The key difference between protomers and capsomeres is that protomers are structural units of oligomeric proteins while capsomeres are individual proteins composed of protomers, which are structural subunits of the viral capsid.
A virus is an intracellular obligate parasite, which is an infectious particle. The capsid is one of the two major components of a virus particle. It is a protein coat that surrounds and protects the viral genome. A capsid comprises of protomers, which are the structural units of oligomeric proteins. Capsomere is a structural subunit of the viral capsid, and it is an aggregation of several protomers as a unit. Thus, protomers self-assemble to form a capsomere and capsomeres self-assemble to form a capsid.
CONTENT
1. Overview and Key Difference
2. What are Protomers
3. What are Capsomeres
4. Similarities Between Protomers and Capsomeres
5. Side by Side Comparison – Protomers vs Capsomeres in Tabular Form
6. Summary
What are Protomers?
Protomers are the structural units of oligomeric proteins. Protomer contains at least two different polypeptide chains. More often, they contain several polypeptides. In viral capsids, protomers self-assemble to form capsomeres, which are the morphological units of capsids. Protomers connect with each other via specific bonds and self assemble.
Figure 01: Protomers
In the tobacco mosaic virus, protomers aggregate to form a two-layered disc to wrap the RNA genome. Thus, these protomers of tobacco mosaic virus finally form a long rigid hollow tube, which gives it a helical appearance. In icosahedral symmetry, protomers aggregate into units of five or six capsomeres and then condense into 20 faces of equilateral triangles and 20 apices.
What are Capsomeres?
Capsomeres are the structural protein subunits of the viral capsid. They are also the morphological subunits of viruses. Structurally, the capsid is an assemblage of capsomeres. Each capsomere has several protomers self-assembled with each other. Moreover, capsomeres are arranged differently in the capsid to provide a shape to the viral capsid. In this regard, helical, icosahedral and complex are the three types of capsomere arrangements in viruses. However, the arrangement of capsomeres is characteristic of a particular virus. Capsomeres join with each other via intercapsomeric triplexes comprised of two copies of one protein and one copy of another.
Figure 02: Capsomeres
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. Moreover, enterovirus has 60 capsomeres in its capsid. Likewise, different viruses have a different number of capsomeres in their protein shell.
Capsomeres fulfil several functions in viruses. Capsomeres protect the viral genome from physical, chemical and enzymatic damages. Moreover, capsomeres are important in introducing a viral genome into the host by adsorbing readily to host cell surfaces.
What are the Similarities Between Protomers and Capsomeres?
- Protomers and capsomeres are protein structures.
- Protomers are subunits of capsomeres while capsomeres are subunits of the viral capsid.
- Both protomers and capsomeres can self assemble.
What is the Difference Between Protomers and Capsomeres?
Protomers are the structural units that make oligomeric proteins while capsomeres are the morphological units of viral capsids. So, this is the key difference between protomers and capsomeres. Furthermore, protomers aggregate to form capsomeres while capsomeres aggregate to form capsid.
Below infographic summarizes the difference between protomers and capsomeres.
Summary – Protomers vs Capsomeres
Protomers are subunits of capsomeres while capsomeres are protein subunits of the viral capsid. Thus, capsomere is an aggregation of protomers while capsid is an aggregation of capsomeres. Arrangement of protomers or arrangement of capsomeres gives shape or symmetry to a particular virus. Based on capsomere arrangement, helical and icosahedral are two shapes or symmetries seen in viruses. So, this summarizes the difference between protomers and capsomeres.
Reference:
1.“Capsid.” Wikipedia, Wikimedia Foundation, 3 Sept. 2019, Available here.
Image Courtesy:
1. “Virus capsid T number” By Ranjan V. Mannige and Charles L. Brooks III – Mannige RV, Brooks CL III, 2010 “Periodic Table of Virus Capsids: Implications for Natural Selection and Design.” PLoS ONE 5(3): e9423. doi:10.1371/journal.pone.0009423 (CC BY 2.5) via Commons Wikimedia
2. “Adenovirus 3D schematic” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia