title: Gaucher disease due to saposin C deficiency is an inherited lysosomal disease caused by rapidly degraded mutant proteins.
abstract: Saposin (Sap) C is an essential cofactor for the lysosomal degradation of glucosylceramide (GC) by glucosylceramidase (GCase) and its functional impairment underlies a rare variant form of Gaucher disease (GD). Sap C promotes rearrangement of lipid organization in lysosomal membranes favoring substrate accessibility to GCase. It is characterized by six invariantly conserved cysteine residues involved in three intramolecular disulfide bonds, which make the protein remarkably stable to acid environment and degradation. Five different mutations (i.e. p.C315S, p.342_348FDKMCSKdel, p.L349P, p.C382G and p.C382F) have been identified to underlie Sap C deficiency. The molecular mechanism by which these mutations affect Sap C function, however, has not been delineated in detail. Here, we characterized biochemically and functionally four of these gene lesions. We show that all Sap C mutants are efficiently produced, and exhibit lipid-binding properties, modulatory behavior on GCase activity and subcellular localization comparable with those of the wild-type protein. We then delineated the structural rearrangement of these mutants, documenting that most proteins assume diverse aberrant disulfide bridge arrangements, which result in a substantial diminished half-life, and rapid degradation via autophagy. These findings further document the paramount importance of disulfide bridges in the stability of Sap C and provide evidence that accelerated degradation of the Sap C mutants is the underlying pathogenetic mechanism of Sap C deficiency.
authors.affiliation: Department of Haematology, Oncology and Molecular Medicine.
authors.forename: Marialetizia
authors.initials: M
authors.lastname: Motta
authors.affiliation: Department of Cell Biology and Neuroscience and.
authors.forename: Serena
authors.initials: S
authors.lastname: Camerini
authors.affiliation: Department of Haematology, Oncology and Molecular Medicine.
authors.forename: Massimo
authors.initials: M
authors.lastname: Tatti
authors.affiliation: Department of Cell Biology and Neuroscience and.
authors.forename: Marialuisa
authors.initials: M
authors.lastname: Casella
authors.affiliation: National Center for Rare Diseases, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Torreri
authors.affiliation: Department of Cell Biology and Neuroscience and.
authors.forename: Marco
authors.initials: M
authors.lastname: Crescenzi
authors.affiliation: Department of Haematology, Oncology and Molecular Medicine.
authors.forename: Marco
authors.initials: M
authors.lastname: Tartaglia
authors.affiliation: Department of Haematology, Oncology and Molecular Medicine, rosa.salvioli@iss.it.
authors.forename: Rosa
authors.initials: R
authors.lastname: Salvioli
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D005776:Gaucher Disease / Q000235:genetics* / Q000378:metabolism*; D015870:Gene Expression; D006801:Humans; D008247:Lysosomes / Q000378:metabolism*; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D055550:Protein Stability; D021381:Protein Transport; D059748:Proteolysis; D049231:Saposins / Q000737:chemistry / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism*; D016415:Sequence Alignment
chemical_list: D049231:Saposins