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The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease virus capsid has faces consisting of three proteins named VP1–3. [6] Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope.
The circular genome of a representative polyomavirus, WU polyomavirus, with the late region at right indicating positions of the VP1, VP2, and VP3 genes. [4]All three capsid proteins are expressed from alternative start sites on a single transcript of the "late region" of the circular viral chromosome (so named because it is transcribed late in the process of viral infection).
The capsid, having a focused role of protecting the genome in addition to immune recognition evasion. [13] The viral capsid is known for its protection of RNA before it is inserted into the host cell, unlike the viral envelope which protects the protein capsid. [14]
Many copies of a single viral protein or a number of different viral proteins make up the capsid, and each of these viral proteins are coded for by one gene from the viral genome. The structure of the capsid allows the virus to use a small number of viral genes to make a large capsid. [3]
Varidnaviria is a realm of viruses that includes all DNA viruses that encode major capsid proteins that contain a vertical jelly roll fold.The major capsid proteins (MCP) form into pseudohexameric subunits of the viral capsid, which stores the viral deoxyribonucleic acid (DNA), and are perpendicular, or vertical, to the surface of the capsid.
However, some DNA phages such as T4 may have large genomes with hundreds of genes; the size and shape of the capsid varies along with the size of the genome. [73] The largest bacteriophage genomes reach a size of 735 kb. [74] Schematic view of the 44 kb T7 phage genome. Each box is a gene. Numbers indicate genes (or rather open reading frames).
The portal is formed during initial capsid assembly and interacts with scaffolding proteins that construct the procapsid. [4] [5] [6] When the capsid is nearly complete, the viral DNA enters the capsid (i.e., the DNA is encapsidated) by a mechanism involving the portal and a DNA-binding protein complex similar to bacteriophage terminase. [7]
Virus genomes (either DNA or RNA) are extremely tightly packed into the viral capsid. [4] [5] Many viruses are therefore little more than an organised collection of nucleoproteins with their binding sites pointing inwards.