HIV-1 envelope glycoproteins: Which glycoprotein serves as the external ‘‘spike’’ antigen of HIV-1 that binds to CD4 and co-receptors on host cells?

Introduction / Context:
HIV-1 entry into host cells depends on envelope glycoproteins that recognize and attach to CD4 and co-receptors (CCR5 or CXCR4). Knowing which protein lies on the virion surface helps interpret diagnostic assays and vaccine targets.

Given Data / Assumptions:

• HIV-1 envelope is synthesized as gp160, then cleaved into gp120 (surface) and gp41 (transmembrane).
• ‘‘Spike’’ typically refers to the exterior ligand that engages host receptors.
• We must identify the specific glycoprotein acting as the spike antigen.

Concept / Approach:
gp120 is the external surface glycoprotein that binds CD4 first and then a chemokine co-receptor. gp41 is the transmembrane fusion subunit. p24 is an internal capsid antigen and not an envelope spike.

Step-by-Step Solution:
Recall gp160 cleavage → gp120 (surface), gp41 (transmembrane).Match ‘‘spike’’ and receptor binding to gp120.Exclude gp41 (fusion), gp36/gp140 (not standard HIV-1 envelope naming), and p24 (internal).Select ‘‘gp 120’’.

Verification / Alternative check:
Assays measuring anti-gp120 antibodies or neutralization titers focus on the receptor-binding surface moiety, confirming its role as the spike.

Why Other Options Are Wrong:

• gp41: Anchors the complex and mediates fusion, not primary receptor binding.
• p24: Core capsid protein; not a spike.
• gp36/gp140: Not the canonical HIV-1 naming used for the receptor-binding spike.

Common Pitfalls:
Confusing the binding and fusion subunits; assuming p24 is external because it is commonly detected in labs.

Final Answer:
gp 120.