Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport.

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE).

Herpesvirus nuclear egress: Pseudorabies Virus can simultaneously induce nuclear envelope breakdown and exit the nucleus via the envelopment-deenvelopment-pathway.

Herpesvirus replication takes place in the nucleus and in the cytosol. After entering the cell, nucleocapsids are transported to nuclear pores where viral DNA is released into the nucleus. After gene expression and DNA replication new nucleocapsids are assembled which have to exit the nucleus for virion formation in the cytosol.

Functional characterization of nuclear trafficking signals in pseudorabies virus pUL31.

Unlabelled: The herpesviral nuclear egress complex (NEC), consisting of pUL31 and pUL34 homologs, mediates efficient translocation of newly synthesized capsids from the nucleus to the cytosol. The tail-anchored membrane protein pUL34 is autonomously targeted to the nuclear envelope, while pUL31 is recruited to the inner nuclear membrane (INM) by interaction with pUL34. A nuclear localization signal (NLS) in several pUL31 homologs suggests importin-mediated translocation of the protein.

Identification of conserved amino acids in pUL34 which are critical for function of the pseudorabies virus nuclear egress complex.

Unlabelled: Nuclear egress of herpesvirus capsids is mediated by a conserved heterodimeric complex of two viral proteins, designated pUL34 and pUL31 in herpes simplex virus and pseudorabies virus (PrV). pUL34, a tail-anchored membrane protein, is targeted to the nuclear envelope and recruits pUL31 to the inner nuclear membrane (INM) to provide the docking and envelopment machinery for the nascent capsid. While the less conserved C-terminal part of pUL34 is required for correct positioning of the nuclear egress complex (NEC) at the INM, the conserved N-terminal part functions as a docking site for pUL31.

Mapping of sequences in Pseudorabies virus pUL34 that are required for formation and function of the nuclear egress complex.

The nuclear egress complex (NEC) is required for efficient translocation of newly synthesized herpesvirus nucleocapsids from the nucleus to the cytosol. It consists of the type II membrane protein pUL34 which interacts with pUL31 at the inner nuclear membrane (INM). To map regions within pUL34 required for nuclear membrane targeting and pUL31 interaction, we constructed deletion/substitution mutations.