The herpes simplex virus type-1 origin binding protein. DNA helicase activity

The herpes simplex virus type 1 (HSV-1) origin binding protein, the product of the UL9 gene, catalyzes the unwinding of DNA in the 3'-5' direction. Helicase activity is coupled to the hydrolysis of ATP or dATP and to a lesser extent to CTP, dCTP, or UTP. It requires a divalent cation (Mg²⁺ > Mn²⁺ > Ca²⁺) with an optimum at 2.5 mM MgCl₂. Activity is optimal at high pH (8.5-9.5) and high temperature (45 °C) and is inhibited at ionic strengths > 50 mM NaCl. The helicase activity is specifically stimulated by the HSV-1-encoded single-stranded DNA-binding protein, ICP8, which increases both the rate and extent of helicase activity. Helicase action appears to be stoichiometric, requiring a DNA-dependent assembly of a multimeric UL9 protein complex. Under optimal conditions, the rate of DNA unwinding is approximately 75 base pairs/min.