Immunoglobulin (Ig) fold architecture: Which description best characterizes the Ig fold found in immunoglobulins and many Ig-superfamily proteins?

Introduction / Context:
The immunoglobulin fold is a widely used structural motif across immune receptors and cell-adhesion molecules. Recognizing its β-sandwich architecture helps connect sequence domains to 3D structure and function.

Given Data / Assumptions:

• Ig domains exist in heavy and light chains and many non-Ig proteins.
• Typical domain: two antiparallel β-sheets forming a β-sandwich (β-barrel-like).
• Strand counts commonly described as 3+4.

Concept / Approach:
Each variable (V) or constant (C) domain adopts the Ig fold. The motif is not exclusive to IgG and appears throughout the Ig superfamily (for example, TCR, MHC-related, cell adhesion molecules). An IgG molecule contains multiple Ig domains (more than six), so fixed “six only” is incorrect.

Step-by-Step Solution:
Identify architectural hallmark: two antiparallel β-sheets.Note common strand count: one sheet ~4 strands, the other ~3.Exclude exclusivity to IgG and incorrect copy numbers.

Verification / Alternative check:
Structural databases and textbook schematics depict the conserved 3+4 β-sandwich with intradomain disulfide bridging.

Why Other Options Are Wrong:

• Only in IgG / α-helical bundle: Contradicts known β-sheet nature and distribution.
• Exactly six times in IgG: IgG typically has 12 Ig domains (8 heavy, 4 light).

Common Pitfalls:
Confusing “β-barrel” terminology with porins; here it refers to the β-sandwich architecture of Ig domains.

Final Answer:
A β-barrel composed of two antiparallel β-sheets (typically one with three and one with four strands).