Control of VP16 Translation by the Herpes Simplex Virus Type 1 Immediate-Early Protein ICP27

Abstract

Herpes simplex virus (HSV) ICP27 is an essential and multifunctional regulator of gene expression that modulates the synthesis and maturation of viral and cellular mRNAs. Processes that are affected by ICP27 include transcription, pre-mRNA splicing, polyadenylation, and nuclear RNA export. We have examined how ICP27 influences the expression of the essential HSV tegument protein and transactivator of immediate-early gene expression VP16. We monitored the effects of ICP27 on the levels, nuclear export, and polyribosomal association of VP16 mRNA and on the amount and stability of VP16 protein. Deletion of ICP27 reduced the levels of VP16 mRNA without altering its nuclear export or the stability of the encoded protein. However, the translational yield of the VP16 mRNA produced in the absence of ICP27 was reduced 9-to 80-fold relative to that for wild-type infection, suggesting a defect in translation. In the absence of ICP27, the majority of cytoplasmic VP16 mRNA was not associated with actively translating polyribosomes but instead cosedimented with 40S ribosomal subunits, indicating that the translational defect is likely at the level of initiation. These effects were mRNA specific, as polyribosomal analysis of two cellular transcripts (glyceraldehyde-3-phosphate dehydrogenase and ␤-actin) and two early HSV transcripts (thymidine kinase and ICP8) indicated that ICP27 is not required for efficient translation of these mRNAs. Thus, we have uncovered a novel mRNA-specific translational regulatory function of ICP27.Lytic infection by herpes simplex virus type 1 (HSV-1) is characterized by shutoff of host protein synthesis and the temporally regulated expression of three classes of viral genes: immediate-early (IE), early (E), and late (L) (reviewed in reference 66). The infectious cycle is initiated by the virion transactivator VP16, which upon delivery into host cells acts in concert with cellular factors to induce the expression of the five IE genes. Four of these encode nuclear regulatory proteins (ICP0, ICP22, ICP4, and ICP27) that orchestrate the timely expression of the E and L genes. ICP27 plays an indispensable role in the viral life cycle (41, 67). ICP27 is necessary for efficient expression of some E genes, including a subset of those that encode proteins required for DNA replication (83). Hence, ICP27-null mutants display a partial defect in viral DNA replication. ICP27 also promotes the expression of most L genes (11, 41, 45,62,63,67,75) and is required for the so-called delayed shutoff of host protein synthesis (19,20,67,79). How ICP27 performs its various functions remains poorly defined; however, it is becoming increasingly evident that ICP27 modulates virtually every aspect of mRNA metabolism, including primary transcription, polyadenylation, splicing, nuclear export, and mRNA stability (reviewed below).It was initially assumed that the effects of ICP27 on HSV-1 gene expression were mediated primarily at the transcriptional level. Indeed, ICP27 is required for the transcription of at least two viral l...

Viral and cellular mRNA-specific activators harness PABP and eIF4G to promote translation initiation downstream of cap binding

“…Consistent with its role as an mRNA-specific translational activator (7,8,14), ICP27 binds RNA, cosediments with polyribosomes (14), and can be isolated with components of the translational machinery (10,15). Because ICP27 can stimulate translation in the absence of other viral proteins when tethered to the 3′ UTR of reporter mRNAs (14), its interaction with the translational machinery must either be direct or mediated indirectly by other cellular proteins, further supporting the idea that it may share a mechanism of action with cellular regulators of translation.…”

“…ICP27 regulates the translation of a subset of viral mRNAs, including the viral mRNAs encoding VP16 and ICP5 (7,8), via an uncharacterized mechanism. This effect is mRNA-specific, because the translation of other studied viral and cellular mRNAs is not ICP27-dependent (7,8). We posited that elucidating this mechanism may provide insight into cellular mRNA-specific regulators because HSV-1 mRNAs are, like most cellular mRNAs, capped, polyadenylated, and translated via the canonical capdependent pathway described above.…”

“…Because studies of viruses have proved highly informative for the understanding of cellular translational regulation, we examined the mechanism of action of ICP27, an essential herpes simplex virus type 1 (HSV-1) protein that is a key regulator of host and viral gene expression (6). ICP27 regulates the translation of a subset of viral mRNAs, including the viral mRNAs encoding VP16 and ICP5 (7,8), via an uncharacterized mechanism. This effect is mRNA-specific, because the translation of other studied viral and cellular mRNAs is not ICP27-dependent (7,8).…”

Human cytomegalovirus UL69 protein facilitates translation by associating with the mRNA cap-binding complex and excluding 4EBP1

“…Our attention was drawn to the predicted interaction of pUL69 with poly(A)-binding protein C1 (PABPC1) and eukaryotic initiation factor 4A1 (eIF4A1), because the HSV pUL69 homolog, ICP27, has been shown to interact with initiation factors, including PABP (20), and facilitate the translation of a subset of virus-coded mRNAs (21)(22)(23). PABPC1 and eIF4A1 are both constituents of the mRNA cap-binding complex, raising the possibility that pUL69 interacts with them to modulate translation.…”

Three Arginine Residues within the RGG Box Are Crucial for ICP27 Binding to Herpes Simplex Virus 1 GC-Rich Sequences and for Efficient Viral RNA Export

“…ICP27 interacts with a number of cellular proteins, and it binds RNA (35). One of the functions that ICP27 performs is to escort viral mRNAs from the nucleus to the cytoplasm for translation (2,3,5,10,13,21,34). ICP27 binds viral RNAs (5, 34) and interacts directly with the cellular mRNA export receptor TAP/NXF1 (2, 21), which is required for the export of HSV-1 mRNAs (20,21).…”