Self-assembly helical viruses

Completely different mechanisms are involved in the self-assembly of the tobacco mosaic virus (TMV). This virus consists of single-strand RNA, which is surrounded by 2,130 identical protein units, each of which consists of 158 amino acid residues. A virus particle, which requires the tobacco plant as a host, has a rodlike structure with helical symmetry ( Stanley needles ). It is 300 nm long, with a diameter of 18nm. The protein and RNA fractions can be separated, and the viral... 

We have already dealt with some general aspects of biochemical self-assembly in Section 2.10 including the remarkable formation of viral capsids. There are some biochemical examples, however, that translate readily into supramolecular chemical concepts and have been pivotal in defining the field. One such system is the tobacco mosaic virus, a virus that is very harmful to a variety of crops including tobacco, tomato, pepper, cucumbers and species such as ornamental flowers. This system consists of a helical virus particle measuring some 300 X 18 nm (Figure 10.6). A central strand of RNA is sheathed by 2130 identical protein subunits, each of which contains 158 amino acids. What is remarkable about... 

TMV consists of a cylindrical coat of 2,130 identical protein subunits enclosing a long RNA molecule of 6,400 nucleotides. In 1955, it was shown that the coat protein subunits and the RNA could be dissociated but would, under appropriate conditions, spontaneously self-assemble to reform fully active virus particles. This process is multistage, the critical intermediate being a 34-unit two-layered protein disc which, upon binding the RNA, is converted to a helical structure with 16.33 protein subunits per turn (Fig. 5-2). In the absence of the RNA, the protein may be polymerized into helical tubes of indefinite length. The presence of the RNA aids the polymerization process and results in a virus particle with a fixed length of 300 nm. 

As will be discussed later, in many ways, strict self-assembly is a restatement of Afinison s Thermodynamic Hypothesis and encompasses all processes in which components assemble reversibly. The archetype of strict self-assembly is tobacco mosaic virus (TMV). The TMV particles consist of a single helical strand of RNA embedded in a right-handed helix composed of identical coat protein subunits. The TMV particles will dissociate into their component parts by changes in pH, temperature, or even pressure. However, once the correct conditions are again attained, protein subunits and RNA spontaneously reassemble into virus particles that are indistinguishable from the original TMV. 

Hence, the bottom-up procedures from a molecule to create new structures have become an important approach. This approach has already been used by Nature. Most of the structures of biological systems have been made by self-assembly and self-organization of specific molecules. The tobacco mosaic virus, for example, is rod-shaped—300 nm long and 18 nm in diameter—and has a mass of about 40,000 kDa [2]. The 2130 identical subunits in the protein coat are closely packed in a helical array around an RNA molecule consisting of 6390 nucleotides. Dissociated tobacco mosaic virus subunits and RNA can reassemble, under suitable conditions, into a virus that is identical with the original in structure and function. 

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