CRISPR-Cas9 PAM Recognition: It's More Than Just Direct Contact

Expanding the range of Protospacer Adjacent Motifs (PAMs) recognized by CRISPR-Cas9 is essential for broadening genome-editing applications. This study examined the principles of PAM recognition in three Cas9 variants—VQR, VRER, and EQR—that target noncanonical PAMs.

Utilizing a sophisticated combination of molecular dynamics simulations, graph-theory, and centrality analyses, the researchers found that efficient PAM recognition extends beyond simple direct contacts. As lead author Vieyra notes in the paper, "We show that efficient recognition is not dictated solely by direct contacts between PAM-interacting residues and DNA but also by a distal network that stabilizes the PAM-binding domain and preserves long-range communication with REC3, a hub that relays signals to the HNH nuclease." Further investigation revealed a key role for the D1135 V/E substitution. This substitution enables stable DNA binding by K1107 and preserves key DNA phosphate locking interactions via S1109, thereby securing stable PAM engagement.

In contrast, variants carrying only R-to-Q substitutions at PAM-contacting residues, though predicted to enhance adenine recognition, destabilized the PAM-binding cleft, perturbed REC3 dynamics, and disrupted allosteric coupling to the HNH nuclease. These findings collectively establish that PAM recognition is a sophisticated process demanding local stabilization, distal coupling, and entropic tuning, rather than being a simple consequence of base-specific contacts.

This framework provides guiding principles for engineering Cas9 variants. By applying this understanding, scientists can design new tools with expanded PAM compatibility and improved editing efficiency.