N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase (NAGPA)

Abstract

N-Acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase (phosphodiester α-GlcNAcase, also known as “uncovering enzyme”) is a type I membrane-spanning glycoprotein enzyme of the Golgi apparatus. It exists as a homotetramer (272 kDa) composed of two dimers, each containing a pair of disulfide-linked monomers of 68 kDa. The enzyme catalyzes the second step in the formation of the mannose 6-phosphate recognition marker on lysosomal enzyme oligosaccharides. It removes the “covering” GlcNAc residue from GlcNAc-P added during the first step to C-6-hydroxyl groups of selected mannose residues by the enzyme UDP-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (phosphotransferase). The Man-6-P moiety exposed by phosphodiester α-GlcNAcase action is responsible for the specific, high-affinity binding of lysosomal enzymes to one of the two mannose 6-phosphate receptors in the trans-Golgi network (TGN) that transport the lysosomal enzymes to endosomes and subsequently to lysosomes. Phosphodiester α-GlcNAcase activity is present in all tissues of higher eukaryotes examined but is absent in the slime mold Dictyostelium discoideum and in Acanthamoeba castellanii, each of which contains phosphotransferase activity (Couso et al. 1986).