...encoding their own translation system is something apparently beyond the coding capacity of even the largest viruses identified to date, the mimiviruses, which infect amoeba and fall into the same ...family of nucleocytoplasmic large DNA viruses (NCLDVs) as poxviruses. ...these decoded lysines cause ribosome stalling, signaling decay of the mRNA 5. Modification of RACK1 by the viral B1 kinase contributes to the ability of polyA leaders to function in infected cells. eIF, eukaryotic initiation factor; m-7-GTP, 7-Methylguanosine-5’-triphosphate; RACK1, receptor for activated C kinase 1; UTR, untranslated region; VacV, vaccinia virus; vRNAP, viral RNA polymerase. https://doi.org/10.1371/journal.ppat.1006634.g001 Poxviruses and translational control The study of poxviruses was fundamental to the discovery of the cap and polyA tail that we now know to be present on most eukaryotic mRNAs. While some of their mysteries have begun to be solved, there is undoubtedly much more to learn about how these enigmatic elements function. ...what might once have been seen as the random generation of odd leaders through “erroneous” slippage of the viral RNA polymerase is now clearly part of a well-orchestrated strategy to confer translational advantages to viral mRNAs.
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.
Microtubules (MTs) form a rapidly adaptable network of filaments that radiate throughout the cell. These dynamic arrays facilitate a wide range of cellular processes, including the capture, ...transport, and spatial organization of cargos and organelles, as well as changes in cell shape, polarity, and motility. Nucleating from MT-organizing centers, including but by no means limited to the centrosome, MTs undergo rapid transitions through phases of growth, pause, and catastrophe, continuously exploring and adapting to the intracellular environment. Subsets of MTs can become stabilized in response to environmental cues, acquiring distinguishing posttranslational modifications and performing discrete functions as specialized tracks for cargo trafficking. The dynamic behavior and organization of the MT array is regulated by MT-associated proteins (MAPs), which include a subset of highly specialized plus-end-tracking proteins (+TIPs) that respond to signaling cues to alter MT behavior. As pathogenic cargos, viruses require MTs to transport to and from their intracellular sites of replication. While interactions with and functions for MT motor proteins are well characterized and extensively reviewed for many viruses, this review focuses on MT filaments themselves. Changes in the spatial organization and dynamics of the MT array, mediated by virus- or host-induced changes to MT regulatory proteins, not only play a central role in the intracellular transport of virus particles but also regulate a wider range of processes critical to the outcome of infection.
Being dependent upon host transport systems to navigate the cytoplasm, viruses have evolved various strategies to manipulate cytoskeletal functions. Generally, viruses use the actin cytoskeleton to ...control entry and short-range transport at the cell periphery and exploit microtubules (MTs) for longer-range cytosolic transport, in some cases to reach the nucleus. While earlier studies established the fundamental importance of these networks to successful infection, the mechanistic details and true extent to which viruses usurp highly specialized host cytoskeletal regulators and motor adaptors is only beginning to emerge. This review outlines our current understanding of how cytoskeletal regulation contributes specifically to the early stages of viral infection, with a primary focus on retroviruses and herpesviruses as examples of recent advances in this area.
Actin functions in a wide range of attachment and entry processes that are critical to the very earliest stages of virus infection. Actin remodeling also prevents superinfection, whereby viruses are prevented from entering cells that are already infected.
Once inside the cell, virus particles transition from actin to microtubules (MTs) through actin–MT cross-linking factors and specialized MT plus-end tracking proteins (+TIPs) to initiate retrograde transport to the nucleus.
Many viruses modulate MT dynamics by targeting regulatory signaling pathways and/or +TIP complexes to promote early infection.
Interactions between viral capsids and MT motors and/or adaptor proteins regulate the bidirectional motility of incoming viral particles.
Opposing forces exerted by cytoskeletal motors play a role in capsid disassembly or ‘uncoating’ of some viruses.
Although viruses require cellular functions to replicate, their absolute dependence upon the host translation machinery to produce polypeptides indispensable for their reproduction is most ...conspicuous. Despite their incredible diversity, the mRNAs produced by all viruses must engage cellular ribosomes. This has proven to be anything but a passive process and has revealed a remarkable array of tactics for rapidly subverting control over and dominating cellular regulatory pathways that influence translation initiation, elongation, and termination. Besides enforcing viral mRNA translation, these processes profoundly impact host cell-intrinsic immune defenses at the ready to deny foreign mRNA access to ribosomes and block protein synthesis. Finally, genome size constraints have driven the evolution of resourceful strategies for maximizing viral coding capacity. Here, we review the amazing strategies that work to regulate translation in virus-infected cells, highlighting both virus-specific tactics and the tremendous insight they provide into fundamental translational control mechanisms in health and disease.
Dynamic microtubules (MTs) continuously explore the intracellular environment and, through specialized plus end-tracking proteins (+TIPs), engage a variety of targets. However, the nature of cargoes ...that require +TIP-mediated capture for their movement on MTs remains poorly understood. Using RNA interference and dominant-negative approaches, combined with live cell imaging, we show that herpes simplex virus particles that have entered primary human cells exploit a +TIP complex comprising end-binding protein 1 (EB1), cytoplasmic linker protein 170 (CLIP-170), and dynactin-1 (DCTN1) to initiate retrograde transport. Depletion of these +TIPs completely blocked post-entry long-range transport of virus particles and suppressed infection ∼5,000-fold, whereas transferrin uptake, early endosome organization, and dynein-dependent movement of lysosomes and mitochondria remained unaffected. These findings provide the first insights into the earliest stages of viral engagement of MTs through specific +TIPs, akin to receptors, with therapeutic implications, and identify herpesvirus particles as one of a very limited number of cargoes absolutely dependent on CLIP-170-mediated capture to initiate transport in primary human cells.
Ribosomes have the capacity to selectively control translation through changes in their composition that enable recognition of specific RNA elements. However, beyond differential subunit expression ...during development, evidence for regulated ribosome specification within individual cells has remained elusive. Here we report that a poxvirus kinase phosphorylates serine/threonine residues in the human 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 with 5' untranslated regions that contain adenosine repeats, so-called polyA-leaders. Structural and phylogenetic analyses revealed that although RACK1 is highly conserved, this loop is variable and contains negatively charged amino acids in plants, in which these leaders act as translational enhancers. Phosphomimetics and inter-species chimaeras have shown 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 polymerase confer a translational advantage. Our findings provide insight into ribosome customization through trans-kingdom mimicry and the mechanics of species-specific leader activity that underlie poxvirus polyA-leaders.
End-binding proteins (EBs) are widely viewed as master regulators of microtubule dynamics and function. Here, we show that while EB1 mediates the dynamic microtubule capture of herpes simplex virus ...type 1 (HSV-1) in fibroblasts, in neuronal cells, infection occurs independently of EBs through stable microtubules. Prompted by this, we find that transforming acid coiled-coil protein 3 (TACC3), widely studied in mitotic spindle formation, regulates the cytoplasmic localization of the microtubule polymerizing factor chTOG and influences microtubule plus-end dynamics during interphase to control infection in distinct cell types. Furthermore, perturbing TACC3 function in neuronal cells resulted in the formation of disorganized stable, detyrosinated microtubule networks and changes in cellular morphology, as well as impaired trafficking of both HSV-1 and transferrin. These trafficking defects in TACC3-depleted cells were reversed by the depletion of kinesin-1 heavy chains. As such, TACC3 is a critical regulator of interphase microtubule dynamics and stability that influences kinesin-1-based cargo trafficking.
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•TACC3 regulates microtubule plus-end growth and HSV-1 infection in interphase cells•Loss of TACC3 functionality results in aberrant detyrosinated microtubule arrays•Effects of TACC3 on stable microtubule arrays affect cell shape and cargo sorting•Cargo sorting defects caused by TACC3 loss are rescued by impairing kinesin-1 activity
While EB proteins are widely studied as master regulators of microtubule plus-end dynamics, Furey et al. report EB-independent regulation of microtubule arrays and cargo trafficking by the transforming acid coiled-coil-containing protein, TACC3. By controlling the formation of detyrosinated stable microtubule networks, TACC3 influences kinesin-1-based sorting of both host and pathogenic cargoes.
While beta-amyloid (Aβ), a classic hallmark of Alzheimer's disease (AD) and dementia, has long been known to be elevated in the human immunodeficiency virus type 1 (HIV-1)-infected brain, why and how ...Aβ is produced, along with its contribution to HIV-associated neurocognitive disorder (HAND) remains ill-defined. Here, we reveal that the membrane-associated amyloid precursor protein (APP) is highly expressed in macrophages and microglia, and acts as an innate restriction against HIV-1. APP binds the HIV-1 Gag polyprotein, retains it in lipid rafts and blocks HIV-1 virion production and spread. To escape this restriction, Gag promotes secretase-dependent cleavage of APP, resulting in the overproduction of toxic Aβ isoforms. This Gag-mediated Aβ production results in increased degeneration of primary cortical neurons, and can be prevented by γ-secretase inhibitor treatment. Interfering with HIV-1's evasion of APP-mediated restriction also suppresses HIV-1 spread, offering a potential strategy to both treat infection and prevent HAND.
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