This is one of the SIMPLEST FORMS of viral capsid: the protein is "wound on" to the viral nucleic acid (generally ssRNA, though M13 and other filamentous phage virions contain circular ssDNA) in a simple HELIX, like a screw (see the diagram for tobacco mosaic virus, below).
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| Diagram of TMV structure: protein subunits are all in equivalent crystallographic positions, related by a right-hand "screw translation" | Image reconstruction of TMV |
| courtesy L Stannard | 8th Report ICTV |
In the case of TMV this is the entire virion: this is also the case for all RODLIKE and FILAMENTOUS virions where no membranes are involved. This includes all Tobamoviridae, Potyviridae, and Closteroviridae, but NOT Filoviridae, like Ebola virus (see here).
In other cases, filamentous helical nucleocapsids may be enclosed
within matrix protein and a membrane studded with spike proteins:
excellent examples of this are PARAMYXOVIRIDAE, detailed images of which
can be found here, at
Linda Stannard's site, and
for ORTHOMYXOVIRIDAE here in this
tutorial.
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| "What is a Virus?" | Basic Virion Constituents | Isometric Nucleocapsids |
| Tutorial Contents |
