Repeating one or a few capsid proteins in a helical arrangement is one of the simplest and most efficient ways of building a virus particles. Short particles built in this way have rod-like structures and tend to be quite rigid. Tobacco mosaic virus (TMV) is a classic example of a short, rod-like particle and is discussed in detail in Chapter 2. This architecture works well for viruses with small genomes as the hydrodynamic shear forces which can build up on a short rod are limited and probably not enough to damage the particle and expose the vulnerable genome it protects. But what about viruses with bigger genomes? They are forced to build longer helices to fit in all the nucleic acid that makes up their genetic material. Then they must make a choice - bend, or risk breaking.
Filovirus particles such as Ebolavirus and Marburg virus can be very long indeed - approximately 100 nm in diameter but sometimes thousands of nanometres long. Some nice structural work in recent years has revealed in fine detail structure of these virus particles:
Structure of Ebola virus [video]
A recent short review article builds on this work and compares the structure and organization of filovirus particles with members of the paramyxovirus and rhabdovirus families as well as with example of TMV. These comparisons reveal similarities and differences in these helical structures across the spectrum of organization, from a helical coat-protein–RNA complex of TMV with only one protein, to that of filovirus, which has five different proteins:
How do filovirus filaments bend without breaking? (2013) Trends Microbiol. pii: S0966-842X(13)00153-4. doi: 10.1016/j.tim.2013.08.001
Abstract: Viruses of the Mononegavirales have helical nucleocapsids containing a single-stranded negative-sense RNA genome complexed with the nucleoprotein and several other virus-encoded proteins. This RNA-protein complex acts as the template for replication and transcription during infection. Recent structural data has advanced our understanding of how these functions are achieved in filoviruses, which include dangerous pathogens such as Ebola virus. Polyploid filoviruses package multiple genome copies within strikingly long filamentous viral envelopes, which must be flexible to avoid breakage of the 19kb non-segmented genomic RNA. We review how the structure of filoviruses and paramyxoviruses permits this morphological flexibility in comparison to rhabdoviruses that have short, bullet-shaped virions with relatively rigid envelopes.