Searching DNA via a "Monkey Bar" mechanism: the significance of disordered tails.

The search through nonspecific DNA for a specific site by proteins is known to be facilitated by sliding, hopping, and intersegment transfer between separate DNA strands, yet the driving forces of these protein dynamics from the molecular perspective are unclear. In this study, molecular features of the DNA search mechanism were explored for three homologous proteins (the HoxD9, Antp, and NK-2 homeodomains) using a simple computational model in which protein-DNA interactions are represented solely by electrostatic forces. In particular, we studied the impact that disordered N-terminal tails (N-tails), which are more common in DNA-binding proteins than in other proteins, have on the efficiency of DNA search. While the three homeodomain proteins were found to use similar binding interfaces in specific and nonspecific interactions with DNAs, their different electrostatic potentials affect the nature of their sliding dynamics. The different lengths and net charges of the N-tails of the homeodomains affect their motion along the DNA. The presence of an N-tail increases sliding propensity but slows linear diffusion along the DNA. When the search is performed in the presence of two parallel DNA molecules, a direct transfer, which is facilitated by the protein tail, from one nonspecific DNA to another occurs. The tailed proteins jump between two DNA molecules through an intermediate in which the recognition helix of the protein is adsorbed to one DNA fragment and the N-tail is adsorbed to the second, suggesting a "monkey bar" mechanism. Our study illustrates how the molecular architecture of proteins controls the efficiency of DNA scanning.