NeWS & AnaLySiS
Under the Lens Coming together during viral assembly Christof Hepp and Nicole C. Robb
This month’s Under the Lens discusses how these questions by imaging Nipah virus (NiV) only a small number (7–14 molecules) super-resolution microscopy has been and HIV-1 assembly at nanometre-scale of Env glycoproteins are present on indi- used to answer fundamental questions resolution. vidual released particles, this incorporation about the assembly mechanisms of Single-molecule localization microscopy is likely to be tightly regulated. The authors enveloped viruses. What, if anything, helps methods take advantage of the sequential showed that Env predominantly clusters at the major components of a virus particle activation and ultra-precise localization the neck region of VLPs, suggesting that Env find each other? of photoswitchable fluorophores to recon- is incorporated into the Gag lattice at a late struct a super-resolved image. Liu et al.1 uti- time point during assembly. Spinning disc Enveloped viruses are surrounded by a lipid lized 3D stochastic optical reconstruction confocal imaging of infected cells showed bilayer, which is formed as nascent virions microscopy (STORM) imaging combined that Env was predominantly retained intra- bud through cellular membranes. Virus with sample drift correction to image host cellularly, possibly to prolong incorporation assembly at the plasma membrane is an essen- cells transfected with plasmids that express into VLPs and limit its density on individual tial part of the viral life cycle, and its regula- the NiV matrix protein (M), attachment gly- virus particles. Furthermore, single-particle tion is a potential target for antiviral therapies. coprotein (G) and fusion glycoprotein (F). tracking revealed that the Env-CT domain It is generally accepted that the accumulation The researchers found that clusters of F and G decreased Env mobility within the plasma of viral matrix proteins at the plasma mem- did not significantly co-localize and were ran- membrane, supporting a model in which brane is sufficient to drive the formation of domly distributed on the plasma membrane steric trapping supports the incorporation of virus-like particles (VLPs), while viral glyco- regardless of the presence or absence of M. Env into nascent particles. Whereas Liu et al.’s proteins are incorporated into the particles Moreover, imaging of purified VLPs showed study provides evidence for stochastic incor- to mediate host cell entry. Viral glycopro- that the concentration of F and G on the poration of glycoproteins into NiV virions, teins are thought to be directed to assembly virion surface was dependent on cell surface Buttler et al.’s results point to a tightly regu- sites through interactions with viral matrix expression levels, suggesting that the incorpo- lated assembly pathway that limits cell surface proteins; however, precise information on ration levels of these proteins is unregulated. exposure and the incorporation of Env into the distribution, organization and dynamics These findings suggest a model for the assem- HIV-1 virions. of the viral components is needed to deter- bly of NiV particles in which the incorpora- In summary, the comparison of these two mine whether this association is random or tion of glycoproteins into nascent virions is studies highlights important differences in the result of specific regulatory interactions. not driven by M, but is rather an undirected, enveloped virus assembly mechanisms, and Two recent studies have sought to address stochastic process. illustrates the strength of super-resolution In a separate study, Buttler et al.2 used microscopy techniques in taking advantage interferometric photoactivation localiza- of precise spatial information to discover tion microscopy (iPALM), which uses the subtle regulation mechanisms that would photon signal collected from two objec- not be observed using conventional light tives simultaneously to gain precise 3D microscopy. information, to obtain super-resolved Christof Hepp* and Nicole C. Robb* images of cell-associated HIV-1 Clarendon Laboratory, Department of Physics, particles. During HIV-1 infection, University of Oxford, Oxford, UK. the matrix domain of the Gag *e-mail: [email protected] polyprotein oligomerizes into https://doi.org/10.1038/s41579-018-0102-4 a lattice to drive the formation 1. Liu, Q. et al. A stochastic assembly model for Nipah of VLPs at assembly sites, and virus revealed by super-resolution microscopy. Nat. Commun. 9, 3050 (2018).
viral envelope (Env) glycoproteins 2. Buttler, C. A. et al. Single molecule fate of HIV-1 assemble within this lattice3. Steric envelope reveals late-stage viral lattice incorporation. Nat. Commun. 9, 1861 (2018). trapping of the long cytoplasmic tail 3. Freed, E. O. HIV-1 assembly, release and maturation. domain of Env (Env-CT) between Nat. Rev. Microbiol. 13, 484 (2015). Gag proteins is thought to retain Competing interests Springer Nature Limited Springer Nature Credit: Philip Patenall/ Credit: Env at assembly sites, and because The authors declare no competing interests.
NatuRe Reviews | Mic ro bi ology volume 16 | DECEMBER 2018 | 721