The structural basis for 2'-5'/3'-5'-cGAMP synthesis by cGAS

Area of Science:

• Biochemistry
• Molecular Biology
• Immunology

Background:

• Cyclic GMP-AMP synthase (cGAS) is a key sensor of cytosolic double-stranded DNA (dsDNA).
• Activation of cGAS triggers innate immune responses.
• Understanding the cGAS catalytic mechanism is crucial for immunology and drug development.

Purpose of the Study:

• To elucidate the unifying catalytic mechanism of cGAS.
• To determine the structural basis for cGAS substrate specificity.
• To compare NTP and linkage specificities between mouse and human cGAS.

Main Methods:

• X-ray crystallography of cGAS at various reaction stages.
• Biochemical assays to study enzyme kinetics and metal ion dependence.
• Comparative analysis of mouse and human cGAS.

Main Results:

• Inactive apo-cGAS adopts multiple conformations, binding NTPs nonproductively.
• dsDNA binding induces cGAS dimerization and locks the active site for productive catalysis.
• A network of interactions ensures stepwise cGAMP synthesis and specificity.
• Manganese ions are preferred over magnesium ions for catalysis.
• Mouse cGAS exhibits stricter NTP and linkage specificities than human cGAS.

Conclusions:

• cGAS employs an adaptable, lock-and-key-like mechanism for catalysis.
• Structural insights reveal how cGAS discriminates between cognate and non-cognate substrates.
• Differences in specificity between mouse and human cGAS are elucidated.