The conserved helicase motifs of the herpes simplex virus type 1 origin-binding protein UL9 are important for...

The UL9 gene of herpes simplex virus encodes a protein that specifically recognizes sequences within the viral origins of replication and exhibits helicase and DNA-dependent ATPase activities. The specific DNA binding domain of the UL9 protein was localized to the carboxy-terminal one-third of the molecule (H. M. Weir, J. M. Calder, and N. D. Stow, Nucleic Acids Res. 17:1409-1425, 1989). The N-terminal two-thirds of the UL9 gene contains six sequence motifs found in all members of a superfamily of DNA and RNA helicases, suggesting that this region may be important for helicase activity of UL9. In this report, we examined the functional significance of these six motifs for the UL9 protein through the introduction of site-specific mutations resulting in single amino acid substitutions of the most highly conserved residues within each motif. An in vivo complementation test was used to study the effect of each mutation on the function of the UL9 protein in viral DNA replication. In this assay, a mutant UL9 protein expressed from a transfected plasmid is used to complement a replication-deficient null mutant in the UL9 gene for the amplification of herpes simplex virus origin-containing plasmids. Mutations in five of the six conserved motifs inactivated the function of the UL9 protein in viral DNA replication, providing direct evidence for the importance of these conserved motifs. Insertion mutants resulting in the introduction of two alanines at 100-residue intervals in regions outside the conserved motifs were also constructed. Three of the insertion mutations were tolerated, whereas the other five abolished UL9 function. These data indicate that other regions of the protein, in addition to the helicase motifs, are important for function in vivo. Several mutations result in instability of the mutant products, presumably because of conformational changes in the protein. Taken together, these results suggest that UL9 is very sensitive to mutations with respect to both structure and function, perhaps reflecting its multifunctional character.