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Cyclic Adenosine Monophosphate (cAMP) is an essential second messenger that activates protein kinase A (PKA) and regulates various biological processes. A single epinephrine molecule binds to GPCR and activates several heterotrimeric G proteins, each stimulating multiple adenylyl cyclase, amplifying the signal, and synthesizing large numbers of cAMP molecules. Small changes in cAMP concentration affect PKA activity. The binding of four cAMP molecules induces a conformational change in PKA,...
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Some GPCRs transmit signals through adenylyl cyclase (AC), a transmembrane enzyme. AC helps synthesize second messenger cyclic adenosine monophosphate (cAMP). AC catalyzes cyclization reaction and converts ATP to cAMP by releasing a pyrophosphate. The pyrophosphate is further hydrolyzed to phosphate by the enzyme pyrophosphatase, which drives cAMP synthesis to completion. However, cAMP is rapidly degraded to 5′ AMP by the enzymes phosphodiesterase (PDE), preventing overstimulation of...
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Membrane lipids such as phosphatidylinositol (PI) are precursors for several membrane-bound and soluble second messengers. Specific kinases phosphorylate PI and produce phosphorylated inositol phospholipids. One such inositol phospholipids are the  phosphatidylinositol-4,5 bisphosphate [PI(4,5)P2], present in the inner half of the lipid bilayer. Upon ligand binding, GPCR stimulates Gq proteins to turn on phospholipase Cꞵ. Activated phospholipase Cꞵ cleaves PI(4,5)P2 and...
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Heterotrimeric G proteins are guanine nucleotide-binding proteins. As the name suggests, heterotrimeric G proteins are composed of three subunits: alpha, beta, and gamma. They remain GDP-bound or GTP-bound inside the cells and switch between inactive/active states. The Gα subunit possesses the nucleotide-binding pocket that binds guanine nucleotides and switches between GDP or GTP-bound states. In contrast, the Gꞵ and Gγ subunits are always bound together with high...
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Once a ligand binds to a receptor, the signal is transmitted through the membrane and into the cytoplasm. The continuation of a signal in this manner is called signal transduction. Signal transduction only occurs with cell-surface receptors, which cannot interact with most components of the cell, such as DNA. Only internal receptors can interact directly with DNA in the nucleus to initiate protein synthesis. When a ligand binds to its receptor, conformational changes occur that affect the...
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The structural basis for 2'-5'/3'-5'-cGAMP synthesis by cGAS.

Shuai Wu1, Sandra B Gabelli1,2,3,4, Jungsan Sohn5,6,7

  • 1Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD, USA.

Nature Communications
|May 13, 2024
PubMed
Summary
This summary is machine-generated.

Cyclic GMP-AMP synthase (cGAS) uses a precise lock-and-key mechanism to synthesize cGAMP, activating innate immunity against DNA. This study reveals the detailed catalytic steps and specificities of cGAS.

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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.