Domain within herpes simplex virus 1 scaffold proteins required for interaction with portal protein in infected cells and incorporation of the portal vertex into capsids

The portal vertex of herpesvirus capsids serves as the conduit through which DNA is inserted during the assembly process. In herpes simplex virus (HSV), the portal is composed of 12 copies of the U,6 gene product, pU(L)6. Previous results identified a domain in the major capsid scaffold protein, ICP35, required for interaction with pU,6 and its incorporation into capsids formed in vitro (G. P. Singer et al., J. Virol. 74:6838-6848, 2005). In the current studies, PU(L)6 and scaffold proteins were found to coimmunoprecipitate from lysates of both HSV-infected cells and mammalian cells expressing scaffold proteins and PU(L)6. The coimmunoprecipitation of PU(L)6 and scaffold proteins was precluded upon deletion of codons 143 to 151 within U(L)26.5, encoding ICP35. While wild-type scaffold proteins colocalized with PU(L)6 when transiently coexpressed as viewed by indirect immunofluorescence, deletion of UL26.5 codons 143 to 151 precluded this colocalization. A recombinant herpes simplex virus, vJB11, was generated that lacked U(L)26.5 codons 143 to 151. A virus derived from this mutant but bearing a restored U(L)26.5 was also generated. vJBI1 was unable to cleave or package viral DNA, whereas the restored virus packaged DNA normally. vJBI1 produced ample numbers of B capsids in infected cells, but these lacked normal levels of pU(L)6. The deletion in U(L)26.5 also rendered PU(L)6 resistant to detergent extraction from vJ1311-infected cells. These data indicate that, as was observed in vitro, amino acids 143 to 151 of ICP35 are critical for (i) interaction between scaffold proteins and PU(L)6 and (ii) incorporation of the HSV portal into capsids.