The processing of lysosomal enzymes in the Golgi

In the cis- Golgi one or more mannose residues of the high-mannose oligosaccharides of the lysosomal enzymes become phosphorylated on C 6 . The phosphorylated mannose residues are the chemical signal that targets protein to lysosome.

The phosphorylation is a two-step procedure :

1. phosphorylation by N-Acetylglucosamine phosphotransferase. This enzyme uses only lysosomal proteins as substrates. It recognizes the protein part of lysosomal glycoprotein (not the oligosaccharide). The recognized domain contains several amino acid residues, including a lysine residue which is present on the surfaces of all lysosomal enzymes.
2. removal of GlcNAc by N-Acetylglucosaminyl-1-phosphodiester-alpha-N-Acetylglucosaminidase.

There is a specific receptor for mannose-6-phosphate residue in the trans-Golgi reticulum. After the lysosomal proteins are bound to the receptors they become localized to a small region of membrane which is coated on its cytosolic face by the fibrous protein- clathrin. Clathrin is involved in the forming of the transport vesicle.

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Pathway for non-lysosomal enzymes

N-glycosylation

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