The key difference between capsid and capsomere is that capsid is the protein coat that surrounds and protects the viral genome while capsomere is the structural subunit of a viral capsid and aggregation of several protomers as a unit.
Viruses are intracellular parasites. They all are infectious and cause different types of diseases to plants, animals, protists, bacteria and fungi. There are two main components that make a virus. They are the protein shell and the nucleic acid genome. Protein shell, also known as the capsid, is made up of proteins. The main function of the capsid is to protect the viral genome and aid in the infection process. A capsid consists of capsomeres, which are the structural and morphological subunits of the viral capsid. Capsomeres self assemble nicely and give shape to the viral capsid in each virus particle. Structurally, capsomere is an aggregation of several protomers as a unit.
CONTENT
1. Overview and Key Difference
2. What is Capsid
3. What is Capsomere
4. Similarities Between Capsid and Capsomere
5. Side by Side Comparison – Capsid vs Capsomere in Tabular Form
6. Summary
What is Capsid?
The capsid is one of the two major components in a virus. It is the protein coat that surrounds the viral genome. A capsid consists of several oligomeric structural subunits of proteins called protomers. Several protomers (5 to 6) collectively make individual protein subunits called capsomeres. Capsomeres are organized in a precise and highly repetitive pattern around the nucleic acid. These capsomeres are the smallest morphological units of a capsid. They are only visible under the electron microscope. A single virion has a large number of capsomeres.
Protein capsid can be arranged in different shapes. There are three basic shapes as helical, icosahedral or polyhedral and complex arrangement. The majority of viruses have helical or icosahedral capsid structures. Some viruses, especially bacteria infecting viruses (bacteriophages), have complicated capsid structures. Capsomeres are organized in a spiral manner in helical viruses. In icosahedral viruses, capsomeres are arranged in 20 equilateral triangular faces.
Figure 01: Capsid
Protein capsid carries out several functions. It mainly protects the genetic material of the virus particle. It also helps in transferring virus particles between host organisms. Moreover, capsid aids in specificity and viral infectivity since it has spikes. Spikes are glycoprotein protrusions which can bind with certain receptors on the host cell.
What is Capsomere?
Capsomeres are the structural protein subunits of the viral capsid. In fact, they are the morphological subunits of the viral capsid. Structurally, a capsid is an assemblage of capsomeres. Each capsomere has several protomers self-assembled with each other. Moreover, capsomeres are arranged differently in the capsid in order to provide a shape to the viral capsid. Helical, icosahedral and complex are three types of capsomere arrangements in viruses. However, the arrangement of capsomeres is unique for a particular virus.
Figure 02: Capsomere
Capsomeres join each other via intercapsomeric triplexes comprised of two copies of one protein and one copy of another. Furthermore, each virus has a finite number of capsomeres. Hepatitis B virus has an icosahedral capsid containing 180 capsomeres. Recombinant adenovirus has a capsid containing 252 capsomeres. Herpesviruses have 162 capsomeres in their capsids. Enterovirus has 60 capsomeres in its capsid. Likewise, different viruses have different numbers of capsomeres in their protein shell.
Capsomeres fulfil several functions in viruses. They protect the viral genome from physical, chemical and enzymatic damages. Moreover, capsomeres are important in introducing a viral genome into hosts by adsorbing readily into the host cell surfaces.
What are the Similarities Between Capsid and Capsomere?
- Capsid and capsomeres are found in viruses.
- Both are made from proteins.
- Capsomeres are the morphological units of capsids.
- In fact, they are the structural subunits of the capsid.
- Both structures are responsible for giving a shape to the virus.
- Arrangement of both structures is unique to each viral particle.
What is the Difference Between Capsid and Capsomere?
The capsid is the protein coat that surrounds the nucleic acid genome of the virus particle. In contrast, capsomere is the basic morphological subunit of the viral capsid. So, this is the key difference between capsid and capsomere. Moreover, capsomeres self assemble to form the capsid. Meanwhile, protomers self assemble to form a capsomere. Also, a virus has only one capsid, but many capsomeres in its capsid.
Furthermore, the main function of the capsid is to protect the viral genome, but the main function of the capsomere is to make the capsid. Therefore, this is another major difference between capsid and capsomere.
Summary – Capsid vs Capsomere
A capsid is one of the two parts of a virus. It is the protein coat that surrounds the viral genome. But, a viral capsid is made from capsomeres, which are the individual proteins composed of protomers. Thus, capsomeres are the structural subunits of the viral capsid. Arrangement of capsomeres gives shape or symmetry to a particular virus. So, this summarizes the difference between capsid and capsomere.
Reference:
1. Roos, W H, et al. “Viral Capsids: Mechanical Characteristics, Genome Packaging and Delivery Mechanisms.” Cellular and Molecular Life Sciences: CMLS, Birkhäuser-Verlag, June 2007, Available here.
2. “Boundless Microbiology.” Lumen, Available here.
Image Courtesy:
1. “Helical capsid with RNA” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “P24 HIV-capsid” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia