What is the Difference Between Capsid and Capsomere

The main difference between capsid and capsomere is that capsid is the protective protein coat of viruses, whereas capsomere is the smallest subunit of viral capsid.

Capsid and capsomere are two anatomical structures of the virus. Their function is to protect the viral genome.

Key Areas Covered

1. What is Capsid
– Definition, Structure, Function
2. What is Capsomere
– Definition, Structure, Function
3. Similarities Between Capsid and Capsomere
– Outline of Common Features
4. Difference Between Capsid and Capsomere
– Comparison of Key Differences

Key Terms 

Capsid, Capsomere

What is Capsid

Capsid is the protein shell that surrounds the genetic material of the virus. The proteins that make up the capsid are capsid proteins. The three-dimensional morphological subunits in the capsid are capsomeres. Protomers are the oligomeric, repeating structural subunits of the capsid. In addition, capsids occur in three shapes: icosahedral, helical, and prolate. Most viruses are icosahedral and helical in shape. But, some viruses, such as bacteriophages, have more complicated shapes. The capsid is coated with a lipid membrane to form the viral envelope.

Figure 1: Capsid

Furthermore, the main function of the viral capsid is to protect the inner contents of the virus. Generally, capsids protect the virus from extreme temperatures, pH differences, radiation, chemicals, and enzymes. During replication, the protein biosynthesis pathways of the host cell produce new viral capsid proteins.

What is Capsomere

Capsomere is a subunit of capsid. It self-assembles to form the capsid. Protomers aggregate to form capsomeres. The three types of capsomere arrangements are icosahedral, helical, and complex. 

Icosahedral 

An icosahedral capsid is a polyhedron that contains 12 vertices and 20 faces. Also, two types of capsomeres assemble in the icosahedral capsid. They are the pentagonal capsomeres at the vertices and the hexagonal capsomeres at the feces. Twelve pentagonal capsomeres always occur, and the number of hexagons in the capsid depends on virus groups. Capsomeres exhibit a regularly spaced ring with a central hole under the electron microscope.

Figure 2: Icosahedral Capsid

Helical

The capsomere forms a ribbon-like structure, folding into a helix, forming the helical capsid. Also, the characteristics of the protomers determine the diameter of the capsid, while the length of the nucleic acid determines the length of the helical capsid. 

Complex

The complex capsid occurs in two viral groups: poxvirus and rhabdovirus.

Moreover, the host’s cellular enzyme digests the proteins in the capsid and capsomeres, exposing the virus’s genetic material upon the host cell’s entrance. Besides forming the capsid, capsomeres protect the viral genome from chemical, physical, and enzymatic digestion. 

Similarities Between Capsid and Capsomere

• Capsid and capsomere are two structures of the virus.
• Their function is to protect the viral genome.
• They are made up of proteins. 

Difference Between Capsid and Capsomere

Definition

Capsid refers to the protein shell of a virus particle that surrounds its nucleic acid, while capsomere refers to a subunit of the capsid, an outer covering of protein that protects the genetic material of a virus.

Significance

Capsid is the protective protein coat of viruses, while the capsomere is the smallest subunit of the viral capsid.

Function

Capsid protects the viral genome, while capsomere forms the capsid.

Conclusion

In brief, the capsid and capsomere are two anatomical structures of the virus. They are made up of proteins. Capsid is the protective protein coat of the virus. Its function is to protect the viral genome. In comparison, the capsomere is a small subunit of the capsid. Its function is to form the capsid. Therefore, the main difference between capsid and capsomere is their structure and function. 

References:

1. Capsid. Capsid – an overview | ScienceDirect Topics. (n.d.). 
2. Capsomere. Capsomere – an overview | ScienceDirect Topics. (n.d.). 

Image Courtesy:

1. “CMVschema” By Emmanuel Boutet – Own work (CC-BY SA 2.5) via Commons Wikimedia
2. “Adenovirus 3D schematic” By Thomas Splettstoesser – Own Work (CC-BY SA4.0) via Commons Wikimedia