Prosaposin sorting is mediated by oligomerization

The compartmental nature of eukaryotic cells requires sophisticated mechanisms of protein sorting. Prosaposin, the precursor of four sphingolipid activator proteins, is transported from the trans-Golgi network (TGN) to lysosomes as a partially glycosylated (65 kDa) protein with high-mannose/hybrid oligosaccharides. Prosaposin is also found in the extracellular space where it is secreted as a fully glycosylated (70 kDa) protein composed of complex glycans. Although the trafficking of prosaposin to lysosomes is known to be mediated by sortilin, the mechanism of secretion of this protein is still unknown. In this study, we report that prosaposin may covalently aggregate into oligomers. Our results demonstrate that while prosaposin oligomers are secreted into the extracellular space, monomeric prosaposin remains inside the cell bound to sortilin. We also found that deletion of the C-terminus of prosaposin, previously shown to block its lysosomal transport, did not abolish its oligomerization and secretion. On the other hand, elimination of the N-terminus and of each saposin domain inhibited its oligomerization and resulted in its retention as a fully glycosylated protein. In conclusion, we are reporting for the first time that oligomerization of prosaposin is crucial for its entry into the secretory pathway.

► In eukaryotes, the delivery of newly synthesized proteins to the extracellular space and/or lysosomes is dependent on a series of functionally distinct compartments including the endoplasmic reticulum and the Golgi apparatus, which play a role in post-translational modification, sorting and secretion.
► Most proteins are sorted in the trans-Golgi network (TGN) by sorting receptors.
► However, the sorting mechanism of proteins with dual destinations is still unknown.
► Prosaposin, a protein linked to lysosomal storage disorders, is either secreted or transported to lysosomes.
► This investigation reports that monomeric prosaposin is transported to lysosomes bound to sortilin and that oligomerized prosaposin is delivered to the extracellular space.
► In conclusion, this study uncovered for the first time an intracellular trafficking process that might be physiologically relevant in eucharyotic cells.