Biology Reference
In-Depth Information
A-particles sediment at 135S. The latter are most likely an intermediate
of the uncoating process that culminates in RNA expulsion and gener-
ation of empty subviral B-particles with a sedimentation constant of
80S. Subviral particles can also be produced
in vitro
. Exposure to pH
5.0
28,94
results primarily in 135S particles, and heating to 50-56
C for
some minutes preferentially generates 80S particles (Lonberg-Holm,
1973).
94,97
This structural modification is accompanied by changes in
antigenicity manifesting in the appearance of new (C-antigenic) epi-
topes that can be detected with specific antisera and monoclonal anti-
bodies (Lonberg-Holm, 1973).
92,98
As explicitly shown for HRV2,
A-particles are hydrophobic whereas B-particles are not.
7,8
This dis-
agrees with the structural data on empty capsids that suggest the pres-
ence of the hydrophobic N-terminus of VP1 at the viral surface.
99-101
°
Conversion of Major Group HRVs In Vitro
In vitro
, interaction of soluble two-domain ICAM-1 (sICAM) with
some major group viruses (e.g. HRV3 and HRV14, but not HRV16)
above 20
C destabilizes the capsid to different extents in a time-
dependent manner, giving rise to predominantly either 135S or 80S
particles.
102-104
The tip of ICAM-1 is believed to first attach to the floor
and south wall of the canyon with rather low affinity. Upon expulsion
of the pocket factor, if present, more extensive interactions with both
canyon walls become established; the latter requires physiologic tem-
perature.
105,106
Concomitantly, the canyon undergoes conformational
adjustments to better accommodate the receptor. Determination of
the attachment of sICAM-1 to immobilized HRV3 and HRV14 by
Biacore instrumentation demonstrated biphasic kinetics.
83,107
In princi-
ple, this supports the abovementioned mechanism; however, both
kinetic components of the reaction were observed between 10
°
°
C and
25
C, temperatures where the structural changes should not occur.
ICAM-1 presumably acts like an enzyme in diminishing the activation
energy of the transition from the native virion to the subviral parti-
cle.
108
In addition to the receptor, low pH, which prevails in endo-
somes, may affect these structural alterations catalyzed by ICAM-1.
When HRV16-sICAM-1 complexes preformed at 37
°
C were exposed
to pH 6.0, the uncoating reaction was cooperative. For HRV3 that
°
