Role of N-Linked Oligosaccharide Flexibility in Mannose Phosphorylation of Lysosomal Enzyme Cathepsin L

Mannose phosphorylation of N-linked oligosaccharides by UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase is a key step in the targeting of lysosomal enzymes in mammalian cells and tissues. The selectivity of this process is determined by lysine-based phosphorylation signals shared by lysosomal enzymes of diverse structure and function. By introducing new glycosylation sites at several locations on the surface of mouse procathepsin L and modeling oligosaccharide conformations for sites that are phosphorylated, it was shown that the inherent flexibility of N-linked oligosaccharides can account for the specificity of the transferase for oligosaccharides at different locations on the protein. By using this approach, the physical relationship between the lysine-based signal and the site of phosphorylation of mannose residues was determined. The analysis also revealed the existence of additional independent lysine-based phosphorylation signals on procathepsin L, which account for the low level of phosphorylation observed when the primary Lys-54/Lys-99 signal is ablated. Mutagenesis of residues that surround Lys-54 and Lys-99 and demonstration of mannose phosphorylation of a glycosylated derivative of green fluorescent protein provide strong evidence that the cathepsin L phosphorylation signal is a simple structure composed of as few as two well placed lysine residues.