Friday, December 06, 2019

Protein coat of virus

Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.

A fully assembled infectious virus is called a virion. The simplest virions consist of two basic components: nucleic acid (single- or double-stranded RNA or DNA) and a protein coat, the capsid.

All viruses have capsids that enclose and protect their nucleic acid. Each capsid is constructed from identical subunits called capsomers made of protein. The capsid together with the nucleic acid are nucleoscapsid.

The nucleic acid of a virion is enclosed within a protein coat, or capsid, composed of multiple copies of one protein or a few different proteins, each of which is encoded by a single viral gene. Because of this structure, a virus is able to encode all the information for making a relatively large capsid in a small number of genes. The capsid together with the nucleic acid are nucleoscapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins.

Capsids are formed as single or double protein shells and consist of only one or a few structural protein species. Therefore, multiple protein copies must self assemble to form the continuous three-dimensional capsid structure.

What are the functions of capsids or protein coat?
*Protect genome from atmosphere (May include damaging UV-light, shearing forces, nucleases either leaked or secreted by cells).
*Virus-attachment protein-interacts with cellular receptor to initiate infection.
*Delivery of genome in infectious form. May simply “dump” genome into cytoplasm (most +ssRNA viruses) or serve as the core for replication (retroviruses and rotaviruses).
Protein coat of virus
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