Predicting an intrinsic conformational twist in Card1: an in silico study

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Type III CRISPR systems employ cyclic adenosine nucleotides (cA_n) as second messengers to activate CARF-containing effector proteins.
• Card1, a critical auxiliary protein, binds cA₄ through its CARF domain, initiating ssRNA and ssDNA cleavage via its REase domain.

Purpose of the Study:

• To elucidate the molecular mechanism of cA₄ binding to the Card1 CARF domain.
• To investigate the conformational dynamics of Card1 upon cA₄ interaction.
• To provide insights into Type III CRISPR immune mechanisms and optimize CRISPR-based genetic tools.

Main Methods:

• All-atom and coarse-grained molecular dynamics (MD) simulations using AMBER24 and SIRAH force fields.
• Accelerated MD (GaMD) simulations to enhance sampling of large conformational changes.
• Structural analysis using CPPTRAJ to calculate distances, RMSD, PCA, and dihedral angles.

Main Results:

• cA₄ binding is stabilized by extensive hydrogen bonding within the Card1 CARF domain, restricting pocket expansion.
• Card1 undergoes significant conformational changes, including swinging and twisting motions, upon cA₄ binding.
• Despite domain-level flexibility, the Card1 protein chains exhibit high rigidity, with minimal interchain conformational changes.

Conclusions:

• The study provides a detailed molecular understanding of Card1 activation by cA₄.
• Findings offer a theoretical foundation for enhancing CRISPR-based genetic technologies.
• This research deepens our comprehension of the intricate Type III CRISPR immune system.