title: Saposin B binds and transfers phospholipids.
abstract: Saposin B (Sap B) is a member of a family of four small glycoproteins, Sap A, B, C, and D. Like the other three saposins, Sap B plays a physiological role in the lysosomal degradation of sphingolipids (SLs). Although the interaction of Sap B with SLs has been investigated extensively, that with the main membrane lipid components, namely phospholipids and cholesterol (Chol), is scarcely known. Using large unilamellar vesicles (LUVs) as membrane models, we have now found that Sap B simultaneously extracts from the lipid surface neutral [phosphatidylcholine (PC)] and anionic [phosphatidylinositol (PI)] phospholipids, fewer SLs [ganglioside GM1 (GM1) or cerebroside sulfate (CS)], and no Chol. More PI than SL (GM1 or CS) was solubilized from LUVs containing equal amounts of PI and SLs. An increase in PI level had a poor effect on the Sap B-induced solubilization of GM1 or CS but strongly inhibited that of PC. Sap B was able not only to bind, but also to transfer phospholipids between lipid surfaces. Both the phospholipid binding and transfer activities were optimal at low pH values. These results represent the first biochemical analysis of the Sap B interaction with phospholipids. The capacity of Sap B to bind and transfer phospholipids occurs under conditions mimicking the interior of the late endosomal/lysosomal compartment and thus might have physiological relevance.
authors.affiliation: Department of Hematology, Oncology, and Molecular Medicine, Istituto Superiore Sanita, Roma, Italy.
authors.forename: Fiorella
authors.initials: F
authors.lastname: Ciaffoni
authors.forename: Massimo
authors.initials: M
authors.lastname: Tatti
authors.forename: Alessandra
authors.initials: A
authors.lastname: Boe
authors.forename: Rosa
authors.initials: R
authors.lastname: Salvioli
authors.forename: Arvan
authors.initials: A
authors.lastname: Fluharty
authors.forename: Sandro
authors.initials: S
authors.lastname: Sonnino
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: Vaccaro
mesh_terms: D002850:Chromatography, Gel; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008081:Liposomes / Q000737:chemistry; D010743:Phospholipids / Q000378:metabolism*; D049231:Saposins / Q000378:metabolism*; D012995:Solubility; D013107:Sphingolipids / Q000378:metabolism
chemical_list: D008081:Liposomes; D010743:Phospholipids; D049231:Saposins; D013107:Sphingolipids