Catalyzes the second step in the formation of the mannose 6-phosphate targeting signal on lysosomal enzyme oligosaccharides by removing GlcNAc residues from GlcNAc-alpha-P-mannose moieties, which are formed in the first step. Also hydrolyzes UDP-GlcNAc, a sugar donor for Golgi N-acetylglucosaminyltransferases. H2O + N(4)-[6-(N-acetyl-alpha-D-glucosaminyl-1-phospho)-alpha-D-mannosyl-(1->2)-alpha-D-mannosyl-(glycan)]-L-asparaginyl-[protein] = H(+) + N(4)-[6-phospho-alpha-D-mannosyl-(1->2)-alpha-D-mannosyl-(glycan)]-L-asparaginyl-[protein] + N-acetyl-D-glucosamine Protein modification; protein glycosylation. Homotetramer arranged as two disulfide-linked homodimers. Interacts with AP4M1. Cis/medial Golgi. Glycosylated. Contains complex N-linked oligosaccharides with appreciable amounts of sialic acid. The precursor is cleaved and activated in the trans-Golgi network by a furin endopeptidase.