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Crystal structures of saposins A and C

¹ Department of Medical Biophysics, University of Toronto, Toronto M5G 2M9, Canada
² Department of Biochemistry, University of Toronto, Toronto M5S 1A8, Canada
³ Division of Cancer Genomics and Proteomics, Ontario Cancer Institute, Toronto M5G 1L7, Canada

(RECEIVED March 31, 2006; FINAL REVISION April 29, 2006; ACCEPTED May 1, 2006)

Saposins A and C are sphingolipid activator proteins required for the lysosomal breakdown of galactosylceramide and glucosylceramide, respectively. The saposins interact with lipids, leading to an enhanced accessibility of the lipid headgroups to their cognate hydrolases. We have determined the crystal structures of human saposins A and C to 2.0 Å and 2.4 Å, respectively, and both reveal the compact, monomeric saposin fold. We confirmed that these two proteins were monomeric in solution at pH 7.0 by analytical centrifugation. However, at pH 4.8, in the presence of the detergent C₈E₅, saposin A assembled into dimers, while saposin C formed trimers. Saposin B was dimeric under all conditions tested. The self-association of the saposins is likely to be relevant to how these small proteins interact with lipids, membranes, and hydrolase enzymes.

Keywords: saposins; X-ray crystallography; analytical ultracentrifugation; protein–detergent interactions

Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.062256606.

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