Viruses

Wednesday, January 28, 2009 5:02 PM

Virus Host interaction

1st Stage- Attachment

It is a specific binding between viral surface proteins and their receptors on the host cellular surface.

This specificity determines the host range of a virus.

Example, the Human Immunodeficiency Virus (HIV) attacks only human's immune cells (mainly T cells), because its surface protein, gp120, can interact with CD4 and chemokine receptors on the T cell's surface.

2nd Stage- Entry, Direct Cell Membrane Fusion

Viruses take advantage of these processes in a number of ways, after they have attached to the cell surface via binding to a specific receptor.

The simplest is DIRECT MEMBRANE FUSION, where the virion membrane fuses with the cell membrane, and the virion nucleoprotein complex is delivered into the cell cytoplasm directly.

This is generally a pH-independent process, and requires only that the membrane be fluid (ie: temperature in the physiological range), and generally that some divalent cations be present.

The entry process for HIV is shown in the graphic below.

3rd Stage-Uncoating

Uncoating is a process that viral capsid is degraded by viral enzymes or host enzymes.

4th Stage-Replication

Replication involves assembly of viral proteins and genetic materials produced in the host cell.

5th Stage- Assembly

Assembly of infectious virions is dependent on the action of: (a) an aspartyl protease encoded by the viral pol gene and responsible for cleavage of the gag and gag-pol precursors into mature proteins (1,2) and (b) cellular N-protein myristoyl transferase (NMT) which adds myristic acid to the N-terminus of gag, gag-pol and nef viral polyprotein precursors(3).

6th Stage- Release

Viruses may escape from the host cell by causing cell rupture (lysis).

Enveloped viruses such as HIV typically "bud" from the host cell. During the budding process, a virus acquires the phospholipid envelope containing the embedded viral glycoproteins.

Done by: Toh Wan Nee