Biology Reference
In-Depth Information
a plenitude of information has become available, and now SARS-CoV
has become one of the best-studied members among the coronaviruses.
8
SARS-CoV was initially reported to be different from any known
coronaviruses and thus failed to be classified in any of the coronavirus
groups.
4
However, a detailed phylogenetical study including Torovirus
that is also classified in
Coronaviridae
, suggests it to be a member of
group 2, though it is most distant from the other members of this
group.
9
It is also reported that the SARS-CoV genome is a mosaic of
mammalian and avian coronaviruses.
10
Most coronaviruses, other than
SARS-CoV, do not cause a fatal disease. Two species of human coron-
avirus are causative agents of the common cold, and studies on these
viruses were restricted because of their weak pathogenic nature.
1,2
Other coronaviruses infect domestic animals, pets and experimental
animals, but mostly do not cause severe and fatal diseases. However, as
they are important pathogens from an economic point of view, these
animal coronaviruses have been researched extensively.
2
A prototype coronavirus, mouse hepatitis virus (MHV), has been
studied as a model of human disease, such as hepatitis and demyeli-
nating encephalomyelitis.
2
This virus is one of the most studied coro-
naviruses, especially in terms of its molecular mechanism of viral
replication as well as its cell entry.
2,11
In this review, I would like to
describe how MHV and SARS-CoV interact with their respective
receptors and enter cells, as well as the similarity and dissimilarity
between these two coronaviruses.
S Proteins of Coronaviruses
Coronaviruses have morphologically characteristic spikes resembling a
solar corona on the virion surface, for which they were so named. The
spike consists of a globular structure in the outer extremity and a stem-
like structure beneath it. The spike is thought to be composed of three
molecules of spike (S) protein. The S protein is a class I fusion protein
with a molecular weight of 180 to 200 kDa (Fig. 1).
2
After being syn-
thesized, MHV and some other coronavirus S proteins are cleaved
around the middle of the molecule into two subunits, N-terminal S1 and
C-terminal S2, by host cell-derived proteases.
12
There is a high similarity
in the amino acid sequences in S2, but these are quite variable in S1
