Phage anti-CBASS protein simultaneously sequesters cyclic trinucleotides and dinucleotides.

Area of Science:

• Molecular Biology
• Immunology
• Microbial Genetics

Background:

• Cyclic-oligonucleotide-based anti-phage signaling systems (CBASS) are crucial bacterial defense mechanisms against phage infection.
• Phages have evolved anti-CBASS proteins, like Acb2, to counteract these immune systems by sequestering cyclic oligonucleotide signals.
• Understanding these molecular interactions is key to deciphering host-pathogen dynamics.

Purpose of the Study:

• To investigate the molecular mechanism by which Acb2 protein interacts with CBASS signaling molecules.
• To determine the binding affinity and specificity of Acb2 for cyclic dinucleotides (CDNs) and cyclic trinucleotides (CTNs).
• To assess the broad-spectrum inhibitory potential of Acb2 against various CBASS systems.

Main Methods:

• Biochemical assays to characterize Acb2 binding to CDNs and CTNs.
• Inhibition assays to measure the effect of Acb2 on STING activity in human cells.
• Phylogenetic analysis of Acb2 homologs across diverse phage and prophage genomes.

Main Results:

• Acb2 effectively sequesters multiple CDNs produced by CBASS systems, inhibiting stimulator of interferon genes (STING) activity.
• Acb2 binds with high affinity to CBASS cyclic trinucleotides (CTNs) at a distinct site from CDNs, with one hexamer binding two CTNs and three CDNs simultaneously.
• Phylogenetic analysis suggests most Acb2 homologs possess dual binding capabilities for both CTNs and CDNs.

Conclusions:

• Acb2 acts as a broad-spectrum inhibitor of cGAS-based immunity by sequestering nearly all known CBASS signaling molecules.
• The dual binding pockets of Acb2 provide a potent defense mechanism for phages against bacterial CBASS systems.
• This study reveals a sophisticated molecular strategy employed by phages to overcome host immunity.