Capsid Geometry; are we sure all viruses aren’t complex?

Research from imaging viruses via Cryo-EM has elucidated a unique feature of viral morphologies, namely that there appears to be no such thing as a symmetrical virus.  With the discovery last fall that electrons are indeed round, data involving pictures of viruses, bacteria, and proteins now must be reanalyzed to give the true shape of these organisms and molecules. This finding in particular was useful for viruses which were then shown to be asymmetrical. Capsid Morphology is generally understood to be separated into three groups: Icosahedral, Helical, or Complex. At this time, only Poxviridiae are considered to be complex in morphology given their biconcave structure. However, the understanding that electrons are round led researchers to reanalyze viral Cryo-EM imaging data. This analysis, particularly as it applied to the flaviviruses analyzed in this paper, could no longer assume symmetry as the finding the electrons have shape meant that they could have different contributions to overall structure of the structures they make up. It also means that the results of EM and Cryo-EM techniques could yield aberrations in the image associated with the shape of the illuminating electrons. After removing this requirement of the post-image analysis, researchers found asymmetrical distortions on the outer capsid structure of the imaged flaviviruses. This asymmetric behavior has yet to be fully understood, but it has been suggested that the irregular positioning of the nucleocapsid as it exits the cell may be responsible. This is because the new virus must do more work to pinch off the vesicle than to form an indentation in the host cell surface. Additionally, it may be the case that these irregularities are associated with many viruses budding from the surface of the host cell at once, leading to an effect where the budding of some virus particles overlap closely enough that there are irregularities in the capsid as it forms.

Of course, this means quite a lot for naked viruses. To start, the outermost protein essential to viral infectivity is orientationally dependent if the virus is asymmetrical. Given the role that the outermost protein is known to play in the infectious cycle of these viruses, it will be important to quantify the effect, if any, that this new understanding of morphology may have on viral infectivity. Important questions continue to motivate research to determine if there is any population-wide or individual effects of this asymmetric morphology on viruses and the damage they are able to cause in host cells, as well as to determine if this is known to influence the latency of viral infection in host cells.

~Kyle Enriquez

Source:  https://www.livescience.com/63901-irregular-spherical-viruses.html