The phosphorylation pattern of oligosaccharides in secreted procathepsin D is glycosylation site-specific and independent of the expression of mannose 6-phosphate receptors.

Mammalian cells contain two types of mannose 6-phosphate receptors (MPR), MPRs 46 and 300, that contribute with variable efficiency to the sorting of individual lysosomal proteins. To evaluate the role of phosphorylated oligosaccharides for the sorting efficiency by either of the two receptors, the structure of phosphorylated oligosaccharides on lysosomal proteins escaping sorting in cells lacking MPR 46 and/or MPR 300 was analyzed. Procathepsin D was chosen as a model because it is sorted efficiently via MPR 300 and poorly via MPR 46 and contains a distinct and highly heterogenous mixture of phosphorylated oligosaccharides at either of its two N-glycosylation sites. Both MPRs 46 and 300 were found to have a minor but distinct preference for forms of procathepsin D and other lysosomal proteins containing oligosaccharides with two phosphomonoesters. However, the phosphorylation of oligosaccharides in procathepsin D and other lysosomal proteins that escape sorting in control cells or in cells lacking MPR 46 and/or MPR 300 was strikingly similar, and oligosaccharides with two phosphomonoesters represented the major oligosaccharide species. We conclude from these results that the position of the position of the phosphate groups, the structure of the underlying oligosaccharide, and/or the polypeptide backbone of lysosomal proteins have major roles in determining the affinity to MPRs.