Viruses are fully reliant on the translation machinery of their host cells to produce the polypeptides that are essential for viral replication. Consequently, viruses recruit host ribosomes to translate viral mRNAs, typically using virally encoded functions to seize control of cellular translation factors and the host signalling pathways that regulate their activity. This not only ensures that viral proteins will be produced, but also stifles innate host defences that are aimed at inhibiting the capacity of infected cells for protein synthesis. Remarkably, nearly every step of the translation process can be targeted by virally encoded functions. This Review discusses the diverse strategies that viruses use to subvert host protein synthesis functions and regulate mRNA translation in infected cells.

Although the activity of the translation initiation factor eIF4F is regulated in part by translational repressors (4E-BPs) that prevent incorporation of eIF4E, the cap-binding protein, into the initiation complex, the contribution of eIF4E phosphorylation to translational control remains controversial. Here, we demonstrate that the herpes simplex virus-1 (HSV-1) ICP0 gene product, a multifunctional transactivator of viral gene expression with ubiquitin E3 ligase activity that is important for vegetative replication and reactivation of latent infections, is required to stimulate phosphorylation of eIF4E as well as 4E-BP1, and promote assembly of eIF4F complexes in infected cells. Furthermore, 4E-BP1 is degraded by the proteasome in an ICP0-dependent manner, establishing that the proteasome can control 4E-BP1 steady-state levels. Preventing eIF4E phosphorylation by inhibiting the eIF4E kinase mnk-1 dramatically reduced viral replication and the translation of viral polypeptides in quiescent cells, providing the first evidence that phosphorylation of eIF4E by mnk-1 is critical for viral protein synthesis and replication. Thus, in marked contrast to many viruses that inactivate eIF4F, HSV-1 stimulates eIF4F complex assembly in quiescent, differentiated cells; moreover, this is important for viral replication, and may be crucial for HSV-1 to initiate its productive growth cycle in resting cells, such as latently infected neurons.

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Intracellular transport of cargos, including many viruses, involves directed movement on microtubules mediated by motor proteins. While a number of viruses bind motors of opposing directionality, how they associate with and control these motors to accomplish directed movement remains poorly understood. Here we show that human immunodeficiency virus type 1 (HIV-1) associates with the kinesin-1 adaptor protein, Fasiculation and Elongation Factor zeta 1 (FEZ1). RNAi-mediated FEZ1 depletion blocks early infection, with virus particles exhibiting bidirectional motility but no net movement to the nucleus. Furthermore, both dynein and kinesin-1 motors are required for HIV-1 trafficking to the nucleus. Finally, the ability of exogenously expressed FEZ1 to promote early HIV-1 infection requires binding to kinesin-1. Our findings demonstrate that opposing motors both contribute to early HIV-1 movement and identify the kinesin-1 adaptor, FEZ1 as a capsid-associated host regulator of this process usurped by HIV-1 to accomplish net inward movement toward the nucleus.

Summary Stable microtubule (MT) subsets form distinct networks from dynamic MTs and acquire distinguishing posttranslational modifications, notably detyrosination and acetylation. Acting as specialized tracks for vesicle and macromolecular transport, their formation is regulated by the end-binding protein, EB1, which recruits proteins that stabilize MTs. We show that HIV-1 induces the formation of acetylated and detyrosinated stable MTs early in infection. Although the MT depolymerizing agent nocodazole affected dynamic MTs, HIV-1 particles localized to nocodazole-resistant stable MTs and infection was minimally affected. EB1 depletion or expression of an EB1 carboxy-terminal fragment that acts as a dominant negative inhibitor of MT stabilization, prevented HIV-1-induced stable MT formation and suppressed early viral infection. Furthermore, we show that the HIV-1 matrix protein targets the EB1-binding protein, Kif4 to induce MT stabilization. Our findings illustrate how specialized MT-binding proteins mediate MT stabilization by HIV-1 and the importance of stable MT subsets in viral infection.

As a viral opportunistic pathogen associated with serious disease among the immunocompromised and congenital defects in newborns, human cytomegalovirus (HCMV) must engage the translational machinery within its host cell to synthesize the viral proteins required for its productive growth. However, unlike many viruses, HCMV does not suppress the translation of host polypeptides. Here, we examine how HCMV regulates the cellular cap recognition complex eIF4F, a critical component of the cellular translation initiation apparatus that recruits the 40S ribosome to the 5 end of the mRNA. This study establishes that the cap binding protein eIF4E, together with the translational repressor 4E-BP1, are both phosphorylated early in the productive viral growth cycle and that the activity of the cellular eIF4E kinase, mnk, is critical for efficient viral replication. Furthermore, HCMV replication also induces an increase in the overall abundance of eIF4F components and promotes assembly of eIF4F complexes. Notably, increasing the abundance of select eIF4F core components and associated factors alters the ratio of active eIF4F complexes in relation to the 4E-BP1 translational repressor, illustrating a new strategy through which members of the herpesvirus family enhance eIF4F activity during their replicative cycle.

Despite their self-sufficient ability to generate capped mRNAs from cytosolic DNA genomes, poxviruses must commandeer the critical eukaryotic translation initiation factor 4F (eIF4F) to recruit ribosomes. While eIF4F integrates signals to control translation, precisely how poxviruses manipulate the multisubunit eIF4F, composed of the cap-binding eIF4E and the RNA helicase eIF4A assembled onto an eIF4G platform, remains obscure. Here, we establish that the poxvirus infection of normal, primary human cells destroys the translational repressor eIF4E binding protein (4E-BP) and promotes eIF4E assembly into an active eIF4F complex bound to the cellular polyadenylate-binding protein (PABP). Stimulation of the eIF4G-associated kinase Mnk1 promotes eIF4E phosphorylation and enhances viral replication and protein synthesis. Remarkably, these eIF4F architectural alterations are accompanied by the concentration of eIF4E and eIF4G within cytosolic viral replication compartments surrounded by PABP. This demonstrates that poxvirus infection redistributes, assembles, and modifies core and associated components of eIF4F and concentrates them within discrete subcellular compartments. Furthermore, it suggests that the subcellular distribution of eIF4F components may potentiate the complex assembly.

Ribosomes have the capacity to selectively control translation through changes in their composition that enable recognition of specific RNA elements1. However, beyond differential subunit expression during development2,3, evidence for regulated ribosome specification within individual cells has remained elusive1. Here, we report that a poxvirus kinase phosphorylates serine/threonine residues in the small ribosomal subunit protein, Receptor for Activated C Kinase (RACK1) that are not phosphorylated in uninfected cells or cells infected by other viruses. These modified residues cluster in an extended loop in RACK1, phosphorylation of which selects for translation of viral or reporter mRNAs whose 5’ untranslated regions (UTRs) contain adenosine repeats, so-called polyA-leaders. Structural and phylogenetic analysis revealed that although RACK1 is highly conserved, this loop is variable and contains negatively charged amino acids in plants, where these leaders act as translational enhancers for poorly understood reasons. Phosphomimetics and inter-species chimeras demonstrated that negative charge in the RACK1 loop dictates ribosome selectivity towards viral RNAs. By converting human RACK1 to a charged, plant-like state, poxviruses remodel host ribosomes so that adenosine repeats erroneously generated by slippage of the viral RNA polymerase4 confer a translational advantage. Our findings uncover ribosome customization through a novel trans-kingdom mimicry and the mechanics of species-specific leader activity that underlie the enigmatic poxvirus polyA-leaders4.