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.

Gaucher disease (GD) is characterized by accumulation of glucosylceramide (GC) in the cells of monocyte/macrophage system. The degradation of GC is controlled by glucosylceramidase (GCase) and saposin (Sap) C, a member of a family of four small glycoproteins (Saps A, B, C and D), all derived by proteolytic processing of a common precursor, prosaposin (PSAP). Saps contain six cysteine residues, forming three disulfide bridges, that affect their structure and function. Sap C is an essential activator of GCase and its deficit impairs the GCase activity causing GD. In the present study the biological properties of cells from four recently described GD patients carrying mutations in the Sap C domain of the PSAP gene have been characterized. Two patients had mutations involving a cysteine residue, whereas the other two had a L349P mutation. It was found that: (i) in the four Sap C-deficient cells PSAP was normally processed and sorted, the lack of Sap C being mainly due to the Sap C instability in late endosomal/lysosomal environment; (ii) the decrease/absence of Sap C affected the GCase intracellular localization; (iii) the lowest level of Sap C and enhanced autophagy were observed in the cells, which carried a Sap C mutation involving a cysteine residue; (iv) the four Sap C-deficient fibroblasts stored GC, ceramide and cholesterol, the last two lipids being clearly localized in lysosomes; (v) a correlation was observed between the type of Sap C mutation and the Gaucher phenotype: apparently, mutations involving cysteine residues lead to a neurological variant of GD.

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.

Gaucher disease is generally caused by a deficiency of the lysosomal enzyme glucocerebrosidase. The degradation of glycosphingolipids requires also the participation of sphingolipid activator proteins. The prosaposin PSAP gene codes for a single protein which undergoes post-translational cleavage to yield four proteins named saposins A, B, C and D. Saposin (SAP-) C is required for glucosylceramide degradation, and its deficiency results in a variant form of Gaucher disease. In this report, we present clinical, biochemical, and molecular findings in a 36-year-old man and his 30-year-old sister with non-neuronopathic Gaucher disease due to SAP-C deficiency. Very high levels of chitotriosidase activity, chemokine CCL18, and increased concentration of glucosylceramide in plasma and normal beta-glucosidase activity in skin fibroblasts were observed in the patients. A molecular genetics study of the PSAP gene enabled the identification of one missense mutation, p.L349P, located in the SAP-C domain and another mutation, p.M1L, located in the initiation codon of the prosaposin precursor protein. The presented findings describe the first cases where the non-neuronopathic Gaucher disease has been definitely demonstrated to be a consequence of SAP-C deficiency. Three previously described cases in the literature displayed a Gaucher type 3 phenotype.

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-␤-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.