Crystal Structure of the Mannose-6-Phosphate Uncovering Enzyme

Most lysosomal hydrolytic enzymes reach their destination via the mannose-6-phosphate (M6P) pathway. The enzyme N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase (NAGPA, or "uncovering enzyme") catalyzes the second step in the M6P tag formation, namely the removal of the masking N-acetylglucosamine (GlcNAc) portion. Defects in this protein are associated with non-syndromic stuttering. To gain a better understanding of the function and regulation of this enzyme, we determined its crystal structure. The propeptide binds in a groove on the globular catalytic domain, blocking active site access. High-affinity substrate binding is enabled by a conformational switch in an active site loop. The protein recognizes the GlcNAc and phosphate portions of its substrate, but not the mannose moiety of the glycan. Based on enzymatic and ¹H-NMR analysis, a catalytic mechanism is proposed. Crystallographic and solution scattering analyses suggest that the C-terminal domain forms a long flexible stem that extends the enzyme away from the Golgi membrane.