Herpes simplex virus type 1 protein IE63 affects the nuclear export of virus intron-containing transcripts

Functional Domains within the Nucleus of a Cell Infected with HSV-1

HSV-1 is a nuclear replicating DNA virus capable of establishing both lytic and latent infections in mammalian cells. Expression of the more than 80 HSV genes (the majority of which do not contain introns) requires complex coordination of viral and cellular factors both temporally, at appropriate points during the infectious cycle, and spatially as the virus transcription, replication and DNA packaging factories develop in the cell nucleus. Whilst the HSV genome encodes sufficient proteins to sustain viral DNA replication, it is reliant upon its host cell for RNA polymerase II and RNA processing machinery, in addition to an increasing number of cellular cofactors, for gene expression. As HSV establishes a lytic infection, cellular gene expression and splicing are inhibited as cellular chromatin is displaced and a dramatic reorganisation of the host cell nucleus occurs. The formation of large protein-rich factories synthesising viral RNA and replicating and packaging the viral genomes is the most striking alteration. In addition to the synthetic factories, large clumps of cellular and viral intron-containing RNAs accumulate in the nucleus as a result of the inhibition of splicing, at locations which colocalise with splicing factors, but are separate from transcription sites. An essential HSV protein IE63, discussed here, has been identified with a role in the organisation of the nucleus at many levels including replication and transcription site formation, splicing factor organisation and the transport of RNA. This review is a summary of our present understanding of the organisation of the HSV infected cell nucleus, relating viral genomes, RNA, DNA and proteins in the context of the nucleus. However this is a rapidly evolving field and new factors (both viral and cellular) involved in the regulation of these functional domains are constantly being identified. Copyright 1997 by John Wiley & Sons, Ltd.

The interaction between human cytomegalovirus immediate-early gene 2 (IE2) protein and heterogeneous ribonucleoprotein A1

Human cytomegalovirus (HCMV) is an ubiquitous pathogen which causes significant illness in immunocompromised individuals. The immediate-early gene 2 (IE2) protein of HCMV plays an important role in the regulation of virus replication. Previous studies have shown that the IE2 protein is able to interact with several cellular proteins, but many of the IE2 interacting partners remain unidentified. By utilizing the yeast two-hybrid system, the heterogeneous ribonucleoprotein A1 (hnRNP A1) was identified as an IE2 interacting protein. The interaction was confirmed via the in vitro binding of bacterial expressed glutathione S-transferase (GST) IE2 fusion protein with the in vitro translated hnRNP A1. The mutational analysis showed that both the N-terminal half (1-290 residues) and C-terminal half (291-579 residues) of IE2 protein can interact with the hnRNP A1, indicating that more than one region of IE2 protein are involved in the binding with hnRNP A1.

Differentiation of the shutoff of protein synthesis by virion host shutoff and mutant gamma (1)34.5 genes of herpes simplex virus 1

vhs protein is the product of the UL41 open reading frame of herpes simplex virus 1. The protein, made late in infection, is packaged into virions and, in newly infected cells, shuts off host protein synthesis by degrading mRNA. gamma (1)34.5 gene encodes a protein which precludes total shutoff of protein synthesis after the onset of viral DNA synthesis in infected cells of human derivation. The experiments reported here were designed to test the hypothesis that in cells infected with gamma (1)34.5- mutant the total shutoff of protein synthesis reflects the failure to alter the function of vhs made late in infection. Hence, double mutants, vhs- and gamma (1)34.5 should not cause total shutoff of protein synthesis. The mutants constructed to test the hypothesis were (i) viruses lacking 1 kbp from the coding domain of gamma (1)34.5 and carrying lacZ inserted into the coding domain of UI41, (ii) viruses with deletions in gamma (1)34.5 genes, (iii) viruses with lacZ inserted into UL41, and (iv) viruses in which the sequences of the deleted or interrupted genes were restored. We report that viruses with wild-type UL41 gene shut off the synthesis of actin, whereas viruses with interrupted genes made amounts of actin comparable to those of mock-infected cells. However, late in infection, protein synthesis in human neuroblastoma cells infected with the gamma (1)34.5- mutants was shut off irrespective of the status of the UL41 gene. Conversely, the phenotype of UI41 viruses with wild-type gamma (1)34.5 gene could not be differentiated from those of wild-type virus in the same assays. These studies indicate that the functions of the UL41 and gamma (1)34.5 genes and their products are independent of each other.

Nuclear sites of herpes simplex virus type 1 DNA replication and transcription colocalize at early times postinfection and are largely distinct from RNA processing factors

We have visualized the intracellular localization of herpes simplex virus (HSV) type 1 replication and transcription sites in infected HeLa cells by using direct labelling methods. The number of viral transcription foci increases in a limited way; however, the number of replication sites increases in a near-exponential manner throughout infection, and both replication and transcription sites are found buried throughout the nuclear interior. Simultaneous visualization of viral transcription and replication foci shows that the two processes colocalize at early times, but at later times postinfection, there are additional sites committed solely to replication. This contrasts with the situation in adenovirus-infected cells in which, throughout replication, sites of transcription are adjacent to but do not colocalize with sites of viral DNA replication. The data for an increase in HSV transcription sites suggest an initial phase of replication of input genomes which are then transcribed. Sites of HSV replication colocalize with viral DNA replication and packaging proteins but are largely distinct from the punctate distribution of small nuclear ribonucleoprotein particles. Very high multiplicities of infection have shown an upper limit of some 18 viral transcription foci per nucleus, suggesting cellular constraints on transcription site formation. Use of virus replication mutants confirms that the labelled foci are sites of viral RNA and DNA synthesis; in the absence of viral DNA replication functions, no replication foci and only a limited number of transcription foci were present. Absence of a packaging function had no apparent effect on transcription or replication site formation, illustrating that DNA packaging is not a prerequisite for ongoing DNA synthesis. Further, the essential HSV protein IE63 is required for efficient replication site formation at later times postinfection but is not required for transcription foci formation.