Hexameric helicase deconstructed: interplay of conformational changes and substrate coupling.

Hexameric helicases are molecular motor proteins that utilize energy obtained from ATP hydrolysis to translocate along and/or unwind nucleic acids. In this study, we investigate the dynamic behavior of the Simian Virus 40 hexameric helicase bound to DNA by performing molecular dynamics simulations employing a coarse-grained model. Our results elucidate the two most important molecular features of the helicase motion. First, the attractive interactions between the DNA-binding domain of the helicase and the DNA backbone are essential for the helicase to exhibit a unidirectional motion along the DNA strand. Second, the sequence of ATP binding at multiple binding pockets affects the helicase motion. Specifically, concerted ATP binding does not generate a unidirectional motion of the helicase. It is only when the binding of ATP occurs sequentially from one pocket to the next that the helicase moves unidirectionally along the DNA. Interestingly, in the reverse order of sequential ATP binding, the helicase also moves unidirectionally but in the opposite direction. These observations suggest that in nature ATP molecules must distinguish between different available ATP binding pockets of the hexameric helicase in order to function efficiently. To this end, simulations reveal that the binding of ATP in one pocket induces an opening of the next ATP-binding pocket and such an asymmetric deformation may coordinate the sequential ATP binding in a unidirectional manner. Overall, these findings may provide clues toward understanding the mechanism of substrate translocation in other motor proteins.