m6p is a key 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 n-acetylglucosamine-1-phosphate transferase 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 network. there, the m6p moiety is recognized and bound by mannose 6-phosphate receptor (mpr) proteins at ph 6.5-6.7.