Modulation of the herpes simplex virus type-1 UL9 DNA helicase by its cognate single-strand DNA-binding protein, ICP8

The mechanism of stimulation of a DNA helicase by its cognate single-strand DNA binding protein was examined using herpes simplex virus type-1 UL9 DNA helicase and ICP8. UL9 and ICP8 are two essential components of the viral replisome that associate into a complex to unwind the origins of replicatio n. The helicase and DNA stimulated ATPase activities of UL9 are greatly ele vated as a consequence of this association. Given that ICPS acts as a singl e strand DNA;binding protein, the simplest model that can account for its s timulatory effect predicts that it tethers UL9 to the DNA template, thereby increasing its processivity. In contrast to the prediction, data presented here show that the stimulatory activity of ICP8 does not depend on its sin gle-strand DNA binding activity. Our data support an alternative hypothesis in which ICPS modulates the activity of UL9. Accordingly, the data show th at the ICP8 binding site of UL9 constitutes an inhibitory region that maint ains the helicase in an inefficient ground state. ICP8 acts as a positive r egulator by neutralizing this region. ICP8 does not affect substrate bindin g, ATP hydrolysis, or the efficiency of translocation/DNA unwinding. Rather , we propose that ICP8 increases the efficiency with which substrate bindin g and ATP hydrolysis are coupled to translocation/DNA unwinding.