Biology Reference
In-Depth Information
reconstructions, no expansion of the VP3
-cylinder was seen. Such
an expansion would be required to generate a channel sufficiently
large for RNA egress. It is thus believed that the
β
-cylinder of at least
one, but not of all 12, five-fold axes enlarges to allow for transit of the
RNA (Fig. 8). Since cryo-EM image reconstruction is an averaging
technique, changes at single vertices would escape detection. The data
might indicate that,
in vivo
, the A-particle is handed over from ICAM-
1 to the membrane, where it directly interacts with the lipids via the
hydrophobic N-terminus of VP1, possibly aided by the myristoylated
N-terminus of VP4. However, it should be kept in mind that the rigid
ICAM-1 molecule has a length of
β
19 nm and, consequently, the virus
would be held at this distance from the endosomal membrane, thus
making such a scenario unlikely. Rather, the movement of ICAM-1 by
30
∼
concomitant with the conformational modification
101
might result
in membrane destabilization. This process could be directly coupled
with RNA uncoating and penetration into the cytosol (Fig. 7).
In the case of HRV2, cryo-EM reconstruction of empty capsids
generated by heating to 55
°
C revealed, in addition to the 4% expansion
of the capsid, the presence of density below the pseudo three-fold axis
that was interpreted as stemming from the exiting N-terminus of VP1.
However, there was no density attributable to VP4, suggesting that it
was completely absent.
100
However, in HRV2, the VP3
°
-cylinders had
expanded, making it highly probable that the RNA can exit at any of the
12 five-fold axes (Fig. 8). The high flexibility of LDLR and LRP and the
presumed attachment to the ligand-binding domain via several ligand-
binding modules wound around each five-fold axis might, in principle,
permit the virus to come close to the membrane. This would permit
contacts between the N-termini of VP1 and the endosomal membrane
to be established with the receptor still remaining attached to the virus.
β
-cylinders might enlarge and give way for the RNA. VP4 is probably
almost entirely lost in HRV2, but VP4 might remain bound to HRV14 at the
five-fold axes or at the pseudo three-fold axes (HRV3). Schemes are based on
data from Hewat (empty HRV2
100
and empty HRV14
99
) and from Xing
(empty HRV3 with bound soluble two-domain ICAM-1
101
). RNA release
might start with the 5
12
β
-terminus as indicated by Vpg, but recent studies with
atomic force microscopy suggest instead that the 3
′
′
-end exits first.
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