Attachment and entry of viruses into cells

LECTURE 9:

Viro100: Virology3 Credit hoursNUST Centre of Virology & Immunology

Waqas Nasir Chaudhry

• Virus aim to get replicate • To achieve it, virus need to enter into the cell• In general replication of virus can be divided

1. Attachment of a virion to a cell2. Entry into the cell3. Transcription of virus genes into messenger RNA

molecules (mRNAs)4. Translation of virus mRNAs into virus proteins5. Genome replication6. Assembly of proteins and genomes into virions7. Exit of the virions from the cell

3

VIRAL LIFE CYCLE

ATTACHMENT

PENETRATION HOSTFUNCTIONS

ASSEMBLY(MATURATION)

Transcription

REPLICATION

RELEASE

UNCOATING

Translation

MULTIPLICATION

Cell receptors and co-receptors

• A virion attaches via one or more of its surface proteins to specific molecules on the surface of a host cell

• These cellular molecules are known as receptors and the recognition of a receptor by a virion is highly specific, like a key fitting in its lock

• It has been found that some viruses need to bind to a second type of cell surface molecule (a co-receptor) in order to infect a cell.

HIV entry process. (a) CD4–gp120 binding, (b) gp120-co-receptor interaction and (c) viral and cellular membrane fusion

• Receptors and co-receptors are cell surface molecules, usually glycoproteins, with a wide range of functions that include

– acting as receptors for chemokines* and growth factors

– mediating cell-to-cell contact and adhesion

*Chemokines are a family of small proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines

Virus attachment sites

• Each virion has multiple sites that can bind to receptors, and each site is made up of regions of one or more protein molecules

• The virus attachment sites of some naked viruses are on specialized structures, such as the fibres and knobs of adenoviruses

• while the virus attachment sites of enveloped viruses are on the surface glycoproteins

Adenovirus virion Enveloped Viruses

• Some virion surface proteins that bear the virus attachment sites are able to bind strongly to red blood cells of various species

• Cause them to clump, a phenomenon known as haemagglutination

• The proteins responsible for aemagglutination are called haemagglutinins

• Examples are influenza viruses and measles virus

Haemagglutination

Evidence that a cell surface moleculeis a virus receptor

• Block the cell surface with antibodies• Soluble derivatives of the molecule block virus

binding/infectivity• The normal ligand for the molecule blocks

virus binding/infectivity• Introduction of the gene encoding the

molecule into virus resistant cells, and expression of that gene, makes those cells susceptible to infection

Block the cell surface molecule with antibodies

Soluble derivatives of the molecule block virus binding/infectivity

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Polio Vp1 interacting with CD155p1

Influenza Virus Receptor

Sialic acid is always the last sugar in a chain that is attached to a protein. On the right is the chemical structure of sialic acid; the next sugar, to the right, is galactose. Influenza virions attach to cells when the HA grabs onto the very small sialic acid.

Influenza Virus Receptor