key: cord-0002185-ay91z3vv authors: Ravindran, Madhu Sudhan; Tsai, Billy title: Viruses Utilize Cellular Cues in Distinct Combination to Undergo Systematic Priming and Uncoating date: 2016-04-07 journal: PLoS Pathog DOI: 10.1371/journal.ppat.1005467 sha: 5d3e915d8571f298da4ff2e929d8475d68dc559f doc_id: 2185 cord_uid: ay91z3vv nan endosomal pH in turn triggers additional structural alterations to the viral glycoprotein gB, promoting fusion of viral and endosomal membranes that releases the capsid into the cytosol (step ii) [8] ; HIV-1 entry, by contrast, is thought to be pH-independent [9] . For HSV-1, the action of molecular motors (dynein and kinesin) at the nuclear pore is essential to disassemble and release the viral genome (step iii) [10] . It should be noted that the entry mechanisms of HIV-1 and HSV-1 have been reported to be cell-type specific [11, 12] . Nonetheless, unlike HIV's use of receptor-enzyme-mechanical cues, HSV-1 uses a modified combination, in which receptor-chemical-mechanical cues are instead exploited to deliver the viral genome into the host. Remarkably, receptor engagement at the plasma membrane does not appear to initiate uncoating of Polyomaviridae, a nonenveloped DNA virus responsible for many human diseases ranging from nephropathy to cancer. In fact, for members of this virus family, such as the archetype SV40, uncoating is initiated in the endoplasmic reticulum (ER). Specifically, upon endocytosis, SV40 is routed to the ER, where protein disulfide isomerase (PDI) members Receptor-Enzyme-Mechanical: HIV-1 binding to its receptor structurally alters GP120, inducing membrane fusion (step i) and capsid release into the cytosol. Cytosolic peptidyl-isomerase conformationally alters the capsid (step ii), which is then trafficked to the nuclear pore by motor proteins to execute mechanical disassembly (step iii). (B) Receptor-Chemical-Mechanical: Herpes simplex virus-1 (HSV-1) engagement to its receptors alters the structural proteins (step i), which then induce endocytosis. The low pH endocytic compartment further alters the structural proteins (step ii) to promote fusion and capsid escape into the cytosol, where engagement with motor protein causes disassembly (step iii). (C) Enzyme-Mechanical: SV40 binds to its glycolipid receptor and reaches the endoplasmic reticulum (ER) unaltered via endocytic route. In the ER, the protein disulfide isomerase (PDI)-family of isomerases/reductases rearrange the disulphide bonds (step i) to structurally alter the virus. The viral capsid is then engaged by cytosolic disaggregation machinery (step ii), which extracts and simultaneously disassembles the viral particle. (D) Receptor-Chemical-Mechanical: Binding of human adenovirus-2 (HAdV2) to its receptors imposes mechanical strain due to drifting motion of the receptors (step i). The destabilized virus undergoes further structural distortion at low endosomal pH, which probably assists in capsid release into the cytosol (step ii). In the cytosol, the destabilized capsid engages the motor protein, which transports the capsid to the nuclear pore to undergo mechanical disruption (step iii), leading to genome release. Note: small Roman numerals (i, ii, and iii) represent virus coopting host cues. The background colors of the Roman numerals categorize them into receptor or enzyme (green), chemical (red), and mechanical (yellow). isomerize and reduce the viral capsid disulfide bonds (Fig 1C, step i) . These reactions destabilize the capsid and expose the hidden hydrophobic proteins VP2/3, allowing the virus to insert into the ER membrane [13] . The membrane-inserted virus subsequently reorganizes different ER membrane factors (BAP31, DnaJB14) to create a cytosol entry site [14, 15] . Importantly, during cytosol entry, a membrane-associated disaggregation machinery (Hsc70, Hsp105, and DnaJB14) extracts SV40 into the cytosol in a step coupled to the further disassembly of the viral particle (step ii) [16] . From the cytosol, the partially disassembled viral particle transports into the nucleus and releases its genome in this compartment. Thus, an enzymatic reaction (localized in the ER lumen) followed by a mechanical force (encoded by the cytosolic disaggregation complex) is the cue combination used to uncoat this nonenveloped virus. Another example of host cue and viral uncoating interplay is observed in the nonenveloped Adenoviridae (AdV) family. The species C viruses HAdV-C2/5 are the best-studied viruses from this family. While this virus is responsible for mild respiratory infections, it can also cause life-threatening diseases in immunocompromised individuals. AdV contains a highly stable capsid that encases its viral DNA genome [17] . Infection typically begins when the viral fiber and penton base proteins interact with the Coxsackievirus adenovirus receptor (CAR) and αvβ3/αvβ5 integrin coreceptors. These receptor interactions disrupt the viral architecture due to mechanical strain imposed on the virus. The mechanical tension results when the viral core capsid is tethered to stationary integrins, while the fibers are simultaneously bound to CAR molecules that actively drift on the plasma membrane. This capsid destabilization causes detachment of the fibers and exposure of protein IV (Fig 1D, step i) [18] . The structurallyprimed virion then undergoes clathrin-dependent endocytosis to reach the endosome, where a pH-dependent step enables viral escape into the cytosol (step ii) [19] . Upon cytosol entry, AdV uses motor-driven, microtubule-based transport to reach the nucleus and dock on the nuclear pore complex. Here, a second mechanical force generated by the kinesin motor disassembles the virus, allowing the viral genome to be released into the nucleus (step iii) [20] . Hence, for the highly stable AdV, initial receptor engagement (leading to mechanical disruption) followed by a chemical cue and then a mechanical cue coordinately uncoat this virus. Although the four examples illustrated above clearly demonstrate a complex relationship between viruses and host cues used during uncoating, a general uncoating strategy leading to genome delivery can nonetheless be observed. For many viruses, receptor engagement at the plasma membrane (that imparts viral conformational changes) is the first cue that primes viral uncoating. Proteolytic processing by host proteases localized on the plasma membrane (that also leads to viral structural alterations) can likewise be used to initiate uncoating before entry, as seen in the case of rotavirus and SARS-coronavirus (see Table 1 for more examples). After gaining entry into the host, low pH is often used as the subsequent cue to further uncoat the virus. However, enzyme-and/or chaperone-mediated cues can similarly be utilized within the host to trigger viral disassembly. Finally, in many instances, mechanical cues generated by molecular machines that convert the energy stored in nucleotides to mechanical forces, including motor proteins, disaggregation machinery, and the proteasome complex, are recruited to complete the uncoating process. It is interesting to note that, for the more stable AdV [21] , mechanical cues that can impart powerful destabilizing forces disassemble these viral particles to cause genome release. In fact, the stability of viruses has also been implicated in the selection of host cues. For instance, the human nonenveloped RNA rhinovirus (HRV), a Picornaviridae family member, is classified into a major and a minor group based on receptor usage [22] . Because the major group (HRV-14/3) is thought to be more stable than the minor group (HRV-2/16), the major group requires uncoating by receptor-induced priming followed by low pH-mediated disassembly, while the minor group only requires chemical stimuli to uncoat (Table 1 ) [23] . While there are (and will continue to be) exceptions to the viral uncoating strategy that we have described in this short article, our intention is to organize the known disassembly mechanisms of approximately 30 different viruses from many virus families that are used to deliver the viral genome into the host. By depicting a general pattern, we hope this information may be useful for the broader virology community in deciphering the uncoating mechanism for a virus within the same family for which the uncoating strategy is known (see Table 1 for uncoating step marked as not determined [ND] ). For instance, does the Merkel cell polyomavirusthe causative agent for the aggressive skin cancer Merkel cell carcinoma-exploit the same uncoating mechanism as other members of the Polyomaviridae family? Additionally, can we apply the uncoating program used by members of the Coronaviridae family to MERS coronavirus, a recently discovered member of this family that causes severe respiratory diseases? Finally, from a practical viewpoint, clarifying detailed viral uncoating mechanisms will continue to pave the way for identifying new therapeutic agents, as already successfully found in the discovery of many antiviral compounds that act primarily by inhibiting the viral uncoating process [24] . How viruses and toxins disassemble to enter host cells. Annual review of microbiology DNA virus uncoating Uncoating of non-enveloped viruses. Current opinion in virology HIV: cell binding and entry. Cold Spring Harbor perspectives in medicine Target cell type-dependent modulation of human immunodeficiency virus type 1 capsid disassembly by cyclophilin A HIV-1 uncoating is facilitated by dynein and kinesin 1 Structure of unliganded HSV gD reveals a mechanism for receptor-mediated activation of virus entry Low-pH-dependent changes in the conformation and oligomeric state of the prefusion form of herpes simplex virus glycoprotein B are separable from fusion activity pH-independent HIV entry into CD4-positive T cells via virus envelope fusion to the plasma membrane Plus-and minus-end directed microtubule motors bind simultaneously to herpes simplex virus capsids using different inner tegument structures HIV enters cells via endocytosis and dynamin-dependent fusion with endosomes Viral entry mechanisms: cellular and viral mediators of herpes simplex virus entry Simian Virus 40 depends on ER protein folding and quality control factors for entry into host cells A cytosolic chaperone complexes with dynamic membrane J-proteins and mobilizes a nonenveloped virus out of the endoplasmic reticulum BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol A Non-enveloped Virus Hijacks Host Disaggregation Machinery to Translocate across the Endoplasmic Reticulum Membrane Nuclear import of adenovirus DNA in vitro involves the nuclear protein import pathway and hsc70 Drifting motions of the adenovirus receptor CAR and immobile integrins initiate virus uncoating and membrane lytic protein exposure Stepwise dismantling of adenovirus 2 during entry into cells Kinesin-1-mediated capsid disassembly and disruption of the nuclear pore complex promote virus infection How cells tune viral mechanics-insights from biophysical measurements of influenza virus Structural studies of two rhinovirus serotypes complexed with fragments of their cellular receptor Uncoating of human rhinoviruses. Reviews in medical virology Antiviral agents: structural basis of action and rational design We thank Martin Engelke (University of Michigan) for extensive discussion and critical review of this manuscript.