**Explanation:**

The protein coat of a virus is called the **capsid**. The capsid is a protective shell made up of protein subunits called capsomeres. It encloses the genetic material of the virus, which can be either DNA or RNA.

**Function of the Capsid:**

The main function of the capsid is to protect the viral genome from the external environment, including physical and chemical agents. It acts as a shield to prevent the degradation of the genetic material and helps in the transmission of the virus from one host to another.

**Structure of the Capsid:**

The capsid can have various shapes, including helical, icosahedral, or complex. The helical capsid is formed by protein subunits arranged in a spiral structure, while the icosahedral capsid is made up of triangular faces and forms a spherical shape. Complex capsids have a combination of both helical and icosahedral structures.

**Capsomeres:**

Capsomeres are the individual protein subunits that make up the capsid. They self-assemble to form the overall structure of the capsid. The arrangement of these capsomeres determines the shape of the capsid.

**Virus Envelope:**

In some viruses, an additional layer called the viral envelope may surround the capsid. The viral envelope is derived from the host cell's membrane and contains host cell proteins. It helps the virus to escape the host's immune system and facilitates the entry of the virus into host cells.

**Examples:**

- The human immunodeficiency virus (HIV) has an icosahedral capsid surrounded by a viral envelope.
- The tobacco mosaic virus (TMV) has a helical capsid.

In conclusion, the protein coat of a virus is called the capsid, which is made up of protein subunits called capsomeres. The capsid plays a crucial role in protecting the viral genome and facilitating the transmission of the virus.