The canyon hypothesis. Hiding the host cell receptor attachment site on a viral surface from immune surveillance.

The three-dimensional structure of human rhinovirus 14 has a deep surface depression or "canyon" encircling each of the twelve 5-fold vertices. The canyon's surface is inaccessible to the broad antigen binding region of antibodies, permitting conservation of residues that might be required for host cell receptor recognition without danger of attack by the host's immune system. In contrast, the exposed surface features, where neutralizing antibodies are known to bind, change rapidly under pressure from the host's immune system. It was, therefore, hypothesized that this depression was the site of receptor attachment. Similar, but smaller, depressions had been observed previously on both the hemagglutinin and neuraminidase spikes of influenza virus. These have also been shown to be the site of host cell interaction. Although support for the canyon hypothesis was only circumstantial in the first place, there are now extensive confirmatory data. These include site-specific mutations of residues in the canyon and conformational changes induced in the canyon by the binding of small organic molecules, all of which alter receptor attachment. The strategy used in human rhinovirus 14 to protect the viral receptor attachment site from immune surveillance may be utilized not only in other picornaviruses but also in many other types of viruses including human immunodeficiency virus.