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since the N-terminal of the trimer continues to move ahead and
push dsRNA even further inside the cavity, the steric conflict is
resolved by melting the dsRNA.
4.
The 3´ terminus of the vRNA comes out of the trimer where it
is immediately engaged by the viral polymerase, while the 5´ termi-
nus remains captured by the N-trimer.
The scheme presented here has an analogy in the mode of activity
of cellular DNA topoisomerase.
30
Coiled-coil helices of this enzyme
first recognize dsDNA; the protein then moves along causing dsDNA
to move down to other parts of the protein where they are opened or
chopped. Our observation on high sequence (and hence structural)
homology between the N-terminal, coiled-coil-forming region of the
hantaviral N protein and DNA topoisomerase
21
is in line with this rea-
soning. There is also an example of an RNA chaperone (the L1 pro-
tein of the mammalian retrotransposon) that acts as a stable trimer
formed by means of the N-terminal coiled-coils.
31
Figure 5 shows how the N protein could encapsidate RNA (in this
case, RNA from insect cells infected with a recombinant baculovirus
expressing the N protein of Puumala hantavirus). The image on the
background shows RNP complexes, which appear as dark lines sur-
rounded by bright thick boundaries on both sides. A framed part of the
RNP is shown in detail at the bottom (left) as an image pair. The dark
line represents the aligned hollow spaces formed by the middle portions
of neighboring N-monomers interacting with RNA. In the further
zoomed version at the bottom (right) two N-monomers (shown on
one of the duplicate images in red and yellow) are positioned in the
middle of the field. They appear as dumbbell shapes, with their termini
touching each other. The termini appear bright while the middle por-
tions of the neighboring N-monomers surround a darker area. The dark
line running in the middle, across the chain of these hollow spaces
between the monomers (drawn in blue), is most likely the place where
RNA is associated with the N-protein. This arrangement resembles the
models of RNPs of vesicular stomatitis and rabies viruses.
13,14,29
Large round structures were found abundant in the cryo-EM
micrographs of recombinant hantaviral N protein (Fig. 6). These
coiled ring-like (sometimes, arch-like) structures are very similar, both
