Comparative lipid binding study on the cerebroside sulfate activator (saposin B)

Cerebroside sulfate activator (saposin B) is a small protein involved in glycosphingolipid metabolism. It binds certain membrane lipids, making them available to water-soluble enzymes. Defects in this protein are responsible for a form of metachromatic leukodystropy, a progressive neurodegenerative condition. The protein participates in the catabolism of a number of lipids but does show lipid binding selectivity. However, the basis of this selectivity is unclear. Here we assess the relative binding of a number of lipids compared to cerebroside sulfate (sulfatide). We utilize a competitive binding paradigm, in which the lipids compete for protein under favorable conditions and are then switched to a condition in which the complex is stable. This study is unique in that a single molecular species of the activator is employed, and an expanded selection of natural and semisynthetic membrane lipids is surveyed. No simple "binding rule" can be ascertained from these data, but ligands with longer and/or more complex lipoidal and polar adducts appear to be favored.