Structural and mechanistic insights into a lysosomal membrane enzyme HGSNAT involved in Sanfilippo syndrome

• Published in: Nature communications 2024-06, Vol.15 (1), p.5388-10, Article 5388
• Main Authors: Zhao, Boyang, Cao, Zhongzheng, Zheng, Yi, Nguyen, Phuong, Bowen, Alisa, Edwards, Robert H., Stroud, Robert M., Zhou, Yi, Van Lookeren Campagne, Menno, Li, Fei
• Format: Article
• Language: English
• Subjects: 101/28; 13/109; 631/45/173; 631/535/1258/1259; 631/57/2283; 631/80/221; 631/80/304; 82/16; 82/80; 82/83; Access control; Acetyl Coenzyme A - chemistry; Acetyl Coenzyme A - metabolism; Acetylation; Acetyltransferase; Acetyltransferases - chemistry; Acetyltransferases - genetics; Acetyltransferases - metabolism; Aspartic acid; Catalytic Domain; Cryoelectron Microscopy; Enzymes; Glucosamine; Glucosamine - chemistry; Glucosamine - metabolism; Glycosidases; Heparan sulfate; Heparitin Sulfate - metabolism; Histidine; Humanities and Social Sciences; Humans; Intracellular Membranes - metabolism; Lysosomal storage diseases; Lysosomes - enzymology; Lysosomes - metabolism; Membranes; Models, Molecular; Mucopolysaccharidosis; Mucopolysaccharidosis III - enzymology; Mucopolysaccharidosis III - genetics; Mucopolysaccharidosis III - metabolism; multidisciplinary; Mutation; N-Acetyltransferase; Reaction mechanisms; Science; Science (multidisciplinary); Structure-function relationships
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-024-49614-1

Heparan sulfate (HS) is degraded in lysosome by a series of glycosidases. Before the glycosidases can act, the terminal glucosamine of HS must be acetylated by the integral lysosomal membrane enzyme heparan-α-glucosaminide N -acetyltransferase (HGSNAT). Mutations of HGSNAT cause HS accumulation and consequently mucopolysaccharidosis IIIC, a devastating lysosomal storage disease characterized by progressive neurological deterioration and early death where no treatment is available. HGSNAT catalyzes a unique transmembrane acetylation reaction where the acetyl group of cytosolic acetyl-CoA is transported across the lysosomal membrane and attached to HS in one reaction. However, the reaction mechanism remains elusive. Here we report six cryo-EM structures of HGSNAT along the reaction pathway. These structures reveal a dimer arrangement and a unique structural fold, which enables the elucidation of the reaction mechanism. We find that a central pore within each monomer traverses the membrane and controls access of cytosolic acetyl-CoA to the active site at its luminal mouth where glucosamine binds. A histidine-aspartic acid catalytic dyad catalyzes the transfer reaction via a ternary complex mechanism. Furthermore, the structures allow the mapping of disease-causing variants and reveal their potential impact on the function, thus creating a framework to guide structure-based drug discovery efforts. HGSNAT is a critical lysosomal membrane enzyme involved in the devastating lysosomal storage disease Sanfilippo syndrome. Here, Zhao et. al. reveal the mechanism for transmembrane acetylation catalyzed by HGSNAT with a series of cryo-EM structures.