capsid

A small to medium-sized bug that typically feeds on plants; occasionally predatory; c.10 000 species, distributed worldwide. (Order: Heteroptera. Family: Miridae.)

A capsid is the outer shell of a virus. The capsid serves three main purposes :

It protects the genetic material of the virus. It determines if a cell is suitable for infection. It starts the actual infection by attaching and "opening" the target cell and injecting the genetic material of the virus into the cell.

Once the virus has infected the cell, it will sooner or later start replicating itself, using the "infrastructure" of the infected cell. During this process, the capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell. Some viruses will also take a portion of the host cell's cell membrane with them when they depart, enclosing the proteinaceous capsid with viral proteins projecting through it. helical capsids, icosahedral (isometric) capsids, or enveloped. In helical symmetry, the protein subunits are arranged around the circumference of a circle to form a disk.

Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into families.