Appendix 2. Newcastle disease virus: classification and important viral proteins

Classification of the virus	Morphology of the virus
Family: Paramyxoviridae	Spherical shape although pleomorphism is common
Subfamily: Paramyxovirinae	100 to 300 nanometres in diameter
Genus: Rubulavirus	Enveloped

Important viral proteins

Two proteins of Newcastle disease virus are inserted in the envelope. See Figure 23. They are the haemagglutinin/neuraminidase protein and the fusion protein. These two proteins are important in determining the virulence of the virus and how the virus infects host cells.

The haemagglutinin/neuraminidase protein

This protein has two sections that are of interest.

1. The haemagglutinin section. This is an attachment protein and binds to receptors on the outside of the membrane of host cells including red blood cells. Attachment of the virus to red blood cells is an important property used in the laboratory to detect the presence of the virus and to detect antibodies to the virus.

2. The neuraminidase section is the active site of an enzyme that aids in the release of the virus from the membrane of host cells. The activity of this enzyme affects the time taken for the virus to elute from red blood cells.

The fusion protein

This protein fuses the virus envelope to the membrane of the host cell. This allows penetration of the host cell by the viral genome.

In order for fusion to occur, the shape of the native fusion protein must be changed. This change happens when a host cell protease cleaves or cuts the protein at a specific cleavage site. After this has happened, the fusion protein is activated and can now fuse to the membrane of the cell.

The sequence of the amino acids around the cleavage site determines the range of proteases that can activate cleavage of the protein. This sequence therefore determines the virulence.

Molecular analysis of the Newcastle disease virus genome

A link between the amino acid sequence of the cleavage site of the fusion protein and virulence of Newcastle disease virus isolates has been established. Molecular techniques are now used to indicate the virulence of new isolates of Newcastle disease virus. This involves the use of Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) and sequencing of the DNA product. The amino acid sequence of the viral protein can then be deduced and analyzed.

The complete viral genome of the I-2 strain of Newcastle disease virus has now been sequenced. (Personal communication: Philemon Wambura.)

Figure 23: Diagrammatic representation of Newcastle disease virus