Submitted on December 19, 2005
Revised on January 31, 2006
Accepted on February 6, 2006
Saposin B binds and transfers phospholipids
Fiorella Ciaffoni, Massimo Tatti, Alessandra Boe, Rosa Salvioli, Arvan Fluharty, Sandro Sonnino, and Anna Maria Vaccaro
Hematology, Oncology and Molecular Medicine, Istituto Superiore di Sanità, Roma 00161
Corresponding Author: annamaria.vaccaro{at}iss.it
Saposin (Sap) B is a member of a family of four small glycoproteins, Sap A, B, C and D. As the other three saposins, Sap B plays a physiological role in the lysosomal degradation of sphingolipids (SLs). While the interaction of Sap B with SLs has been extensively investigated, that with the main membrane lipid components, namely phospholipids and cholesterol, is scarcely known. Utilizing 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, less SLs (ganglioside GM1 or cerebroside sulfate, CS), and no cholesterol. More PI than SL (GM1 or CS) was solubilized from LUVs containing equal amounts of PI and SLs. An increase of the PI level had poor effect on the Sap B-induced solubilization of GM1 or CS, while 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 condition mimicking the interior of the late endosomal/lysosomal compartment and thus might have physiological relevance.
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