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J. Biol. Chem., Vol. 276, Issue 9, 6840-6845, March 2, 2001
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From the Department of Biochemistry and Molecular Biology,
University of Miami School of Medicine, Miami, Florida 33101-6129
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 replication. The helicase and
DNA-stimulated ATPase activities of UL9 are greatly elevated as a
consequence of this association. Given that ICP8 acts as a
single-strand DNA-binding protein, the simplest model that can account
for its stimulatory 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 single-strand DNA binding activity. Our
data support an alternative hypothesis in which ICP8 modulates the
activity of UL9. Accordingly, the data show that the ICP8-binding site
of UL9 constitutes an inhibitory region that maintains the helicase in
an inefficient ground state. ICP8 acts as a positive regulator by
neutralizing this region. ICP8 does not affect substrate binding, ATP
hydrolysis, or the efficiency of translocation/DNA unwinding. Rather,
we propose that ICP8 increases the efficiency with which substrate
binding and ATP hydrolysis are coupled to translocation/DNA unwinding.
To whom correspondence should be addressed: Dept. of Biochemistry
and Molecular Biology, University of Miami School of Medicine, P.O. Box
016129, Miami, FL 33101-6129. Tel.: 305-243-2934; Fax: 305-243-3955;
E-mail: pboehmer@molbio.med.miami.edu.
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