Biology Reference
In-Depth Information
almost completely lost the ability to associate with rafts (nonraft HA).
The mutant influenza viruses that had nonraft HA were shown to be
morphologically indistinguishable from wt virus by electron microscopy,
but importantly they contained smaller amounts of HA and replicated
poorly in cells.
17
These data showed that HA needs to associate with
rafts for influenza virus to replicate efficiently. Barman
et al
. (2004)
performed a similar experiment with NA.
36
Recombinant influenza
viruses were generated in which NA contained a series of alanine sub-
stitutions in the TM domain and CT, or the TM domain of NA was
replaced with that of a transferin receptor.
36
These mutations within
the TM domain and CT had multiple effects on NA function and
virus replication. They affected raft-association, expression, transport,
and enzyme activity of NA. Reduced incorporation of NA and an altered
morphology of budding virions were also caused by the NA muta-
tions.
36
Each NA mutant virus had a different set of defects. However,
generally speaking, mutations of the ectodomain-proximal amino
acids of the TM domain progressively lowered affinity of NA with
rafts, reduced NA enzyme activity, and attenuated virus growth.
36
Because of the multiple effects of NA mutation, it was unclear which
defect was directly due to the lowered affinity with rafts. However, it
is likely that NA of influenza virus needs to associate with rafts for
proper maturation, transport, and efficient incorporation into virions,
to allow efficient virus replication.
HA Fusion and Lipid Rafts
How does HA promote virus replication by interacting with rafts? HA
functions as homotrimers and is responsible for virus binding and sub-
sequent membrane fusion. HA is synthesized as a single polypeptide,
HA
0
, and is subsequently cleaved by trypsin-like cellular proteases
(
B
) At the plasma membranes, HA and NA concentrate in raft
microdomains. M1 is also accumulated in rafts by interacting with cytoplas-
mic tails of HA and NA. On the other hand, M2 is largely excluded from
rafts. RNP cores associate with rafts intrinsically or by binding with M1, pro-
moting the assembly processes of budding virions. Views from the outside
(
top
) and from the inside (
bottom
) of the cells are shown.
