The interaction of herpes simplex type 1 virus origin-binding protein (UL9 protein) with Box I, the high affinity...

The herpes simplex type 1 (HSV-1) origin binding protein, the UL9 protein, exists in solution as a homodimer of 94-kDa monomers. It binds to Box I, the high affinity element of the HSV-1 origin, Oris, as a dimer. The UL9 protein also binds the HSV-1 single strand DNA-binding protein, ICP8. Photocross-linking studies have shown that although the UL9 protein binds Box I as a dimer, only one of the two monomers contacts Box I. It is this form of the UL9 homodimer that upon interaction with ICP8, promotes the unwinding of Box I coupled to the hydrolysis of ATP to ADP and Pi. Photocross-linking studies have also shown that the amount of UL9 protein that interacts with Box I is reduced by its interaction with ICP8. Antibody directed against the C-terminal ten amino acids of the UL9 protein inhibits its Box I unwinding activity, consistent with the requirement for interaction of the C terminus of the UL9 protein with ICP8. Inhibition by the antibody is enhanced when the UL9 protein is first bound to Box I, suggesting that the C terminus of the UL9 protein undergoes a conformational change upon binding Box I.