Re: Is H2H spread of a novel swine H3N2 occurring in the United States?
Re: Is H2H spread of a novel swine H3N2 occurring in the United States?
I think it could be helpful for all of us to read this brief description by R. Webster and other of the internal structure of Influenza A viruses. Full free PDF is available for consultation at the URL indicated below.
Source:
http://mmbr.asm.org/cgi/reprint/56/1/152
MICROBIOLOGICAL REVIEWS, Mar. 1992, p. 152-179 Vol. 56, No. 1
0146-0749/92/010152-28$02.00/0
Copyright ? 1992, American Society for Microbiology
Evolution and Ecology of Influenza A Viruses
ROBERT G. WEBSTER,* WILLIAM J. BEAN, OWEN T. GORMAN, THOMAS M. CHAMBERS,t AND YOSHIHIRO KAWAOKA
Department of Virology and Molecular Biology, St. Jude Children's Research Hospital,
332 North Lauderdale, P.O. Box 318, Memphis, Tennessee 38101
(...)
STRUCTURE AND FUNCTION OF THE INFLUENZA VIRUS VIRION
Current knowledge of the molecular biology of influenza
viruses has been recently reviewed in extensive detail by
Lamb (95), and individual references are not given. This
section presents a summary of that knowledge as background
information.
Components of the Virion
Influenza A viruses are members of the Orthomyxoviridae
family. They are differentiated from type B and C influenza
viruses on the basis of the identity of the major internal
protein antigens, the nucleoprotein (NP) and matrix (Ml)
proteins. On initial isolation, influenza A viruses are small
(80 to 120 nm in diameter), pleomorphic particles that later
become generally spherical. These particles consist of a
host-derived lipid bilayer envelope in which the virus-encoded
glycoproteins HA and neuraminidase (NA) and M2
are embedded; an inner shell of matrix protein; and, at the
center, the nucleocapsids of the viral genome (Fig. 1). The
genome of influenza A viruses consists of eight unique
segments of single-stranded RNA, which are of negative
polarity (i.e., complementary to the mRNA sense). The
RNA is loosely encapsidated by multiple NP molecules.
Complexes containing the three viral polymerase proteins
(PB1, PB2, and PA) are situated at the ends of the nucleocapsids.
To be infectious, a single virus particle must contain each
of the eight unique RNA segments. Available evidence
suggests that incorporation of RNAs into virions is at least
partly random. The random incorporation of RNA segments
allows the generation of progeny viruses containing novel
combinations of genes (i.e., genetic reassortment) when cells
are doubly infected with two different parent viruses.
The eight influenza A viral RNA segments encode 10
recognized gene products. These are PB1, PB2, and PA
polymerases, HA, NP, NA, Ml and M2 proteins, and NS1
and NS2 proteins.
PB2 polymerase. PB2 polymerase is encoded by RNA
segment 1, the slowest-migrating RNA species by gel electrophoresis.
It is a member of the protein complex providing
viral RNA-dependent RNA polymerase activity. It is known
to function during initiation of viral mRNA transcription as
the protein which recognizes and binds the 5' capl structures
of host cell mRNAs for use as viral mRNA transcription
primers. Endonucleolytic cleavage of these cap structures
from host mRNAs is also at least in part a function of PB2.
The role of PB2 in the other virus-directed RNA synthetic
processes, i.e., synthesis of full-length template cRNA and
new negative-sense viral RNA (vRNA), is not known since
these processes do not require host cap priming. Newly
synthesized PB2 proteins migrate to the nucleus of infected
cells.
PB1 polymerase. PB1 polymerase is encoded by RNA
segment 2; it functions in the RNA polymerase complex as
the protein responsible for elongation of the primed nascent
viral mRNA and also as elongation protein for template
RNA and vRNA synthesis. PB1 proteins localize in the
nucleus of infected cells.
PA polymerase. PA polymerase is encoded by RNA segment
3. It also localizes in the infected cell nucleus and is a
member of the RNA-dependent RNA polymerase complex
along with PB1 and PB2, but its role in viral RNA synthesis
is unknown. There is evidence for possible roles as a protein
kinase or as a helix-unwinding protein.
Hemagglutinin. The HA protein is an integral membrane
protein and the major surface antigen of the influenza virus
virion. It is responsible for binding of virions to host cell
receptors and for fusion between the virion envelope and the
host cell. HA is encoded by RNA segment 4. It undergoes
three kinds of posttranslational processing: proteolytic
cleavage, glycosylation, and fatty acid acylation. Newly
synthesized HA is cleaved to remove the amino-terminal
hydrophobic sequence of 14 to 18 amino acids, which are the
signal sequence for transport to the cell membrane. Carbohydrate
side chains are added, whose number and position
vary with the virus strain. Palmitic acid is added to cysteine
residues near the HA carboxy terminus. The final processing
step is cleavage of the HA into two subunits, HAl and HA2
(uncleaved HA is called HAO), connected by disulfide linkages.
This cleavage is accomplished by host-produced
trypsinlike proteases and is required for infectivity because
virus-cell fusion is mediated by the free amino terminus of
HA2. The fully processed HA thus consists of HAl of
(typically) about 324 amino acids plus variable carbohydrate,
and HA2 of (typically) about 222 amino acids plus variable
carbohydrate plus 3 palmitate residues.
HA molecules form homotrimers during maturation. The
three-dimensional structure of a complete HA trimer has
been determined. In essence, each HA molecule consists of
a globular head on a stalk. The head is made up exclusively
of HAl and contains the receptor-binding cavity as well as
most of the antigenic sites of the molecule. The stalk consists
of all of HA2 and part of HAl. The carboxy-terminal region
of HA2 contains the hydrophobic transmembrane sequence
and a terminal cytoplasmic anchor sequence where palmitate
is attached.
Owing to error-prone viral RNA polymerase activity,
influenza virus HA is subject to a very high rate of mutation,
estimated at about 2 x 10-3 base substitutions per position
per virus generation, or about one base substitution in the
HA gene per virus generation. Selection for amino acid
substitutions is driven at least in part by immune pressure, as
the HA is the major target of the host immune response.
Although the amino acids making up the receptor-binding
site, as well as cysteine and most proline residues, are highly
conserved, the remainder of the HA molecule is highly
mutable. In nature there are presently 14 recognized subtypes
of HA (called Hi, H2, etc.), which differ by at least
30% in the amino acid sequence of HAl and which are
serologically not cross-reactive (Table 1). Subtypes may
include several variant strains which are partially serologically
cross-reactive.
Nucleoprotein. NP is encoded by RNA segment 5. It is
transported into the infected cell nucleus, where it binds to
and encapsidates viral RNA. In addition to its structural
role, NP is believed to play a role in the switching of viral
RNA polymerase activity from mRNA synthesis to cRNA
and vRNA synthesis. NP is abundantly synthesized in
infected cells and is the second most abundant protein in the
influenza virus virion. It is phosphorylated; the pattern of
phosphorylation is host cell dependent and may be related to
viral host range restriction. NP is also a major target of the
host cytotoxic T-cell immune response.
Neuraminidase. NA, encoded by RNA segment 6, is also
an integral membrane glycoprotein and a second major
surface antigen of the virion. NA cleaves terminal sialic acid
from glycoproteins or glycolipids. Thus, it functions to free
virus particles from host cell receptors, to permit progeny
virions to escape from the cell in which they arose, and so
facilitate virus spread.
NA is glycosylated and possesses an amino-terminal hydrophobic
sequence which functions both as signal for
transport to the cell membrane and as transmembrane domain;
it is not cleaved away. The distribution of NA has not
been conclusively resolved; immunogold-labeling experiments
suggest that the NA tetramers are not evenly distributed
over the virion envelope, as is HA, but aggregate into
patches or caps. The complete three-dimensional structure
of an NA tetramer, bound to antibody, has been determined.
Like HA, NA is highly mutable with variant selection
partly in response to host immune pressure. Nine subtypes
of NA (called Ni, N2, etc.) have been identified in nature;
they are not serologically cross-reactive (Table 1). Different
variants of several subtypes are known.
Ml protein. Influenza virus RNA segment 7 is bicistronic,
encoding both Ml and M2 proteins. Colinear transcription of
segment 7 yields mRNA for the matrix protein. This is the
most abundant protein in the influenza virus virion. Matrix
protein forms a shell surrounding the virion nucleocapsids,
underneath the virion envelope. In the infected cell it is
present in both cytoplasm and nucleus. It has no known
enzymatic activity, although it has been speculated to play
an important role in initiating progeny virus assembly.
M2 protein. The mRNA for M2 is also transcribed from
RNA segment 7. It is derived from the colinear (Ml)
transcript by splicing. M2 is an integral membrane protein,
whose membrane-spanning domain also serves as a signal
for transport to the cell surface. It is present as a tetramer in
large amounts on the infected cell surface, and a small
amount is found in the virion. It is believed to act as a proton
channel to control the pH of the Golgi during HA synthesis
and to allow acidification of the interior of the virion during
virus uncoating.
Nonstructural NS1 and NS2 proteins. RNA segment 8
encodes the two nonstructural proteins NS1 and NS2. NS1
mRNA is colinear with the vRNA, whereas NS2 mRNA is
derived by splicing. These proteins, particularly NS1, are
abundant in the infected cell (NS1 primarily in the nucleus,
NS2 primarily in the cytoplasm) but are not incorporated
into progeny virions. Both proteins play roles in virus
replication, but those roles have not been fully defined. NS2
appears to modulate the synthesis of NS.
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