Tailed bacteriophages and herpesviruses load their capsids with DNA through a tunnel formed by the portal protein assembly. Here we describe the X-ray structure of the bacteriophage SPP1 portal protein in its isolated 13-subunit form and the pseudoatomic structure of a 12-subunit assembly. The first defines the DNA-interacting segments (tunnel loops) that pack tightly against each other forming the most constricted part of the tunnel; the second shows that the functional dodecameric state must induce variability in the loop positions. Structural observations together with geometrical constraints dictate that in the portal-DNA complex, the loops form an undulating belt that fits and tightly embraces the helical DNA, suggesting that DNA translocation is accompanied by a 'mexican wave' of positional and conformational changes propagating sequentially along this belt.

After penetrating the host cell, the herpesvirus capsid is transported to the nucleus along the microtubule network and docks to the nuclear pore complex before releasing the viral DNA into the nucleus. The viral and cellular interactions involved in the docking process are poorly characterized. However, the minor capsid protein pUL25 has recently been reported to be involved in viral DNA uncoating. Here we show that herpes simplex virus type 1 (HSV-1) capsids interact with the nucleoporin CAN/Nup214 in infected cells and that RNA silencing of CAN/Nup214 delays the onset of viral DNA replication in the nucleus. We also show that pUL25 interacts with CAN/Nup214 and another nucleoporin, hCG1, and binds to the pUL36 and pUL6 proteins, two other components of the herpesvirus particle that are known to be important for the initiation of infection and viral DNA release. These results identify CAN/Nup214 as being a nuclear receptor for the herpesvirus capsid and pUL25 as being an interface between incoming capsids and the nuclear pore complex and as being a triggering element for viral DNA release into the nucleus.Many nucleus-replicating viruses have evolved different strategies for delivering their genomes into the nucleus of their host cell through the nuclear pores, which provide the only route of transit across the physical barrier of the nuclear envelope. These strategies depend mainly on the nature of the capsid, which acts both as a protective element for the genome and as a delivery agent (for reviews, see references 21 and 60).Alphaherpesviruses are large, double-stranded DNA viruses. Their genomes are contained within a 125-nm-diameter capsid that is surrounded sequentially by a thick proteinaceous layer, called the tegument, and a lipid envelope. The herpes simplex virus type 1 (HSV-1) capsid structure has been extensively studied (66) and is a general model for other alphaherpesviruses. It has icosahedral symmetry with the major capsid protein VP5, forming hexamers and pentamers (termed hexons and pentons) at the faces and vertices, respectively, of the icosahedron. There are 150 hexons and 11 pentons per capsid. At one vertex, the penton is replaced by a portal, a structure common to tailed bacteriophages and herpesviruses, through which the viral DNA is encapsidated and released (7,8). In HSV-1, the portal is a dodecamer of the UL6 gene product, pUL6 (38, 57).The nuclear pore complex (NPC) is a multiprotein complex that selectively controls the passage of material through the nuclear envelope (for a review, see reference 28). The NPC has three structural components: the nuclear basket, the central framework, which is embedded in the nuclear envelope, and the cytoplasmic filaments. The diameter of the cytoplasmic face is ϳ125 nm, whereas the central channel is ϳ60 nm in diameter (3). Its component proteins, termed nucleoporins, perform various roles, being important both in forming a selective gate and in carrying out nucleocytoplasmic transport (41, 55). Several models have been proposed to explain the s...