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Journal of Lipid Research, Vol. 47, 1045-1053, May 2006
Copyright © 2006 by American Society for Biochemistry and Molecular Biology

Saposin B binds and transfers phospholipids

Fiorella Ciaffoni^1,*, Massimo Tatti^1,*, Alessandra Boe^*, Rosa Salvioli^*, Arvan Fluharty, Sandro Sonnino and Anna Maria Vaccaro^2,*

^* Department of Hematology, Oncology, and Molecular Medicine, Istituto Superiore Sanita, 00161 Roma, Italy
Mental Retardation Research Center and Jane and Terry Semel Institute of Neuroscience and Human Behavior, University of California Los Angeles, Los Angeles, CA 90024
Center of Excellence on Neurodegenerative Diseases, Department of Medical Chemistry, Biochemistry, and Biotechnology, University of Milano, 20090 Segrate, Italy

Published, JLR Papers in Press, February 6, 2006.

¹ F. Ciaffoni and M. Tatti contributed equally to this work.

² To whom correspondence should be addressed. e-mail: annamaria.vaccaro{at}iss.it

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.

Abbreviations: Chol, cholesterol; CL, cardiolipin; CS, cerebroside sulfate; GM1, ganglioside GM1; LUV, large unilamellar vesicle; MLV, multilamellar vesicle; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; Sap B, saposin B; SL, sphingolipid; SUV, small unilamellar vesicle

Supplementary key words phospholipid binding • phospholipid transfer • Saposin-membrane interaction

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