Microbiol. cytoplasm of the infected cells, indicating that this deletion affected the nuclear transport of the portal protein. The UL6 protein from the KOS strain (wild (S)-Tedizolid type) and the D-6 mutant were purified from insect cells infected with recombinant baculoviruses and shown to form ring structures as assessed by sucrose gradient centrifugation and electron microscopy. In contrast, the D-LZ mutant protein formed aggregates that sedimented throughout the sucrose gradient as a heterogeneous mixture and did not yield stable ring structures. A mutant (L429E L436E) in which two of the heptad leucines of the putative zipper were replaced with glutamate residues also failed to form stable rings. Our results suggest that the integrity of the leucine zipper region is important for oligomer interactions and stable ring formation, which in turn are required for genome encapsidation. The formation of mature capsids containing the linear double-stranded herpes simplex virus type 1 (HSV-1) genome is a complex process involving a preassembled protein shell (procapsid), concatemeric viral DNA, and seven other viral proteins believed to play a role in cleavage and packaging of the genome (1, 10). The procapsid contains three shell proteins, VP5, VP19C, and VP23, and the viral scaffolding proteins (encoded by UL26 and UL26.5). During wild-type (WT) infection, (S)-Tedizolid three capsid forms are observed: A capsids, which have initiated the encapsidation process and have lost both scaffold and DNA; B capsids, which contain the scaffold but no DNA; and DNA-containing mature C capsids, which have lost scaffold during the process of taking up DNA. The cleavage and packaging machinery consists of the UL6 portal ring, a two-subunit terminase (UL15 and UL28), a protein which seals the capsid after DNA uptake has occurred (UL25), and three other proteins which (S)-Tedizolid play less well defined roles in the process (UL17, UL32, and UL33). Defects in any of these genes except the UL25 gene result in the accumulation of B capsids. The protein encoded by the UL6 gene is present in all three forms of viral capsids (19) and is believed to form a ring structure at a unique vertex which serves as the portal for the entry of viral DNA into the HSV-1 capsid. The UL6 protein is essential for cleavage of the replicated genome, and it can localize to the nucleus in the absence of other viral proteins (19, 20). The UL6 protein is also required for the efficient association of the terminase subunit, UL15, with the capsid (21, 33). Coimmunoprecipitation experiments with recombinant proteins showed that UL6 forms a complex with UL15 and UL28 (30). Newcomb et al. purified the UL6 protein from recombinant baculovirus-infected insect cells and showed in vitro that it forms oligomeric rings (17). The (S)-Tedizolid ring (average of 12 subunits) encloses a channel approximately (S)-Tedizolid 5 nm in diameter and resembles portals from bacteriophages such as T4, P22, SPP1, and 29. Trus et al. determined a low-resolution structure of the HSV-1 portal from cryoelectron microscopy images of UL6 rings (29). The rings average 15 nm Gata3 in diameter and show narrow flanges on the outside, resembling the shape of a conical propeller. Using electron tomography of HSV-1 capsids from which the pentons were removed, Chang et al. showed the portal to be partly buried at a unique fivefold axis within the capsid shell (5). Recently, Cardone et al. confirmed the location of the HSV-1 portal at a unique capsid vertex; their tomographic analysis of A capsids, however, suggested that a major portion of the portal is present outside the capsid floor (3). The apparent discrepancy between these studies may be due to differences in the type of capsids used and the treatment of capsids with guanidine in the former study. The results from all these studies indicate that UL6 forms the portal ring and probably acts as the docking site for the terminase, which would be expected to bring the viral genome to the capsids prior to cleavage and encapsidation. Structural studies have provided information about other portal rings. For instance, the three-dimensional structure of the 29 portal (connector, gp10) was determined by X-ray crystallography (24) and shown to consist of a cone-shaped dodecamer with a central channel lined by long -helices and.