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and requires a larger insertion of polypeptides between strands of
the
-sandwich if the subunit has the same overall structure as that
of the insect nodaviruses. In fact, the striking gaps between the inner
and the outer regions of the capsid are more consistent with a two-
domain subunit than the integrated insertions seen in insect nodaviruses.
The organization of conserved (residues 83-216) and variable (residues
235-315) regions of the subunit leads to a similar conclusion when it
is compared to coat protein sequence alignments of insect nodaviruses
where there are no such linear regions of conservation and variability.
It was shown that the surface portion of the insect nodavirus subunits
is considerably more variable in sequence than the
β
-sandwich, but
since this region is formed by a variety of insertions between strands
of the
β
-sheets, it does not stand out in the sequence alignments. In
contrast, sequence comparison of the insect tetraviruses shows a strik-
ing region of variability that is linear and corresponds to the insertion
of an entire domain between two strands of the
β
β
-sandwich that forms
the
T
4 particle shell. Thus the linearity of the variable domain in
the fish nodavirus coat protein suggests that it may correspond to a
contiguous external domain in fish virus particles, thus indicating a
different subunit organization than observed in the insect nodaviruses.
Such an organization would be more similar to that of the coat
protein subunits of the plant virus tomato bushy stunt virus, where
residues 104 to 268 are in the contiguous
=
-sandwich shell-forming
domain while residues 273-386 form the protruding domain on the
particle outer surface.
The details of the morphology and the radial density distribution
of MGNNV differ dramatically from those of PaV. The maximum
diameter of MGNNV is approximately 380 Å, significantly larger than
the PaV capsid at 360 Å. The density distribution of the MGNNV
map shows two shells, at radii corresponding to protein, separated by
low density. The outer shell is between radii of 154 Å and 192 Å, with
a maximum at 168 Å; the inner shell is between radii of 112 Å and
154 Å, with a maximum at 135 Å. The MGNNV map is contoured
at a level such that the resultant volume of these two shells is equal
to the expected volume of its protein capsid, which is formed by
180 copies of a 37 kDa subunit. It is noteworthy that the interior
β
