Capsid architecture: In the simplest icosahedral capsid, the capsomere located at each of the 12 vertices and surrounded by five neighbors is called what?

Introduction:
Many viruses adopt icosahedral symmetry to build robust capsids from repeating subunits. Understanding the naming of architectural elements such as pentons and hexons is crucial in structural virology.

Given Data / Assumptions:

• An icosahedron has 12 vertices.
• At each vertex lies a capsomere surrounded by five others.
• We must identify the correct term for this vertex capsomere.

Concept / Approach:
In icosahedral capsids, capsomeres at fivefold axes (vertices) are called pentons. Capsomeres at sixfold positions are termed hexons. This naming reflects local symmetry and packing rather than the entire particle’s symmetry number.

Step-by-Step Solution:
Recall that an icosahedron has 12 fivefold vertices. Associate fivefold positions with penton capsomeres. Select “penton” for the vertex capsomere surrounded by five neighbors.

Verification / Alternative check:
Classic adenovirus models and other icosahedral maps distinguish penton base (at vertices) from hexon facets; this convention is standard across textbooks.

Why Other Options Are Wrong:

• Polyhedra / Icosahedral unit: Generic terms, not the specific capsomere name.
• Helical turn: Applies to helical symmetry, not icosahedral.
• Portal complex: A specialized vertex in some tailed phages, not a general vertex capsomere term.

Common Pitfalls:
Confusing overall particle symmetry with local capsomere terminology; “penton” specifically denotes fivefold vertex units.

Final Answer:
The vertex capsomere is a penton.