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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 p24 capsid protein is the most abundant HIV protein with each virus containing approximately 1,500 to 3,000 p24 molecules. [1] It is the major structural protein within the capsid , and it is involved in maintaining the structural integrity of the virus and facilitating various stages of the viral life cycle, including viral entry into host ...
In Human papillomavirus (HPV), two late proteins are involved in capsid formation: a major (L1) and a minor (L2) protein, in the approximate proportion 95:5%. L1 forms a pentameric assembly unit of the viral shell in a manner that closely resembles VP1 from polyomaviruses. Intermolecular disulphide bonding holds the L1 capsid proteins together. [3]
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).
Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy. [43] [44] In general, there are five main morphological virus types: Helical
The capsid of some viruses are enclosed in a membrane called the viral envelope. In most cases, the viral envelope is obtained by the capsid from the host cell's plasma membrane when a virus leaves its host cell through a process called budding. [4]
In some cases, the virus with an envelope will form an endosome within the host cell. [10] There are three main types of viral glycoproteins: Envelope proteins, membrane proteins, and spike proteins (E, M, and S). [11] The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host.
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]