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are arranged as a hollow-centred, right-handed spiral with 16 1 / 3 proteins per turn and
131 turns in all, the complete virus measuring 300 nm length
18 nm diameter. The
RNA is also arranged in a spiral, about 4 nm from the axis and therefore just within
the protein's space, and a hole about 2 nm across runs through down the axis of the
whole structure ( Figure 3.10 ).
If purified TMV coat protein and RNA are mixed together in vitro , they self-assemble to
form morphologically normal and infective virus particles. 23 The proteins alone, however,
will not self-assemble to form a complete coat structure (at least, not at physiological pH)
but instead form smaller oligomers, mainly di-and trimers, and some one-layered non-helical
discs of 17 subunits and two-layered non-helical discs of 34 subunits. Promotion of the rest of
the assembly requires association of one of these discs with RNA. The association is specific,
and requires an unusual stemmed-loop structure which is present around nucleotide 5476 of
TMV's genome (the figure is correct for the vulgare strain of the virus). The requirement for
this stemmed loop ensures that TMV coat protein is used only to package TMV RNA and is
not wasted on general RNA from the host cell.
During normal assembly, the stemmed loop enters the hole in the middle of a two-layered
disc, so that both free ends of the RNA dangle out from the same side. The strength of the
interaction between the protein and the stemmed loop causes the RNA-RNA duplex to
melt and to generate more single-stranded RNA. This associates with more of the protein
in the centre of the hole, until a complete turn binds. The total energy change involved in
this causes the disc to flip to its helical lock-washer shape, an action that traps the RNA
and that also enables the structure to initiate the addition of more coat protein. Extra protein
seems to attach mainly in the form of discs d a fact that can be demonstrated by running gels
of partially completed virus particles. The result is a discrete ladder representing successive
addition of discs rather than the smear that would result if addition were monomer-by-
monomer. As new discs are added, the 5 0 end of the RNA is pulled up through the middle
of the existing structure until it has all been packaged. The remaining projecting length of
the 3 0
end is then covered. Stable attachment of new discs requires the energy change
FIGURE 3.10 Assembly of Tobacco Mosaic Virus (TMV). Protein subunits (green) form di- and trimers spon-
taneously, and also form a few discs (not shown). In the presence of TMV RNA, one of these discs flips to a lock-
washer shape. These lock-washers become two-layered and, once the RNA/lock-washer complex has formed,
further protein subunits are recruited, generally in the form of discs, until the complete virus is assembled.
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