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 Gln-893-Asp-894 bond. Decreased mannose phosphorylation of lysosomal acid hydrolases, craniofacial and cardiac defects. Impaired development of pectoral fins and otic vesicles. Craniofacial defects are due to changes in the timing and localization of both type II collagen and sox9 expression, suggestive of an accelerated chondrocyte differentiation program. Increased level of active cathepsin K (ctsk) 3 days post-fertilization (dpf) because of abnormal processing and activation, leading to morphologic cartilage defects. 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. Contaminating sequence. Potential poly-A sequence.