Downloaded from http://cshperspectives.cshlp.org/ on September 27, 2021 - Published by Cold Spring Harbor Laboratory Press How Viruses Use the Endoplasmic Reticulum for Entry, Replication, and Assembly Takamasa Inoue and Billy Tsai Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, Michigan 48103 Correspondence: [email protected] Tocause infection, avirus enters a host cell, replicates, and assembles, with the resulting new viral progeny typically released into the extracellular environment to initiate a new infection round. Virus entry, replication, and assembly are dynamic and coordinated processes that require precise interactions with host components, often within and surrounding a defined subcellular compartment. Accumulating evidence pinpoints the endoplasmic reticulum (ER) as a crucial organelle supporting viral entry, replication, and assembly. This review focuses on the molecular mechanism by which different viruses co-opt the ER to accomplish these crucial infection steps. Certain bacterial toxins also hijack the ER for entry. An interdisciplin- ary approach, using rigorous biochemical and cell biological assays coupled with advanced microscopy strategies, will push to the next level our understanding of the virus-ER interac- tion during infection. o trigger infection, a virus binds to receptors tion and assembly site. Successful infection is Ton a host cell’s plasma membrane. This in- usually completed when the newly assembled teraction induces virus internalization, and ini- particle is released into the extracellular mi- tiates a complex journey of the viral particle into lieu, in which it can promote another infection the host’s interior that leads to either nonpro- round. Thus, the ability to co-opt a host cell ductive or productive infection (Mercer et al. entry pathway leading to efficient replication 2010). In nonproductive infection, the virus and assembly ultimately dictates the fate of an may be targeted to and trapped in organelles incoming virus. unsupportive of viral membrane fusion or pen- For proper entry, replication, and assembly, etration, events which normally enable the viral viruses often rely on the complex membran- nucleic acid access to the host cytosol or nucle- ous network surrounding and residing within us. Alternatively, the virus could be transported the host cell, such as the plasma, endolysosomal, to a degradative intracellular compartment in and endoplasmic reticulum (ER) membranes. which it is destroyed. In contrast, for productive Selecting the suitable membrane system requires infection, a viral particle must avoid these non- several considerations. To support entry, the productive routes and traffic along a pathway membranous system must possess triggers capa- that allows it to reach the appropriate replica- ble of inducing the necessary conformational Editors: Susan Ferro-Novick, Tom A. Rapoport, and Randy Schekman Additional Perspectives on The Endoplasmic Reticulum available at www.cshperspectives.org Copyright # 2013 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a013250 Cite this article as Cold Spring Harb Perspect Biol 2013;5:a013250 1 Downloaded from http://cshperspectives.cshlp.org/ on September 27, 2021 - Published by Cold Spring Harbor Laboratory Press T. Inoue and B. Tsai changes that facilitate viral membrane fusion or and Skach 2011; Smith et al. 2011). To do so, a penetration (Inoue et al. 2011). Examples of cel- network of ER factors recognizes and retro- lular triggers include receptors, low pH, prote- translocates the misfolded protein to the cyto- ases, chaperones, and reductases. Additionally, sol. On reaching the cytosol, another cohort of because viral replication and assembly often oc- cytosolic factors engage and ubiquitinate the curinthecontextofvirus-inducedmembranous substrate, targeting it to the proteasome for deg- structures derived from host membranes, the radation. membranous network of choice should accom- Strikingly, some of ER’s general properties modate these remodeling reactions (Miller and are beneficial to viruses. For example, as the ER- Krijnse-Locker 2008). Moreover, as avirus com- to-cytosol retro-translocation machinery is an monly manipulates the host immune system to inherent apparatus in the ER, it represents an sustain infection, a membrane’s ability to pro- ideal conduit for certain viruses and bacterial vide the virus with such an opportunity would toxins to enter the cytosol. offer tremendous advantages during the infec- Additionally, ER membrane’s ability to un- tion course (Takeuchi and Akira 2009). dergo constant budding reactions plays a crucial A wealth of data implicates the endoplas- role during viral replication and assembly when mic reticulum (ER), one of the most elaborate viruses deform and rearrange the ER membrane membranous networks in a cell (Shibata et al. to generate ER-derived structures used to sup- 2009), as the organelle many viruses exploit port these processes. during infection. This review focuses on how viruses co-opt the ER to enter, replicate, and POLYOMAVIRUS CO-OPTS THE ER assemble in the target cell. We will also draw DURING ENTRY parallels from the mechanisms by which bacte- rial toxins use the ER for entry. Together, these A decisive virus entry step necessitates the in- insights should unveil clues regarding why coming viral particle to breach a host cell’s many viruses select the ER during infection. membrane barrier (Fig. 1). This barrier could be the plasma (Fig. 1, pathway 1), endolysosome (Fig. 1, pathway 2), or ER (Fig. 1, pathway 3) THE ER membrane. Although many viruses such as in- Structurally, the ER is a continuous membra- fluenza virus, HIV,human poliovirus (PV), and nous system consisting of the nuclear envelope, adenovirus cross the plasma or endolysosome and peripheral sheets and tubules emanating membrane (Fig. 1, pathways 1 and 2), only poly- from it (Voeltz et al. 2002). Recent studies sug- omavirus (Py) family members penetrate the gest the membrane sheets correspond to the ER membrane during entry (Fig. 1, pathway 3). rough ER whereas the tubules represent the As Py lacks a surrounding lipid bilayer that smooth ER (Voeltz et al. 2006; Shibata et al. definesenvelopedviruses,itisclassifiedasanon- 2010). Functionally, the rough ER is responsible enveloped virus. Prominent Py family members for translating secretory and transmembrane include the murine Py (mPy), simian virus 40 proteins, whereas the smooth ER possesses spe- (SV40), and the human BK (BKV), JC (JCV), cialized roles including lipid and glycogen me- and Merkel Cell (MCPy) polyomaviruses. Al- tabolism (reviewed in Hopkins 1978). Once a though mPy and SV40 can induce tumors in newly synthesized protein is translated and fold- experimental animals (Gross 1953; Sweet and ed properly in the ER lumen, it exits the ER to Hilleman 1960), human polyomavirus’s role as reach the Golgi apparatus via membrane bud- human cancer-causing agents is less certain. Per- ding mediated by the COPII complex. By con- haps the strongest link is observed in MCPy, in trast, should a protein misfold in the ER, an which the virus is positively correlated with in- endogenous ER quality control system called cidences of Merkel cell carcinoma, a rare but ag- ER-associated degradation (ERAD) alleviates gressive skin cancer of neuroendocrine origin the build-up of misfolded ER proteins (Brodsky (Feng et al. 2008). Regardless of their role in 2 Cite this article as Cold Spring Harb Perspect Biol 2013;5:a013250 Downloaded from http://cshperspectives.cshlp.org/ on September 27, 2021 - Published by Cold Spring Harbor Laboratory Press Viruses Co-Opt the ER for Infection Virus Virus Virus PM Pathway 1 Endolysosome Pathway 2 ER Pathway 3 Figure 1. Virus entry across different cellular membranes. Toenter cells, viruses penetrate the plasma membrane (PM) (pathway 1), endolysosome membrane (pathway 2), or ER membrane (pathway 3) to reach the cytosol. Whereas many viruses including influenza virus, HIV,and PV breach the plasma or endolysosome membranes, only Py transports across the ER membrane during cell entry. cancer, human polyomaviruses are well-estab- each VP1 within a pentamerextends its carboxy- lished causativeagents forother human diseases, terminal arm to an adjacent pentamer. Calcium including hemorrhagic cystitis and nephropa- ions bound to the virus further strengthen this thy (by BKV), and the fatal demyelinating dis- interpentamer interaction (Stehle et al. 1996). ease progressive multifocal leukoencephalopa- Finally, a network of VP1 disulfide bonds rein- thy (by JCV) (Jiang et al. 2009a). forces its overall architecture (Stehle et al. 1996). Structurally, Py particles are composed of a To initiate infection, Py binds to glycolipid layer of the outer coat protein VP1. This single receptors called gangliosides on the plasma protein, arranged as 72 pentamers, forms the membrane (Smith et al. 2003; Tsai et al. 2003; shell surrounding the viral genome (Liddington Low et al. 2006). After internalization, the virus et al. 1991; Stehle et al. 1994). Each VP1 pen- is transferred first to the endolysosome (Eash tamer engages the internal protein VP2 or VP3 et al. 2004; Querbes et al. 2006; Qian et al. via hydrophobic interactions (Chen et al. 1998). 2009; Engel et al. 2011) and then the ER (Tsai Additionally, VP1 binds directly to its DNA ge- et al. 2003; Gilbert and Benjamin 2004; Qian nome harbored within the viral particle (Car- et al. 2009). Py transport to the ER, a phenom-