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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 nucleocapsid (N) protein is a protein that packages the positive-sense RNA genome of coronaviruses to form ribonucleoprotein structures enclosed within the viral capsid. [ 2 ] [ 3 ] The N protein is the most highly expressed of the four major coronavirus structural proteins . [ 2 ]
Thus, the entire chromosome, i.e. chromatin in eukaryotes consists of such nucleoproteins. [ 2 ] [ 13 ] In eukaryotic cells, DNA is associated with about an equal mass of histone proteins in a highly condensed nucleoprotein complex called chromatin . [ 14 ]
The nucleocapsid of the virion is made up of the nucleic acid and the capsid. Remember that the genomes of most viruses are very small. Genes code for instructions to make proteins, so small genomes cannot code for many proteins. Therefore, the virion capsid consists of one or only a few proteins that repeat over and over to form the structure.
The HIV capsid consists of roughly 2000 copies of the p24 protein. The p24 structure is shown in two representations: cartoon (top) and isosurface (bottom) The p24 capsid protein is the most abundant HIV protein with each virus containing approximately 1,500 to 3,000 p24 molecules. [ 1 ]
It consists of: HBsAg (hepatitis B surface antigen) was the first hepatitis B virus protein to be discovered. [15] It consists of small (S), medium (M) and large (L) protein. [16] HBcAg (hepatitis B core antigen) is the main structural protein of HBV icosahedral nucleocapsid and it has function in replication of the virus. [17]
Inside the envelope, there is the nucleocapsid, which is formed from multiple copies of the nucleocapsid (N) protein, which are bound to the positive-sense single-stranded RNA genome in a continuous beads-on-a-string type conformation. [49] [56] N protein is a phosphoprotein of 43 to 50 kDa in size, and is divided into three conserved domains.
The capsid/nucleocapsid is round with triangulation number T=16 and exhibits icosahedral symmetry. The capsid is isometric and has a diameter of 100–110 nm. [2] The capsid consists of 162 capsomer proteins with a hexagonal base and a hole running halfway down the long axis. The core consists of a fibrillar spool on which the DNA is wrapped.