Structural Research of Leviviridae

The Leviviridae are a family of positive-stranded RNA viruses that infect prokaryotes, consisting of two genera, Allolevivirus and Levivirus. Of these, the RNA-binding capsid protein (CP) of the Levivirus MS2 phage is a tool for many basic science applications and a model for RNA virus assembly. It is evident from recent research that Leviviridae phages are commonly used as indicators of the presence of viruses in wastewater and surface water, as these species favor specific hosts. In 2020, the family was renamed from Leviviridae to Fiersviridae.

Figure 1. Atomic model of Levivirus MS2. (Thongchol J, et al, 2023)

Structure and Composition of Leviviridae Virion

Leviviridae members are shown as unenveloped, spherical viral particles, approximately 26 nm in diameter, with T=3 icosahedral symmetry. Members of the family are small viruses with linear, positive-sense, single-stranded RNA genomes encoding four essential proteins, including the maturation protein (Mat), the CP, the β-subunit of the replicase (Rep), and a single cleavage protein (Lys). The Mat plays an essential role in determining phage maturation. Mat and CP are responsible for the structural integrity of the phage, and Rep replicates the viral genome within the host cell. The newly formed viral progeny is released into the environment via Lys, which is required for host cell lysis.

Research Progress on the Structure of Leviviridae

In the early years, researchers used X-ray crystallography and cryo-electron microscopy (cryo-EM) to resolve the capsid structures of known Leviviridae phages. One of the most widely researched phages was the Escherichia coli phage MS2. The cryo-EM structures of MS2 provided insights into the composition of the virion particles of mature Leviviridae phages. The structure shows that the MS2 virus particle comprises a capsid encapsulating a single-stranded gRNA and a single Mat. The capsid has a nearly icosahedral shape with 89 CP dimers, and the Mat breaks the T = 3 icosahedral symmetries of one of the 2-fold axes.

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Reference

1. Thongchol J, et al. Recent Advances in Structural Studies of Single-Stranded RNA Bacteriophages. Viruses. 2023. 15(10): 1985.