Modification of lysosomal proteins called hydrolases begins in the rough endoplasmic reticulum where a core oligosaccharide is added to the protein.

The hydrolases are then packaged into transport vesicles and transferred to the cis-cisterna of the Golgi apparatus.

Inside the cis-cisterna of the Golgi, the core oligosaccharide is phosphorylated.

One of the mannose residues in the oligosaccharide receives a phosphate by way of two sequential reactions.

The modification of the core oligosaccharide on the hydrolase enzyme results in the creation of an mannose 6-phosphate signal unique to proteins destined for the lysosome. Other signals target other proteins for different destinations.

Upon reaching the trans-Golgi, the M6P portion of the hydrolase binds to M6P receptors embedded in the trans-Golgi membrane.

The membrane of the trans-Golgi then buds off into a vesicle containing the receptors and the bound hydrolases.