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-bin ding protein that recognizes elements in the viral origins of DNA repl ication and possesses DNA helicase activity, It forms an essential com plex with its cognate single-strand DNA-binding protein, ICPS, The DNA helicase activity of the UL9 protein is greatly stimulated as a conse quence 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 ICPS sti mulates 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 I:1 ratio. ICP8 prevented dissociation of UL9 protein from the DNA substr ate, suggesting that it increases its processivity, ICPS specifically stimulated the DNA-dependent ATPase activity of the UL9 protein with D NA cofactors that allow translocation of UL9 protein and those with se condary structure, These data suggest that UL9 protein and ICP8 form a specific complex that translocates along DNA. Within this complex, IC P8 tethers the UL9 protein to the DNA substrate, thereby preventing it s dissociation, and participates directly in the assimilation and stab ilization of the unwound DNA strand, thus facilitating translocation o f the complex through regions of duplex DNA.