Viral Structure

Viruses are composed of two fundamental units, a protein coat and a nucleic acid core. The protein coat is termed a capsid and is composed of subunits called capsomeres. The nucleic acid may be either RNA or DNA and can be either single-stranded, double-stranded, or partially double-stranded. Some viruses contain lipid coverings which are termed envelopes. In most cases, these are aquired from cellular membranes (nuclear, golgi, plasma) when the mature viruses bud from the cell. In at least once case, poxvirues, the virus codes for enzymes that make some of the lipids. Viruses which do not have membranes are termed "naked". The protein coat (capsid) and the nucleic acid together are the nucleocapsid. "Virion" refers to the entire virus. In the case of an enveloped virus, the virion is the entire virus and the nucleocapsid is the capsid and nucleic acid, minus the envelope. In the case of a naked virus, the nucleocapsid and virion are the same thing.

The function of the virion is, fundamentally, to ensure that the viral nucleic acid gets delivered from one cell to another cell. This involves many functions, including protection of the genome from nucleolytic enzymes, delivery of the genome, and interactions of the virus with cells.

Virus nucleocapsids come in two basic shapes, helical and icosahedral. The lone exception are the poxviruses which have a complex nucleocapsid.

Many viruses have projections which are given various names, but are usually called spikes (long thin projections) or peplomers (fatter projections). These range from very thin penton fibers in adenoviruses to thick hemagglutin spikes in orthomyxoviruses. Coronaviruses have a unique peplomer that looks a bit like a clover leaf. Most of these projections are glycoproteins.

Some viruses have additional structures in addition to the protein coat, nucleic acid, envelope and spike. For instance, rhabdoviruses (and other viruses) have a protein lattice just underneath their envelope termed the matrix. The major protein that makes up this layer is often called the "M" protein. The matrix gives added rigidity to the virus. Herpesviruses have and aditional layer underneath the membrane, the tegument. This is a very thick globular layer (unlike the matrix protein). The proteins in the tegument have very specific functions, such as activating the transcription of immediate-early genes, induction of degradation of cellular mRNA, and/or block the cellular mechanism by which viral proteins are presented to the host immune system. Poxviruses have a very thick (complex) protein layer outside their nucleocapsid that is not completely understood at this time.

In addition to the proteins specified above, viruses can also carry other types of proteins that have specific function. These include: