Biology Reference
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with higher concentrations of MgCl
2
, as reported to be the case for
T-trimers that bind tightly at the three-fold axes of the core.
16
This
result supports the hypothesis that a P8-trimer binds to the region of
three-fold symmetry of the viral core at the onset of second-layer
assembly, and its orientation determines the orientation of the P8-
trimers that bind subsequently.
The studies described above showed that both the presence of
regions of conserved amino acids that interact with neighboring cap-
sid proteins and the similar distributions of electrostatic potential on
the proteins are required for binding heterologous proteins. Further-
more, these observations confirmed, by biochemical methods, the
mechanism of assembly of the double-layered capsid with disparate
icosahedral lattices, which was suggested by structural analysis. Such
novel architecture might be representative of other layers of multiple
proteins that form cavities with functional molecules inside them.
Side-by-side Interactions Among P8-trimers:
a Mechanism for Formation of the Second Layer
of the Capsid
Once the orientation of a T-trimer relative to the core has been estab-
lished by specific electrostatic interactions between a P8-trimer and the
core along the three-fold axis, the icosahedral T
13
l
arrangement is
probably generated by subsequent interactions between P8-trimers
and the core, as well as by interactions among the P8-trimers them-
selves. Indeed, two-dimensional crystallographic studies of P8 proteins
have shown that the P8-trimer has structural features that facilitate
side-by-side binding of P8-trimers.
17
X-ray crystallographic studies of intact RDV
2
revealed the atomic
details of this biochemical phenomenon. The electrostatic potential at
the margin of each P8-trimer has obvious patches of positive and neg-
ative charge, allowing clear electrostatic complementarity between
adjacent subunits (Fig. 9). Shape complementarities
18
among trimers
(SC
=
0.26 to 0.49) and interactions between individual P8 proteins
might also stabilize the interactions between adjacent P8-trimers. In
particular, a close interaction between two P8-trimers was evident at
=
