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domains with adjacent subunits via strong interactions that yield a
single trimeric unit. The overall shape of the P8-trimer is prismatic,
with sides of 90 Å and a height of 75 Å (Fig. 2). This trimeric struc-
ture is conserved among viruses that belong to Reoviridae (Fig. 2).
The structure of P8-trimers seems to contradict the idea that a
capsid protein should be thin and flat so that large amounts of
genome and related proteins can be packaged inside a cavity, as pro-
posed for the P3 protein of RDV above. Nonetheless, it is reasonable
to postulate that the second layer is made up of proteins that form a
structure wherein each trimer seals the spaces at the junctions between
individual first-layer proteins. The trimeric structure would be very
suitable for a unit that attaches to a three-fold axis at which three mol-
ecules of P3 make a triangle joint. Furthermore, a triangular structure
would be most suitable for lateral organization on an icosahedral par-
ticle. A trimeric structure provides an easy basis for construction of a
triangular structure by protein molecules. It would be difficult, for
example, for a single protein molecule to create a triangular struc-
ture and then for such structures to bind to one another side by side.
Furthermore, triangular structures composed of trimers have the
advantage that each edge is identical in terms of both conformation
and electrostatic charge (Fig. 9). Thus, each edge of each trimer always
faces a complementary arrangement when two trimers are adjacent
to each other. These features might explain why all second layers of
reoviruses examined to date are based on trimers.
It is interesting to note that the height (z axis) of the protein in
the second layer of the reoviruses that have been examined is similar,
namely, 97 Å in rotavirus, 23 90 Å in BTV 3 and 75 Å in RDV. We
might expect this layer to be thinner from an economical perspective,
as exemplified by the first-layer P3 protein and mentioned above.
However, it is possible that the second-layer protein needs to be of a
certain height to allow tight binding of the trimers that are the build-
ing blocks of the second layer. If the intra-P8 binding in the trimer is
not as tight as that of inter-P8 binding, various types of building block
might be formed and make it difficult to generate an ordered lateral
arrangement of P8-trimers in the second layer of the reoviruses. The
individual components of the trimer subunits of reoviruses appear to
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