How giant viruses mature: New evidence from the medusavirus sheds light
Detailed evaluation of medusavirus morphology within and outside host
cells provides new clues into the maturation of giant viruses

Date:
March 31, 2022
Source:
Tokyo University of Science
Summary:
Medusavirus, a giant virus, is more closely related to eukaryotic
cells than other giant viruses are. In an exciting new study,
scientists have used electron microscopy and time-course analysis
to discover four different types of medusavirus particles within
and outside infected amoeba cells, representing four different
stages of virus maturation.

Their results indicate that the medusavirus has a unique maturation
process, providing new insights into the structural and behavioral
diversity of giant viruses.



FULL STORY ========================================================================== Giant viruses represent a unique group of viruses that are similar in
size to small bacteria. Medusavirus -- a special type of giant virus --
was first isolated from a hot spring in Japan. Interestingly, genetic
studies showed that medusavirus was more closely related to mature
organisms called eukaryotes than to other giant viruses, suggesting that
it may hold the key to understanding eukaryotic evolution. Although the
details of medusavirus morphology and maturation in infected cells have
so far remained elusive, the researchers behind its initial discovery
now have some answers.


==========================================================================
In a recent study published in Journal of Virology, a team of Japanese scientists led by Prof. Kazuyoshi Murata from the National Institutes
of Natural Sciences and Prof. Masaharu Takemura from Tokyo University of Science has revealed, for the first time, a unique four-stage maturation process that the medusavirus undergoes within host cells. Prof. Takemura comments, "From an evolutionary perspective, the medusavirus is extremely interesting, as its replication process and genome are different from
those of other viruses.

Interestingly, medusavirus also has a unique particle structure. In this
study, we wanted to make additional inroads towards elucidating the
biology of this virus by characterizing its morphology and maturation
process." To do this, the researchers used two techniques that allow
the high-resolution visualization of viral infection -- conventional transmission electron microscopy (C-TEM) and cryo-electron microscopy (cryo-EM). Using these techniques, they observed the detailed particle morphology of medusavirus in infected amoeba cells.

Their first and rather surprising discovery was the presence of four
types of medusavirus particles both within and outside the infected
host cells. Based on their features, these particles were named pseudo-DNA-empty (p-Empty, i.e., filled with spongy material but no DNA), DNA-empty (Empty, i.e., no spongy material or DNA), semi-DNA-full (s-Full, i.e., half-filled with DNA), and DNA- full (Full, i.e., completely filled
with DNA) particles.

Subsequently, they performed time-course analysis, in which the gene
expression was measured at several time points during maturation, and discovered that the four types of particles represented four consecutive
stages of viral maturation. They found that unlike in other viruses,
the viral capsid or shell of medusavirus was produced independently
in the host cell's cytoplasm, while the viral DNA was produced in the
nucleus. Further, only empty capsids present near the host nucleus could incorporate viral DNA and become s-Full or DNA-full particles. These
findings suggested that the medusavirus had a unique maturation process.

To observe the detailed structure of the four types of medusavirus
particles, the team used the cryo-EM technique. They found that all the different particle types had a comparable outer structure, with the
presence of three different spikes. The configuration of the capsid
shell was also consistent with the structure of the membrane layer
within the capsid. However, while s-Full and Full particles showed
a complete internal membrane, p-Empty and Empty particles had "open
membrane structures," meaning the membrane had a gap at one end.

"Viruses are smart and can replicate and mature in various ways. Our
findings reveal the unique way in which the medusavirus matures. The open membranes we observed in p-Empty and Empty particles were particularly interesting. We believe that the membrane gaps indicate an incompleteness
and represent a state in which viral particles have not yet matured. The
gaps are likely used to exchange DNA and proteins required for medusavirus maturation and disappear as the virus reaches its final stage,"explains
Prof. Takemura.

These new insights not only demonstrate a novel mechanism of particle
formation and maturation in medusavirus but also shed light on the great structural and behavioral diversity of giant viruses. They represent a
"giant" leap in our knowledge of virus biology and call for further
research into giant viruses, which could help answer numerous questions
about evolution and infection.


========================================================================== Story Source: Materials provided by Tokyo_University_of_Science. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
*
Images_and_an_illustration_of_the_maturation_of_four_types_of_medusavirus
particles.

========================================================================== Journal Reference:
1. Ryoto Watanabe, Chihong Song, Yoko Kayama, Masaharu Takemura,
Kazuyoshi
Murata. Particle Morphology of Medusavirus Inside and Outside the
Cells Reveals a New Maturation Process of Giant Viruses. Journal
of Virology, 2022; DOI: 10.1128/jvi.01853-21 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220331151542.htm

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