ABSTRACT

The viral genome is surrounded by a protective protein coat known as the capsid which is assembled from individual virus-encoded polypeptides. The combination of the genome and the capsid is known as the nucleocapsid. Two basic shapes occur (Fig. 1): (i) icosahedral. In this case the individual polypeptide molecules form a geometrical structure that surrounds the nucleic acid. An example is poliovirus; (ii) filamentous or helical. In this case the polypeptide units are arranged as a helix to form a rod-like structure surrounding the nucleic acid genome. In many viruses the capsid is surrounded by a lipid envelope derived from the host cell membrane as the virus is released from the cell. Virus-encoded glycoproteins and proteins derived from the host cell may be inserted into the membrane. The viral glycoproteins play an important role in facilitating infection by interacting with receptor proteins on the surface of the host cell. Matrix proteins also occur which allow interaction between the nucleocapsid and the lipid envelope. The matrix and the capsid proteins also have other roles associated

Structure of viruses

with virus replication and transcription of viral genes. The structural proteins of a virus are often antigenic and are of interest in the development of vaccines.