The putative terminase subunit of herpes simplex virus 1 encoded by UL28 is necessary and sufficient to mediate interaction between pUL15 and pUL33.

Viral terminases play essential roles as components of molecular motors that package viral DNA into capsids. Previous results indicated that the putative terminase subunits of herpes simplex virus 1 (HSV-1) encoded by U(L)15 and U(L)28 (designated pU(L)15 and pU(L)28, respectively) coimmunoprecipitate with the U(L)33 protein from lysates of infected cells. All three proteins are among six required for HSV-1 DNA packaging but dispensable for assembly of immature capsids. The current results show that in both infected- and uninfected-cell lysates, pU(L)28 coimmunoprecipitates with either pU(L)33 or pU(L)15, whereas pU(L)15 and pU(L)33 do not coimmunoprecipitate unless pU(L)28 is present. The U(L)28 protein was sufficient to stabilize pU(L)33 from proteasomal degradation in an engineered cell line and was necessary to stabilize pU(L)33 in infected cells, whereas pU(L)15 had no such effects. The presence of pU(L)33 was dispensable for the pU(L)15/pU(L)28 interaction in lysates of both infected and uninfected cells but augmented the tendency for pU(L)15 and pU(L)28 to coimmunoprecipitate. These data suggest that pU(L)28 and pU(L)33 interact directly and that pU(L)15 interacts directly with pU(L)28 but only indirectly with pU(L)33. It is logical to propose that the indirect interaction of pU(L)15 and pU(L)33 is mediated through the interaction of both proteins with pU(L)28. The data also suggest that one function of pU(L)33 is to optimize the pU(L)15/pU(L)28 interaction.