Unveiling Cas8 Dynamics and Regulation within a transposon-encoded Cascade-TniQ Complex.

The Cascade-TniQ complex unveiled a new paradigm in biology, demonstrating that CRISPR-associated proteins can direct DNA transposition. Despite the tremendous potential of "knocking in" genes at desired sites, the mechanisms underlying DNA binding and transposition remain elusive. In this system, a conformational change of the Cas8 protein is essential for DNA binding, yet how it occurs is unclear. Here, structural modeling and free energy simulations reconstruct the Cas8 helical bundle and reveal an open-to-close conformational transition at key steps of the complex's function. We show that when Cascade-TniQ binds RNA, the Cas8 bundle changes conformation mediated by the interaction with the Cas7.1 protein. This interaction alleviates unfavorable contacts and synchronizes Cas8's shift with neighboring subunits, lowering the barrier for the transition to the open state, a critical requirement for DNA binding. As DNA fully pairs with RNA, the open state becomes increasingly accessible, favoring interactions with DNA and aiding the formation of an R-loop. These outcomes provide the first dynamic representation of a critical conformational change in one of the largest CRISPR systems and illustrate its role at critical steps of the Cascade-TniQ biophysical function, advancing our understanding of nucleic acid binding and transposition mechanisms.