Mannose 6-phosphate

Mannose-6-phosphate (M6P) is a molecule bound by lectin in the immune system. M6P is converted to fructose 6-phosphate by mannose phosphate isomerase.

M6P is a targeting signal for acid hydrolase precursor proteins that are destined for transport to lysosomes. The M6P tag is added to such proteins in the cis-Golgi apparatus. Specifically, in a reaction involving uridine diphosphate (UDP) and N-acetylglucosamine, the enzyme UDP-N-acetylglucosamine:N-acetylglucosaminyl-1-phosphotransferase catalyzes the N-linked glycosylation of asparagine residues with M6P. Once appropriately marked with the M6P targeting signal, these proteins are moved to the trans-Golgi. There, the M6P moiety is recognized by mannose 6-phosphate receptor (MPR) proteins.

The MPRs traffic in vesicles to the lysosome via an endosome.