Assembling the viral jigsaw: How pieces of a virus come together from different cells to cause infection

Written by: Padmini S. Pillai

Original Article: Sicard et al. eLife 2019
The Gist of It:
Viruses have the power to cause disasters ranging from influenza pandemics to the widespread destruction of crops. Typically, a virus holds its entire genome (all the genetic information it needs to survive) inside a single capsule, which then enters a living cell to produce new virions. Multipartite viruses hold different segments of their genome in separate capsules. It was previously thought that these various segments had to somehow enter the same cell to multiply successfully, but this seems pretty hard to coordinate! What if genome segments enter different cells? Researchers at Université de Montpellier have made a stunning new discovery that these distinct segments can be converted into viral proteins in different cells, and that these infected cells can then share viral proteins, leading to the formation of a complete virus. Sicard and colleagues fluorescently labeled genome segments of faba bean necrotic stunt virus (FBNSV) that encode proteins that orchestrate viral replication, movement, or capsule formation. They then searched for the presence of each segment in individual cells of the faba bean plant using microscopy. The scientists found that segments accumulated in different cells and in different amounts. To determine if the full virus could still come together, they looked for the presence of the genome segment for replication (R) and its protein product M-Rep. Although segment R was only found in 40% of cells, M-Rep was found in nearly 85% of cells, indicating that viral proteins made from distinct segments can be transferred between host plant cells. In other words, infected cells were exchanging virus proteins to make complete virus particles! This study reveals a new way by which one virus can exist in multiple cells and still replicate to cause infection.
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The genome segments of FBNSV enter different cells, but still end up working together to cause an infection!

The Nitty Gritty:
Colocalization of segments of FBNSV was detected using fluorescent probes and confocal microscopy. Visualization of red and green fluorophores from pairs of segments demonstrated that segments were found in different cells and accumulated to varying amounts. There was no correlation between the presence of segment R and segment S (which encodes the encapsidation function) in petioles tested. This was true for R/M and S/M pairs as well, where M is the gene segment encoding the intra-host movement function. This lack of correlation was further confirmed using qPCR. A combination of FISH and immunofluorescence was used to compare the presence of segment S and segment R and its protein product M-Rep. Although segment R was detected in ~40% cells, M-Rep was present in 85% of cells. In contrast, cells that contained segment R no longer had M-Rep detectable.
Original Research Article: Sicard, A., et al. “A multicellular way of life for a multipartite virus.” eLife 8 (2019): e43599. doi: 10.7554/eLife.43599.

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