Biology Reference
In-Depth Information
These data suggest a structural requirement of rafts on virus envelopes
for efficient virus entry. However, in spite of these strong reductions
of infectivity, we found no defect in virus-to-cell fusion activity of
M
CD-treated virus by a fluorescent dequenching fusion assay.
17
One possibility is that membrane fusion between virus envelopes and
cell membranes may be interrupted at the hemifusion stage; that is,
lipid mixing occurred, but subsequent content mixing did not. Due
to the difficulty of direct measurement of the content mixing between
virus and cells, we performed a hemolysis assay.
17
β
CD-
treated viruses showed no defect on hemolysis. Reduced infectivity of
M
Again, M
β
CD-treated viruses may be at least partially explained by an effect
on M2 protein, since the uncoating process supported by the M2 ion
channel function is critical for influenza virus infectivity and choles-
terol was shown to be required for the M2 ion channel activity.
44,45
Further insights into the requirement for cholesterol or lipid micro-
domains on virus envelopes will require further investigation.
β
Rafts Function at the Process of Influenza
Virus Assembly
The process of virus assembly includes recruitment of all structural
viral components at a specific site and segregation of cellular proteins
from the structural viral components. The molecular mechanism of
this process has been poorly understood. The concept of lipid rafts
functioning during assembly as selection and concentration devices of
specific membrane proteins is therefore an attractive one. Many
viruses, including influenza virus, have been shown to use rafts as the
sites of assembly (Table 1).
8,9,11,12
The lipid composition of purified
influenza viruses is rich in cholesterol and sphingolipids, similar to
that of the membrane rafts.
19,23
These data are consistent with the
observations that both envelope glycoproteins HA and NA associate
with rafts. In addition to the TM domains, CTs of HA and NA have
been shown to play an important role in the raft-association and in
virus assembly, most likely by interacting with M1 proteins.
19,46
Particle production of a mutant influenza virus with CT deletions of
both HA and NA was reduced
10-fold.
46
These particles are greatly
elongated or distended
46
and contain a reduced amount of cholesterol
∼
