Structural basis for molecular discrimination by a 3',3'-cGAMP sensing riboswitch

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Cyclic dinucleotides function as crucial second messengers in cellular signaling pathways.
• Bacterial riboswitches represent a class of regulatory elements that bind cyclic dinucleotides.
• The STING (stimulator of interferon genes) pathway in mammals is activated by cyclic dinucleotides, modulating the innate immune response.

Purpose of the Study:

• To determine the structural basis for the recognition of 3',3'-cyclic guanosine monophosphate (cGAMP) by a bacterial riboswitch.
• To investigate the structural differences between the aptamer-bound states of 3',3'-cGAMP and other cyclic dinucleotides.
• To explore the potential for reengineering riboswitch specificity.

Main Methods:

• X-ray crystallography was employed to determine the structures of the riboswitch aptamer domain.
• Biochemical assays were performed to assess ligand binding and specificity.
• Structure-based analysis was used to correlate structural features with ligand recognition.

Main Results:

• The 3',3'-cGAMP riboswitch from Geobacter adopts a unique tuning fork-like architecture.
• The ligand-binding pocket is located at the junction of the riboswitch, accommodating the cyclic dinucleotide.
• Distinct orientations of the riboswitch arms correlate with the bound cyclic dinucleotide (3',3'-cGAMP vs. c-di-GMP).
• Point mutations outside the binding pocket can alter ligand specificity, suggesting a broader regulatory mechanism.

Conclusions:

• The structural insights provide a detailed understanding of how this riboswitch discriminates between different cyclic dinucleotides.
• The findings highlight the potential for engineering riboswitches with altered specificities for novel applications.
• This work contributes to the understanding of bacterial signaling mechanisms and the broader field of RNA-based regulation.