Protein-DNA interactions in high speed AFM: single molecule diffusion analysis of human RAD54

High-speed AFM (atomic force microscopy also called scanning force microscopy) provides nanometre spatial resolution and sub-second temporal resolution images of individual molecules. We exploit these features to study diffusion and motor activity of the RAD54 DNA repair factor. Human RAD54 functions at critical steps in recombinational-DNA repair. It is a member of the Swi2/Snf2 family of chromatin remodelers that translocate on DNA using ATP hydrolysis. A detailed single molecular description of DNA-protein interactions shows intermediate states and distribution of variable states, usually hidden by ensemble averaging. We measured the motion of individual proteins using single-particle tracking and observed that random walks were affected by imaging-buffer composition. Non-Brownian diffusion events were characterized in the presence and in the absence of nucleotide cofactors. Double-stranded DNA immobilized on the surface functioned as a trap reducing Brownian motion. Distinct short range slides and hops on DNA were visualized by high-speed AFM. These short-range interactions were usually inaccessible by other methods based on optical resolution. RAD54 monomers displayed a diffusive behavior unrelated to the motor activity.