Catalyzes the formation of mannose 6-phosphate (M6P) markers on high mannose type oligosaccharides in the Golgi apparatus. M6P residues are required to bind to the M6P receptors (MPR), which mediate the vesicular transport of lysosomal enzymes to the endosomal/prelysosomal compartment. N(4)-[alpha-D-mannosyl-(1->2)-alpha-D-mannosyl-(glycan)]-L-asparaginyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N(4)-[6-(N-acetyl-alpha-D-glucosaminyl-1-phospho)-alpha-D-mannosyl-(1->2)-alpha-D-mannosyl-(glycan)]-L-asparaginyl-[protein] + UMP Hexamer of two alpha, two beta and two gamma (GNPTG) subunits; disulfide-linked. The alpha and/or the beta subunits of the enzyme constitute the catalytic subunits. The DMAP1-binding domain mediates substrate recognition. It specifically recognizes a conformation-dependent protein determinant present in acid hydrolases. The alpha- and beta-subunits are generated by a proteolytic cleavage by MBTPS1 protease at the Lys-907-Asp-908 bond. Severe retinal degeneration, growth retardation and secretory cell lesions. Mice are smaller with a reduced mean body weight and length, along with a reduction in total tissue mass and lean body mass. They show elevated levels of serum lysosomal enzymes, cartilage defects, and display cytoplasmic alterations in secretory cells of several exocrine glands. Stealth proteins are part of a protein family that is conserved from bacteria to higher eukaryotes. Family members were first identified in microbes as proteins that help pathogens to elude the host innate immune system. Microbial stealth proteins are most likely involved in the biosynthesis of exopolysaccharides. Stealth proteins are predicted to function as hexose-1-phosphoryltransferases. Belongs to the stealth family.