title: Structure of human saposin A at lysosomal pH.
abstract: The saposins are essential cofactors for the normal lysosomal degradation of complex glycosphingolipids by acid hydrolase enzymes; defects in either saposin or hydrolase function lead to severe metabolic diseases. Saposin A (SapA) activates the enzyme β-galactocerebrosidase (GALC), which catalyzes the breakdown of β-D-galactocerebroside, the principal lipid component of myelin. SapA is known to bind lipids and detergents in a pH-dependent manner; this is accompanied by a striking transition from a `closed' to an `open' conformation. However, previous structures were determined at non-lysosomal pH. This work describes a 1.8 Å resolution X-ray crystal structure determined at the physiologically relevant lysosomal pH 4.8. In the absence of lipid or detergent at pH 4.8, SapA is observeed to adopt a conformation closely resembling the previously determined `closed' conformation, showing that pH alone is not sufficient for the transition to the `open' conformation. Structural alignments reveal small conformational changes, highlighting regions of flexibility.
authors.affiliation: Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Cambridge Biomedical Campus, Hills Road, Cambridge CB2 0XY, England.
authors.forename: Chris H
authors.initials: CH
authors.lastname: Hill
authors.affiliation: Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Cambridge Biomedical Campus, Hills Road, Cambridge CB2 0XY, England.
authors.forename: Randy J
authors.initials: RJ
authors.lastname: Read
authors.affiliation: Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Wellcome Trust/MRC Building, Cambridge Biomedical Campus, Hills Road, Cambridge CB2 0XY, England.
authors.forename: Janet E
authors.initials: JE
authors.lastname: Deane
mesh_terms: D000595:Amino Acid Sequence; D003460:Crystallization; D018360:Crystallography, X-Ray / Q000379:methods; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008247:Lysosomes; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D049231:Saposins / Q000737:chemistry* / Q000235:genetics*
chemical_list: D049231:Saposins
keywords: GALC; lipid-transfer protein; saposin A; sphingolipid activator protein