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Mycobacterium tuberculosis antigen 85A and 85C structures confirm binding orientation and conserved substrate
Donald R Ronning1, Varalakshmi Vissa, Gurdyal S Besra
1Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843-2128, USA.
The Journal of Biological Chemistry
|June 12, 2004
Summary
The antigen 85 proteins are crucial for Mycobacterium tuberculosis survival by maintaining its cell wall. Structural analysis reveals conserved active sites but variable surface residues, aiding immune evasion.
Area of Science:
- Microbiology
- Structural Biology
- Immunology
Background:
- Mycobacterium tuberculosis survival relies on its hydrophobic cell wall.
- Antigen 85 proteins (85A, 85B, 85C) maintain cell wall integrity via mycolic acid transfer and cord factor synthesis.
- These proteins facilitate macrophage invasion through host immune interactions.
Purpose of the Study:
- To determine the crystal structures of antigen 85C and antigen 85A.
- To identify residues involved in substrate binding.
- To compare the structures of the three antigen 85 homologs.
Main Methods:
- X-ray crystallography was used to resolve the structures of antigen 85C (2.0 A) and antigen 85A (2.7 A).
- Antigen 85C was co-crystallized with octylthioglucoside, a substrate analog.
Main Results:
- The structure of antigen 85C with its substrate analog identified key residues for substrate binding.
- The crystal structure of antigen 85A was elucidated.
- Active sites of all three antigen 85 proteins are highly conserved, suggesting a shared substrate.
- Surface residues outside the active site are variable among the homologs.
Conclusions:
- The structural data provide insights into the enzymatic mechanisms of antigen 85 proteins.
- Conserved active sites indicate a common function in cell wall synthesis.
- Variable surface residues likely play a role in evading the host immune system, highlighting the need for all three homologs.