Herpesviruses replicate their DNA and package this DNA into capsids in the nucleus. These capsids then face substantial obstacles to their release from cells. Unlike other DNA viruses, herpesviruses do not depend on disruption of nuclear and cytoplasmic membranes for their release. Enveloped particles are formed by budding through inner nuclear membranes, and then these perinuclear enveloped particles fuse with outer nuclear membranes. Unenveloped capsids in the cytoplasm are decorated with tegument proteins and then undergo secondary envelopment by budding into trans-Golgi network membranes, producing infectious particles that are released. In this Review, we describe the remodelling of host membranes that facilitates herpesvirus egress.

The wild-type U L 31, U L 34, and U S 3 proteins localized on nuclear membranes and perinuclear virions; the U S 3 protein was also on cytoplasmic membranes and extranuclear virions. The U L 31 and U L 34 proteins were not detected in extracellular virions. U S 3 deletion caused (i) virion accumulation in nuclear membrane invaginations, (ii) delayed virus production onset, and (iii) reduced peak virus titers. These data support the herpes simplex virus type 1 deenvelopment-reenvelopment model of virion egress and suggest that the U S 3 protein plays an important, but nonessential, role in the egress pathway.Herpes simplex virus type 1 (HSV-1) virions contain a linear double-stranded DNA genome of approximately 152 kb that is packaged into an icosahedral capsid shell. An amorphous tegument layer surrounds the capsid and is, in turn, surrounded by an envelope composed of a host-derived lipid bilayer studded with viral integral membrane proteins. After the viral genome is replicated and packaged into capsids within the nucleus, assembled nucleocapsids acquire a primary lipid envelope by budding through the inner nuclear membrane (INM) into the space located between the inner and outer leaflets of the nuclear envelope (25,33). Whereas the derivation of the primary envelope from the INM is widely accepted, the route of transit of the nascent virions from the perinuclear space to the extracellular space is more controversial. An overview of the key players in herpesvirus egress and a comparison of the salient features of the two proposed envelopment models have been recently published (8,25).A single-step model of herpesvirus envelopment was proposed for the prototypical alphaherpesvirus HSV-1 (6,18,35,44). This model proposes that enveloped virions move through the endoplasmic reticulum (ER) and the Golgi apparatus in transport vesicles with concomitant modification of primary virion glycoproteins. The single-step envelopment model is supported by the observations that (i) enveloped particles within vesicles can be readily detected by electron microscopy and in fracture label studies (35, 44) and (ii) virion egress and virion-associated glycoprotein processing are both inhibited in cells treated with the ionophore monensin (18). On the other hand, neither of these observations can exclude the alternative deenvelopment-reenvelopment model. Such a model is supported by mounting ultrastructural and biochemical evidence (3,10,13,14,30,37,41,46,50) and has been proposed for HSV-1, other alphaherpesviruses such as varicella-zoster virus (VZV) and pseudorabies virus (PrV), and betaherpesviruses such as human cytomegalovirus. In this model, primary envelopment occurs by budding through the INM but the primary envelope surrounding the perinuclear virion is lost, presumably by fusion with the outer lamellae of the nuclear envelope. In a second step, reenvelopment occurs by wrapping of the nucleocapsid and its associated tegument with a lipid bilayer originating from a membranous cytoplasmic organelle bearing viral glycoprotein...

The herpes simplex virus type 1 (HSV-1) U L 34 protein is likely a type II membrane protein that localizes within the nuclear membrane and is required for efficient envelopment of progeny virions at the nuclear envelope, whereas the U L 31 gene product of HSV-1 is a nuclear matrix-associated phosphoprotein previously shown to interact with U L 34 protein in HSV-1-infected cell lysates. For these studies, polyclonal antisera directed against purified fusion proteins containing U Herpes simplex virus type 1 (HSV-1) nucleocapsids, like those of all herpesviruses are assembled in the nucleus and acquire a lipid bilayer envelope by budding through the inner nuclear membrane into the perinuclear space (10). Several viral proteins have been implicated in this initial budding event, including the myristylated U L 11 protein, glycoprotein K, which is necessary for envelopment in nondividing cells, and U L 34 protein (2, 21, 37). Of these, only U L 34 protein has been implicated solely in the initial envelopment step, whereas glycoprotein K and U L 11 also play roles in egress through the cytoplasm towards the extracellular space (2, 21, 37).The U L 34 sequence predicts that the protein is oriented as a type II integral membrane protein with an N-terminal cytoplasmic domain of 247 amino acids and a C-terminal transmembrane domain of 22 amino acids (32,35,37). The type II membrane topology of HSV-2 U L 34 protein has recently been addressed (39). This topology predicts that if the transmembrane domain were anchored in the outer nuclear membrane, the bulk of the protein would lie in the cytoplasm, whereas localization in the inner nuclear membrane would place the bulk of the protein within the nucleoplasm.The exact role of U L 34 protein in the envelopment process remains unclear. One possibility is that U L 34 protein interacts directly with capsids and/or tegument components and the nuclear membrane, thereby mediating wrapping of the capsid in the membrane. Alternatively, U L 34 protein may be responsible for recruiting other viral or cellular factors to the site of envelopment. Both hypotheses predict that U L 34 protein should associate with the nuclear envelope. To date, research on the localization of HSV-1 U L 34 protein and its homologues in other herpesviruses has not yielded consistent results. In baculovirus-transduced cells. HSV-1 U L 34 protein is found at the nuclear envelope and in the cytoplasm (46), whereas in HSV-1-infected cells, U L 34 protein is reportedly detectable at the cell surface (35). HSV-2 U L 34 protein has been reported to localize at the endoplasmic reticulum in transfected and in-* Corresponding author. Mailing address: