronger than that of bovine saposin-C. The region around residue 35, which is next to the extremely hydrophilic region, seems to be important to produce an interaction with the enzyme.

Secretion of sphingolipid hydrolase activator precursor, prosaposin

Sphingolipid hydrolases are activated by activator proteins or saposins. The precursor protein has been expected from the studies on the cDNA for saposins. Here we demonstrate that prosaposin occurs in various kinds of human secretory fluids such as cerebrospinal fluid, semen, milk, pancreatic juice, and bile. However, mature type saposins were not detected in these fluids. In human milk the amount of prosaposin changed during the lactating period; it became high in concentration within a few days after delivery, decreased during the transitional milk lactating stage, and then increased again toward the mature milk lactating stage. Prosaposin was released from human platelets in response to stimulation by thrombin, but mature saposins were not. From the time course of the release of prosaposin induced by thrombin and from the fact that weak platelet agonists, ADP, epinephrine, and collagen, did not cause the release of prosaposin, prosaposin secretion from platelets seemed to be from lysosome like granules. We postulate that some prosaposin works as a precursor for saposins in the lysosomes and the other serves as an extracellular protein with other specific roles.

Characterization of a mutation in a family with saposin B deficiency: a glycosylation site defect

Saposins are small, heat-stable glycoproteins required for the hydrolysis of sphingolipids by specific lysosomal hydrolases. Saposins A, B, C, and D are derived by proteolytic processing from a single precursor protein named prosaposin. Saposin B, previously known as SAP-1 and sulfatide activator, stimulates the hydrolysis of a wide variety of substrates including cerebroside sulfate, GM1 ganglioside, and globotriaosylceramide by arylsulfatase A, acid beta-galactosidase, and alpha-galactosidase, respectively. Human saposin B deficiency, transmitted as an autosomal recessive trait, results in tissue accumulation of cerebroside sulfate and a clinical picture resembling metachromatic leukodystrophy (activator-deficient metachromatic leukodystrophy). We have examined transformed lymphoblasts from the initially reported saposin B-deficient patient and found normal amounts of saposins A, C, and D. After preparing first-strand cDNA from lymphoblast total RNA, we used the polymerase chain reaction to amplify the prosaposin cDNA. The patient's mRNA differed from the normal sequence by only one C----T transition in the 23rd codon of saposin B, resulting in a threonine to isoleucine amino acid substitution. An affected male sibling has the same mutation as the proband and their heterozygous mother carries both the normal and mutant sequences, providing additional evidence that this base change is the disease-causing mutation. This base change results in the replacement of a polar amino acid (threonine) with a nonpolar amino acid (isoleucine) and, more importantly, eliminates the glycosylation signal in this activator protein. One explanation for the deficiency of saposin B in this disease is that the mutation may increase the degradation of saposin B by exposing a potential proteolytic cleavage site (arginine) two amino acids to the amino-terminal side of the glycosylation site when the carbohydrate side chain is absent.

Acid beta-glucosidase: enzymology and molecular biology of Gaucher disease

Human lysosomal beta-glucosidase (D-glucosyl-acylsphingosine glucohydrolase, EC 3.2.1.45) is a membrane-associated enzyme that cleaves the beta-glucosidic linkage of glucosylceramide (glucocerebroside), its natural substrate, as well as synthetic beta-glucosides. Experiments with cultured cells suggest that in vivo this glycoprotein requires interaction with negatively charged lipids and a small acidic protein, SAP-2, for optimal glucosylceramide hydrolytic rates. In vitro, detergents (Triton X-100 or bile acids) or negatively charged ganglioside or phospholipids and one of several "activator proteins" increase hydrolytic rate of lipid and water-soluble substrates. Using such in vitro assay systems and active site-directed covalent inhibitors, kinetic and structural properties of the active site have been elucidated. The defective activity of this enzyme leads to the variants of Gaucher disease, the most prevalent lysosomal storage disease. The nonneuronopathic (type 1) and neuronopathic (types 2 and 3) variants of this inherited (autosomal recessive) disease but panethnic, but type 1 is most prevalent in the Ashkenazi Jewish population. Several missense mutations, identified in the structural gene for lysosomal beta-glucosidase from Gaucher disease patients, are presumably casual to the specifically altered posttranslational oligosaccharide processing or stability of the enzyme as well as the altered in vitro kinetic properties of the residual enzyme from patient tissues.

Sphingolipid hydrolase activator proteins and their precursors

Activator proteins for sphingolipid hydrolases (saposins) are small acidic, heat-stable glycoproteins that stimulate the hydrolysis of sphingolipids by lysosomal enzymes. The molecular mass of each stimulator is about 10 kDa, but glycosylated forms of higher mass exist too. The distribution and developmental changes in two saposins and their precursor proteins were studied with the aid of monospecific antibodies against saposin-B and saposin-C. They show a wide distribution in rat organs and forms intermediate between saposin and prosaposin (the precursor protein containing four different saposin units) could be seen. The amount of saposin and the degree of processing from prosaposin are quite different in different tissues. The saposins are the dominant forms in spleen, lung, liver, and kidney, while skeletal muscle, heart, and brain contain mainly precursor forms. In human blood, leukocytes contain mainly saposin, while plasma contains mainly precursor forms and platelets show many forms. Their subcellular distribution was studied using rat liver. The saposins of approximately 20 kDa are dominant in the light mitochondrial, mitochondrial, and microsomal fractions, following the distribution of the activity of a lysosomal marker enzyme. The nuclear fraction exhibits bands corresponding to non-glycosylated saposin. The soluble fraction contained much precursor forms. A developmental study of rat brain showed that the concentration of saposin precursors increased with age.

Molecular cloning of a human co-beta-glucosidase cDNA: evidence that four sphingolipid hydrolase activator proteins are encoded by single genes in humans and rats

Authentic cDNAs encoding the activator protein for acid beta-glucosidase (EC3.2.1.45), co-beta-glucosidase, were cloned from the pCD and lambda gt11 human cDNA libraries. Initial screening with oligonucleotide mixtures encoding amino acid sequences of co-beta-glucosidase identified partial cDNAs which were used to obtain a potentially full-length cDNA from the lambda gt11 library. This clone (2767 bp), EGTISI, contained 5' (38 bp) and 3' (1157 bp) noncoding sequences, a translation initiation site, and an open reading frame encoding 524 amino acids which included a typical hydrophobic signal sequence (16 amino acids). Computer analyses identified three regions of high similarity to co-beta-glucosidase encoded by tandem sequences in EGTISI. Searches revealed that two of these regions encoded peptides of known function; SAP1 (sphingolipid activator protein 1) and protein C (a new sphingolipid activator protein) were encoded by EGTISI sequences 5' and 3', respectively, to those for co-beta-glucosidase. The third region of similarity, encoding a theoretical peptide (undefined function), was located most 5' in the cDNA. EGTISI and its encoded polypeptide had high similarity (77% nucleotide identity and about 80% amino acid similarity) to a rat Sertoli cell cDNA and its encoded sulfated glycoprotein-1. These results indicate that a single highly conserved gene encodes the precursor for four potential sphingolipid activator proteins in rat and man.

Saposin A: second cerebrosidase activator protein

Saposin A, a heat-stable 16-kDa glycoprotein, was isolated from Gaucher disease spleen and purified to homogeneity. Chemical sequencing from its amino terminus and of peptides obtained by digestion with protease from Staphylococcus aureus strain V-8 demonstrated that saposin A is derived from proteolytic processing of domain 1 of its precursor protein, prosaposin. Processing of prosaposin (70 kDa) also generates three other previously reported saposin proteins, B, C, and D, from its second, third, and fourth domains. Similar to saposin C, saposin A stimulates the hydrolysis of 4-methylumbelliferyl beta-glucoside and glucocerebroside by beta-glucosylceramidase and of galactocerebroside by beta-galactosylceramidase, mainly by increasing the maximal velocity of both reactions. Saposin A is as active as saposin C in these reactions. Saposin A has no significant effect on other sphingolipid and 4-methylumbelliferyl glycoside hydrolases tested. Saposin A has two potential glycosylation sites that appear to be glycosylated. After deglycosylation, saposin A had a subunit molecular mass of 10 kDa and was as active as native saposin A. However, reduction and alkylation abolished the activation. A three-dimensional model comparing saposins A and C reveals significant sequence homology between them, especially preservation of conserved acidic and basic residues in their middle regions. Each appears to possess a conformationally rigid hydrophobic pocket stabilized by three internal disulfide bridges, with amphipathic helical regions interrupted by helix breakers.