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. Expressed in the heart, whole brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. The DMAP1-binding domain mediates substrate recognition. It specifically recognizes a conformation-dependent protein determinant present in acid hydrolases (PubMed:23733939). The alpha- and beta-subunits are generated by a proteolytic cleavage by MBTPS1 protease at the Lys-928-Asp-929 bond. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Genetic variations in GNPTAB have been suggested to play a role in susceptibility to persistent stuttering. Stuttering is a common speech disorder characterized by repetitions, prolongations, and interruptions in the flow of speech. Due to the low pH in the endosomal/prelysosomal compartment, the lysosomal enzyme-MPR complex dissociates and then the enzyme is delivered to the lysosome. Between 5% and 20% of newly synthesized lysosomal enzymes escape the binding to the MPR in the Golgi apparatus and are secreted. 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.