The herpes simplex virus type-1 single-strand DNA-binding protein, ICP8, increases the processivity of the UL9 protein DNA helicase.

Herpes simplex virus type-1 UL9 protein is a sequence-specific DNA-binding protein that recognizes elements in the viral origins of DNA replication and possesses DNA helicase activity. It forms an essential complex with its cognate single-strand DNA-binding protein, ICP8. The DNA helicase activity of the UL9 protein is greatly stimulated as a consequence of this interaction. A complex of these two proteins is thought to be responsible for unwinding the viral origins of DNA replication. The aim of this study was to identify the mechanism by which ICP8 stimulates the translocation of the UL9 protein along DNA. The data show that the association of the UL9 protein with DNA substrate is slow and that its dissociation from the DNA substrate is fast, suggesting that it is nonprocessive. ICP8 caused maximal stimulation of DNA unwinding activity at equimolar UL9 protein concentrations, indicating that the active species is a complex that contains UL9 protein and ICP8 in 1:1 ratio. ICP8 prevented dissociation of UL9 protein from the DNA substrate, suggesting that it increases its processivity. ICP8 specifically stimulated the DNA-dependent ATPase activity of the UL9 protein with DNA cofactors that allow translocation of UL9 protein and those with secondary structure. These data suggest that UL9 protein and ICP8 form a specific complex that translocates along DNA. Within this complex, ICP8 tethers the UL9 protein to the DNA substrate, thereby preventing its dissociation, and participates directly in the assimilation and stabilization of the unwound DNA strand, thus facilitating translocation of the complex through regions of duplex DNA.