STRUCTURE AND ANTIGENICITY

STRUCTURE AND COMPOSITION- usually spherical or ovoid, but may also be

long filamentous -about 100 nm in diameter (80–120 nm) -highly pleiomorphic

Influenza viruses-relatively hardy in vitro-may be stored at 0–4°C for weeks without

loss of viability-their infectivity may be destroyed by: Lipid solvents protein denaturants formaldehyde irradiation Both infectivity and hemagglutination

are more resistant to inactivation at alkaline pH than at acid pH.

Influenza A and B viruses-single-stranded, negative-sense RNA

genomes-occur as eight separate segments

Influenza C viruses-contain seven segments of RNA-lack a neuraminidase gene.

• Virion: Spherical, pleomorphic, 80–120 nm in diameter (helical nucleocapsid, 9

nm)

• Composition: RNA (1%), protein (73%), lipid (20%), carbohydrate (6%)

• Genome: Single-stranded RNA, segmented (eight molecules), negative-sense,

13.6 kb overall size

• Proteins: Nine structural proteins, one nonstructural

• Envelope: Contains viral hemagglutinin (HA) and neuraminidase (NA) proteins

• Replication: Nuclear transcription; capped 5' termini of cellular RNA scavenged as

primers; particles mature by budding from plasma membrane

• Outstanding characteristics:

Genetic reassortment common among members of the same genus

Influenza viruses cause worldwide epidemics

Structure and Function of Hemagglutinin

• binds virus particles to susceptible cells

• major antigen against which neutralizing (protective) antibodies

are directed.

• able to agglutinate erythrocytes under certain conditions.

• XRAY Crystallography: reveals the three-dimensional structure

of the gene.

• The primary sequence of HA contains 566 amino acids• A short signal sequence at the amino terminal inserts the

polypeptide into the endoplasmic reticulum; the signal is then removed.

• The HA protein is cleaved into two subunits, HA1 and HA2, that remain tightly associated by a disulfide bridge.

• A hydrophobic stretch near the carboxyl terminal of HA2 anchors the HA molecule in the membrane, with a short hydrophilic tail extending into the cytoplasm.

The HA molecule is folded into a complex structure.

Each linked HA1 and HA2 dimer forms an elongated stalk capped by a large globule.

The base of the stalk anchors it in the membrane.

Five antigenic sites on the HA molecule exhibit extensive mutations.

These sites occur at regions exposed on the surface of the structure, are apparently not essential to the molecule’s stability, and are involved in viral neutralization.

• The HA spike on the virus particle is a trimer composed of three intertwined HA1 and HA2 dimers.

• The trimerization imparts greater stability to the spike than could be achieved by a monomer.

• The cellular receptor binding site (viral attachment site) is a pocket located at the top of each large globule.

• The pocket is inaccessible to antibody.

• The cleavage that separates HA1 and HA2 is necessary for the virus particle to be infectious and is mediated by cellular proteases.

• Influenza viruses normally remain confined to the respiratory tract because the protease enzymes that cleave HA are common only at those sites.

Structure and Function of Neuraminidase

• is a tetramer composed of four

identical monomers

• A slender stalk is topped with a box-

shaped head

• There is a catalytic site for NA on the

top of each head, so that each NA spike

contains four active sites.

functions at the end of the viral replication cycle.a sialidase enzyme that removes sialic acid from

glycoconjugates.facilitates release of virus particles from infected

cell surfaces during the budding process and helps prevent self-aggregation of virions by removing sialic acid residues from viral glycoproteins.

helps the virus negotiate through the mucin layer in the respiratory tract to reach the target epithelial cells.

Antigenic Drift and Antigenic Shift• Minor antigenic changes are

termed antigenic drift; major antigenic changes in HA or NA, called antigenic shift, result in the appearance of a new subtype.

• Antigenic shift is most likely to result in an epidemic.

• Antigenic drift is caused by the accumulation of point mutations in the gene, resulting in amino acid changes in the protein.

Influenza Virus Replication(1) After receptor-mediated endocytosis, the viral

ribonucleoprotein complexes are released into

the cytoplasm and transported to the nucleus,

where replication and transcription take place.

(2) Messenger RNAs are exported to the cytoplasm

for translation.

(3) Early viral proteins required for replication and

transcription, including nucleoprotein (NP) and a

polymerase protein (PB1), are transported back

to the nucleus. RNA polymerase activity of the

PB1 protein synthesizes positive single-stranded

RNA (+ssRNA) from genomic negative single-

stranded RNA (–ssRNA) molecules.

(4)These +ssRNA templates are copied by the RNA

polymerase activity of the PB1 protein.

(5)Some of these new genome segments serve as

templates for the synthesis of more viral mRNA.

Later in the infection, they become progeny

genomes. Viral mRNA molecules transcribed

from some genome segments encode structural

proteins such as hemagglutinin (HA) and

neuraminidase (NA). These messages are

translated by endoplasmic reticulum associated

ribosomes and delivered to the cell membrane.

(6)Viral genome segments are packaged as progeny

virions bud from the host cell .

Phases Of Viral Replication

(1) Virus attachment and entry into the cell

(2) transcription of viral RNA and translation of viral proteins

(3) replication of viral RNA;

(4) assembly of new virions and release from the cell.