Virus architecture vocabulary: The capsomeres of an icosahedral capsid are themselves built from repeating protein subunits called what?

Introduction:
Virus particles are highly ordered assemblies. Understanding the hierarchy of structural units—from proteins to capsomeres to the full capsid—is fundamental in virology, structural biology, and vaccine design. This question tests the correct term for the basic protein subunits that polymerize to form capsomeres in many icosahedral viruses.

Given Data / Assumptions:

• Capsid: the protective protein shell around the viral genome.
• Capsomere: a morphological unit visible by electron microscopy that builds the capsid, often arranged with icosahedral symmetry.
• We seek the name of the protein subunits that assemble into each capsomere.

Concept / Approach:
The standard terminology in classical virology is: protomer (basic structural protein subunit) → several protomers associate to form a capsomere → capsomeres assemble into the capsid. While terms like procapsid and scaffolding proteins exist, they refer to intermediates or helper factors during assembly, not the basic repeating subunit of the capsomere itself.

Step-by-Step Solution:
Step 1: Recall the hierarchy: protomer → capsomere → capsid.Step 2: Distinguish from “procapsid,” a precursor shell that later matures, often aided by scaffolding proteins.Step 3: Exclude invented or incorrect terms such as “caproproteins” or vague phrases that lack acceptance in virology texts.Step 4: Identify “protomers” as the correct unit building capsomeres.

Verification / Alternative check:
Standard microbiology and virology references define protomers as the fundamental polypeptide units forming capsomeres. Electron microscopy and cryo-EM studies support this modular assembly model for many icosahedral viruses.

Why Other Options Are Wrong:

• Caproproteins: non-standard term; not used in virology literature.
• Procapsid scaffolds: refer to assembly intermediates or helper proteins, not the basic repeating subunits.
• None of these: incorrect because “protomers” is correct.
• Peptidomer units: fabricated term without consensus usage.

Common Pitfalls:
Confusing morphological units (capsomeres) with molecular subunits (protomers); assuming “procapsid” is a subunit rather than a pre-assembled shell.

Final Answer:
Protomers.