Assembly of Large Icosahedral Double-Stranded RNA Viruses

Abstract

Double-stranded RNA (dsRNA) viruses are a diverse group of viruses infecting hosts from bacteria to higher eukaryotes. Among the hosts are humans, domestic animals, and economically important plant species. Fine details of high-resolution virion structures have revealed common structural characteristics unique to these viruses including an internal icosahedral capsid built from 60 asymmetric dimers (120 monomers!) of the major coat protein. Here we focus mainly on the structures and assembly principles of large icosahedral dsRNA viruses belonging to the families of Cystoviridae and Reoviridae. It is obvious that there are a variety of assembly pathways utilized by different viruses starting from similar building blocks and reaching in all cases a similar capsid architecture. This is true even with closely related viruses indicating that the assembly pathway per se is not an indicator of relatedness and is achieved with minor changes in the interacting components.

Probing, by Self-Assembly, the Number of Potential Binding Sites for Minor Protein Subunits in the Procapsid of Double-Stranded RNA Bacteriophage φ6

“…This complex is essential for the recognition and encapsidation of viral genomic precursor molecules, genome replication and transcript synthesis, and the formation of the core of the mature virion (41,44).…”

“…The dodecahedral PC framework is composed of 120 copies of protein P1 arranged as 60 asymmetric dimers on a Tϭ1 icosahedral lattice (6,20). This type of capsid organization is common for dsRNA viruses but has not been observed in any other type of virus (41). Purified 6 P1 is a monomer, but the equivalent protein from a related cystovirus, 8, forms tetramers in solution (25,43).…”

New enveloped dsRNA phage from freshwater habitat

“…For instance, the outermost layer of different reoviruses consists of a variety of unrelated proteins (Poranen & Bamford, 2012b). Significant variation is also seen in the surface glycoproteins (haemagglutinin and neuraminase) of influenza viruses.…”

“…The innermost capsid layers, containing RNA polymerization activity, are conserved among dsRNA viruses (Bamford et al, 2002;Luque et al, 2010), whereas the outermost layer shows greater genetic and structural diversity. The flexible composition of the outer layer allows the virus to adapt for transmission and initiation of infection in different host cells (Bamford et al, 2002;Poranen & Bamford, 2012b).…”

“…The inner core of the virion is composed of 120 copies of the major capsid protein (Ktistakis & Lang, 1987;Olkkonen & Bamford, 1987) arranged in a T51 icosahedral lattice (Butcher et al, 1997;Huiskonen et al, 2006). This rare architecture of the innermost protein layer is also seen in eukaryotic dsRNA viruses, for example in the members of the family Reoviridae (Poranen & Bamford, 2012b).…”