High-Resolution Structure of the Lysosomal Heparan-α-Glucosaminide N-Acetyltransferase (HGSNAT) Reveals Insights into Mucopolysaccharidosis IIIC

Основные понятия

The high-resolution cryo-EM structure of the lysosomal membrane protein HGSNAT in complex with acetyl-CoA provides critical insights into the molecular basis of its N-acetyltransferase activity and the impact of mutations that cause the rare lysosomal storage disorder mucopolysaccharidosis IIIC.

Аннотация

The content describes the structure determination and analysis of the lysosomal membrane protein heparan-α-glucosaminide N-acetyltransferase (HGSNAT), which catalyzes an essential acetylation step in the degradation of heparan sulfate (HS), a glycosaminoglycan.
Key highlights:

HGSNAT is a dimeric protein with 11 transmembrane helices and a luminal domain. The transmembrane domain has a unique "transmembrane N-acetyltransferase (TNAT)" fold.
The structure reveals the acetyl-CoA binding site, which is accessible from the lysosomal lumen. Key residues involved in acetyl-CoA binding and catalysis are identified.
Mapping of known disease-causing mutations onto the structure provides insights into how they disrupt HGSNAT structure and function, leading to the lysosomal storage disorder mucopolysaccharidosis IIIC.
The structure suggests possible mechanisms for the HGSNAT-catalyzed acetylation reaction, though the exact mechanism remains debated.
The study also sheds light on the controversial issue of HGSNAT proteolytic processing and oligomerization, which was previously unclear.
Overall, this high-resolution structure of HGSNAT provides critical structural insights into the function of this unique lysosomal enzyme and the molecular basis of the associated genetic disorder.

Статистика

Heparan sulfate (HS) is a glycosaminoglycan comprised of repeating units of N-acetylglucosamine and glucuronic acid.
Dysfunction of enzymes in the HS degradation pathway causes mucopolysaccharidosis III (MPS III), or Sanfilippo's syndrome.
MPS IIIC is caused by dysfunction of the heparan-α-glucosaminide N-acetyltransferase (HGSNAT) enzyme.
HGSNAT catalyzes the essential acetylation of the terminal non-reducing amino group of α-D-glucosamine in HS, which is required for its complete breakdown.
Over 70 unique mutations in the HGSNAT gene have been identified that cause the autosomal recessive lysosomal storage disorder MPS IIIC.

Цитаты

"Acetylation of the terminal non-reducing amino group of α-D-glucosamine of HS is essential for its complete breakdown into monosaccharides and free sulfate."
"HGSNAT is the only enzyme of the GAG pathway that is not a hydrolase. It catalyzes the only known biosynthetic reaction of the GAG degradation pathway within the lysosome, that is, the acetyl-CoA mediated N-acetylation of the terminal non-reducing amino group of α-D-glucosamine."
"So far, over 70 unique mutations in the HGSNAT (TMEM76) gene have been identified. These mutations span the entire sequence and include deletions, nonsense mutations, splice-site variants, and silent and missense mutations."

Ключевые выводы из

Structure of the human heparan-α-glucosaminide N-acetyltransferase (HGSNAT)

by Navratna,V.,... в www.biorxiv.org 10-25-2023

https://www.biorxiv.org/content/10.1101/2023.10.23.563672v3

Дополнительные вопросы

How do the conformational changes in HGSNAT induced by pH shifts between the cytosol and lysosomal lumen impact its catalytic mechanism

The conformational changes in HGSNAT induced by pH shifts between the cytosol and lysosomal lumen play a crucial role in its catalytic mechanism. HGSNAT is involved in the acetylation of heparan sulfate (HS) within the lysosomes, a process essential for its complete breakdown. The enzyme undergoes pH-dependent conformational changes that regulate its activity. At a neutral pH in the cytosol, HGSNAT binds acetyl-CoA, the acetyl donor, and prepares for the acetylation reaction. The binding of acetyl-CoA occurs on the cytosolic side of the enzyme. However, the optimal pH for the acetyltransferase activity of HGSNAT is acidic, around pH 5.5-6.0, in the lysosomal lumen.
The pH shift from neutral in the cytosol to acidic in the lysosomal lumen triggers a series of conformational changes in HGSNAT. These changes are essential for the enzyme to catalyze the transfer of the acetyl group from acetyl-CoA to the terminal non-reducing amino group of α-D-glucosamine in HS. The acidic pH in the lysosomal lumen likely induces structural rearrangements in HGSNAT, facilitating the positioning of the catalytic residues for the acetyltransferase reaction. The protonation of specific amino acid residues, such as H269, is crucial for the catalytic activity of HGSNAT at the acidic pH in the lysosomal lumen. These conformational changes optimize the enzyme's active site for efficient acetyl transfer, leading to the acetylation of HS and its subsequent degradation.

What are the potential therapeutic strategies to rescue the function of HGSNAT mutants that cause misfolding and mistrafficking in mucopolysaccharidosis IIIC

Mucopolysaccharidosis IIIC (MPS IIIC) is caused by mutations in the HGSNAT gene, leading to dysfunctional heparan-α-glucosaminide N-acetyltransferase. These mutations can result in misfolding and mistrafficking of the enzyme, leading to the accumulation of heparan sulfate within lysosomes and the development of MPS IIIC symptoms. To rescue the function of HGSNAT mutants and potentially treat MPS IIIC, several therapeutic strategies can be considered:

Pharmacochaperone Therapy: Small molecules that act as pharmacochaperones can stabilize misfolded HGSNAT mutants, promoting proper folding and trafficking to the lysosomes. These compounds can help restore the enzyme's activity and reduce the accumulation of heparan sulfate.

Enzyme Replacement Therapy (ERT): ERT involves the administration of functional HGSNAT enzyme to compensate for the defective enzyme in MPS IIIC patients. Recombinant HGSNAT can be delivered to lysosomes to enhance the degradation of accumulated heparan sulfate.

Gene Therapy: Gene therapy approaches, such as gene editing or gene replacement, can correct the genetic mutations in the HGSNAT gene, restoring the production of functional enzyme in affected individuals.

Substrate Reduction Therapy: By reducing the production of heparan sulfate precursors, substrate reduction therapy can decrease the substrate load on dysfunctional HGSNAT, potentially alleviating lysosomal storage and MPS IIIC symptoms.

Combination Therapies: Combining different therapeutic approaches, such as pharmacochaperone therapy with ERT or gene therapy, may offer synergistic benefits in treating MPS IIIC and addressing the diverse challenges associated with the disease.

These therapeutic strategies aim to address the underlying molecular defects in HGSNAT mutants, restore enzyme function, and mitigate the lysosomal storage of heparan sulfate in MPS IIIC patients.

Given the unique TNAT fold of HGSNAT, what are the evolutionary origins and functional implications of this novel structural architecture among membrane-bound acetyltransferases

The unique transmembrane N-acetyltransferase (TNAT) fold of HGSNAT represents a novel structural architecture among membrane-bound acetyltransferases. The evolutionary origins and functional implications of this distinct fold provide insights into the enzyme's specialized role in heparan sulfate degradation.
Evolutionary Origins:
The TNAT fold of HGSNAT is not homologous to other known acetyltransferases, indicating a unique evolutionary lineage. The structural features of HGSNAT suggest that it has evolved to specifically catalyze the acetylation of heparan sulfate within the lysosomes. The absence of structural homologs in other organisms and the conservation of key residues in HGSNAT across species highlight its functional importance in glycosaminoglycan metabolism.
Functional Implications:
The TNAT fold of HGSNAT is tailored for its role in the lysosomal degradation pathway of heparan sulfate. The distinct arrangement of transmembrane helices and luminal domains in HGSNAT enables the enzyme to interact with substrates and cofactors in a spatially defined manner. The TNAT fold likely contributes to the enzyme's substrate specificity, catalytic efficiency, and subcellular localization within the lysosomal membrane.
The evolutionary origins and functional implications of the TNAT fold in HGSNAT underscore the enzyme's specialized adaptation for heparan sulfate metabolism and its critical role in maintaining cellular homeostasis. Further studies on the structure-function relationship of HGSNAT will provide valuable insights into the molecular mechanisms underlying lysosomal storage disorders like MPS IIIC.