Structural Basis of PAM-Induced Conformational Changes in SpCas9: A Molecular Dynamics Study

As the most widely utilized CRISPR gene-editing enzyme, SpCas9 has been extensively studied and applied. However, its strict dependence on the canonical NGG PAM sequence significantly restricts its targeting scope. Although recent research has successfully engineered SpCas9 variants capable of recognizing noncanonical (non-NGG) PAMs, these variants still exhibit limitations when binding noncanonical PAMs, including substantially reduced cleavage efficiency. To elucidate the molecular mechanisms underlying noncanonical PAM recognition by SpCas9, we employed molecular dynamics simulations to compare the structural differences within the Cas9-gRNA-DNA ternary complex when bound to various PAM sequences. Our analysis revealed significant conformational changes within SpCas9 upon engagement with noncanonical PAMs and uncovered the regulatory mechanisms underpinning these changes. We further identified key dynamic determinants governing the extensive conformational transitions occurring during the noncanonical PAM binding process. These findings provide insights into the dynamic landscape of noncanonical PAM recognition, offering crucial mechanistic guidance for designing efficient, PAM-compatible Cas9 variants.