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Page 464<br />

could not again be agglutinated <strong>by</strong> either the eluted virus or fresh virus preparations. This activity is now<br />

attributed to neuraminidase, which is one of the two integral membrane glycoproteins of influenza virus<br />

(for reviews see References 24 and 25).<br />

A. Function<br />

Neuraminidase is an exoglycosidase that destroys the hemagglutinin receptor <strong>by</strong> cleaving the αketosidic<br />

linkage of terminal sialic acid [(N-actylneuraminic acid (Neu5Ac))] to an adjacent sugar<br />

[26,27]. Viral hemagglutinin binds specifically to Neu5Ac-containing receptors on the surface of<br />

susceptible cells [28]. Neuraminidase, which also removes terminal sialic acid from a range of<br />

glycoconjugates, plays an important, but not completely understood, role in the viral replication cycle.<br />

Without neuraminidase activity viruses [29] were thought to be immobilized <strong>by</strong> mucosal secretions in<br />

the upper respiratory tract. By removing terminal sialic acid from the sialic-acid-rich mucous layer<br />

[27,30] protecting target cells, neuraminidase could facilitate penetration of the virus to the cell surface.<br />

It has been shown that neuraminidase-deficient virus [31] can still replicate in vivo, albiet at a much<br />

reduced rate [32]. This shows that neuraminidase does not play an essential role in viral entry,<br />

replication, assembly or budding in mice, but has an important role in the spread of the infection <strong>by</strong><br />

preventing aggregation at the cell surface and possible immobilization in the mucin <strong>by</strong> hemagglutinin.<br />

Once replication is initiated in the infected cell, the freshly synthesised viral glycoproteins have to be<br />

desialylated to prevent self-aggregation at the infected host cell surface <strong>by</strong> hemagglutinin binding to<br />

terminal sialic acid on these glycoproteins. Finally on elution of progeny virions from infected cells,<br />

neuraminidase activity is required to facilitate viral escape from the cell surface.<br />

Inactivation or inhibition of neuraminidase during budding has been observed to result in aggregation of<br />

virions on the cell surface [33–35]. Inhibition of this glycohydrolase could provide a means of<br />

controlling this disease <strong>by</strong> slowing the rate of viral attachment and subsequent release of progeny virions<br />

allowing the host immune system to eliminate the virus while the number of infected cells is low.<br />

B. Morphology<br />

There are between 50 to 100 neuraminidase spikes per virion [36] which is approximately 10% of the<br />

visible spikes projecting out of the surface of the virion [37]. These spikes can be removed from the<br />

virus <strong>by</strong> treatment with detergent [38]. Electron microscopic images of the neuraminidase spikes [39]<br />

reveal a mushroom-shaped molecule made up of a boxlike head of about 80 ×<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_464.html [4/9/2004 12:13:17 AM]

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