Crystal structure of saposin B reveals a dimeric shell for lipid binding

Abstract

Saposin B is a small, nonenzymatic glycosphingolipid activator protein required for the breakdown of cerebroside sulfates (sulfatides) within the lysosome. The protein can extract target lipids from membranes, forming soluble protein-lipid complexes that are recognized by arylsulfatase A. The crystal structure of human saposin B reveals an unusual shell-like dimer consisting of a monolayer of ␣-helices enclosing a large hydrophobic cavity. Although the secondary structure of saposin B is similar to that of the known monomeric members of the saposin-like superfamily, the helices are repacked into a different tertiary arrangement to form the homodimer. A comparison of the two forms of the saposin B dimer suggests that extraction of target lipids from membranes involves a conformational change that facilitates access to the inner cavity.T he saposins are a group of four structurally related activator proteins that function in conjunction with hydrolase enzymes for the stepwise breakdown of glycosphingolipids within the lysosome (1, 2). These proteins act by modifying the local environment of target lipids, and ''activate'' the breakdown of their substrates by presenting them in a form in which the lipid scissile bonds are accessible to the active sites of the specific enzymes. Presumably, the hydrolases cannot form a catalytic complex with the glycosphingolipids when these are in unmodified membrane bilayers.Each of the four saposins activates one or more lipid exohydrolases. For example, saposin B (also known as the cerebroside sulfate activator, or CS-Act) facilitates the hydrolysis of the sulfate group from cerebroside sulfate by arylsulfatase A, resulting in the formation of galactosylceramide (3). This glycolipid is then catabolized to ceramide by ␤-galactosylceramidase in a reaction activated by saposin C. There seems to be more than one mechanism of activation by the saposins, and this may further depend on the particular target lipid. Thus, saposin B is able to extract and solubilize cerebroside sulfates from membranes, allowing arylsulfatase A to act on the small, diffusible protein-lipid complexes. Saposin B may also have a physiological role in activating the hydrolysis of the ganglioside GM1 to GM2 by lysosomal ␤-galactosidase, and in this case, the activator may act by modifying the local lipid structure at the membrane surface to allow catalysis to proceed (4).The saposins belong to a large and diverse family of small, cysteine-rich proteins that share a common ability to interact with membranes but act in a wide variety of functions. Other members of the ''saposin-like'' superfamily of proteins include the lung surfactant-associated protein B (SP-B), the tumorolytic protein NK-lysin, granulysin, the pore-forming amoebapores, and the membrane-targeting domain of some enzymes (5, 6). The saposin-like proteins are Ϸ80 residues in length and have a characteristic pattern of conserved cysteines ( Fig. 1A; refs. 7 and 8). To date, the three-dimensional modeling of the saposins and other members of the sapo...

NMR Structures of the C-Terminal Segment of Surfactant Protein B in Detergent Micelles and Hexafluoro-2-propanol

“…Six conserved cysteines form three intrapeptide disulfide bonds and define a fold that has been conserved for an estimated 300 million years (27). The experimentally derived structures of three members of the superfamily, including saposin B [PDB entry 1N69 (28)], saposin C [PDB entry 1M12 (29)], and NK-lysin [PDB entry 1NKL (30)], all display this fold, although the helical splay of each of the proteins may differ.…”

Novel CDNF/MANF family of neurotrophic factors

“…1(B)]. Saposin B, a protein with lipid transfer activity (Vogel et al, 1991), crystallized in an open conformation as a homodimer with bound phospholipid (Ahn et al, 2003), whereas Saposin C crystallized in the presence of detergent as a monomer in an open conformation with hydrophobic pocket exposed to solvent (Hawkins et al, 2005). Also at pH 4, Saposin C crystallized in an open conformation, but as a domain-swapped dimer that may facilitate vesicle fusion (Rossmann et al, 2008).…”

Saposins modulate human invariant Natural Killer T cells self-reactivity and facilitate lipid exchange with CD1d molecules during antigen presentation

“…The crystal structure of saposin B has revealed the presence of a large hydrophobic binding site capable of accommodating a broad range of different lipids (31). Although it is accepted that lipid-loaded saposins promote lipid transfer onto CD1d molecules (9), it remains unclear whether they also accelerate the rate of dissociation of lipids already bound to CD1d molecules.…”