Which glycoprotein of HIV binds first to the CD4 receptor on host cells?

AIIMS CRE 2025

Applied Microbiology & Infection control

Easy

Appeared in: AIIMS CRE 2025

Explanation

gp120 is the surface glycoprotein located on the outer envelope of the HIV virion.
Its primary function is to recognize and bind to the CD4 receptor on the surface of target host cells, such as T-helper lymphocytes.
This binding event is the critical first step in the process of viral entry, preceding interaction with co-receptors and membrane fusion.

Option A: gp41 is a transmembrane protein responsible for the fusion of the viral and host cell membranes, which is a later step in the entry process.
Option C: p24 is an internal capsid protein that forms the core of the virus and encloses the viral RNA. It is not located on the viral surface and does not interact with host cell receptors.
Option D: p17 is an internal matrix protein that provides structural support to the virion, located beneath the viral envelope. It is not involved in receptor binding.

Visual 1: Diagram - A labeled diagram of the HIV virion structure, clearly showing the external position of gp120, the transmembrane position of gp41, and the internal locations of the p24 capsid and p17 matrix.
Visual 2: Flowchart - A step-by-step flowchart illustrating the HIV entry process: 1) gp120 binds to CD4, 2) gp120 binds to a co-receptor (CCR5/CXCR4), 3) gp41 mediates membrane fusion, 4) Viral core enters the cell.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain HIV Viral Entry Mechanism as background academic context rather than a clinical decision trigger.
Understanding that gp120 is the initial point of contact is fundamental to comprehending HIV's tropism for CD4+ T-cells and the resulting immunodeficiency.
This specific molecular interaction is a key target for antiretroviral therapy. Entry inhibitors are a class of drugs designed to block this process, for example, by preventing gp120 from binding to its receptors.
What if? If a person has a genetic mutation in the CCR5 co-receptor (e.g., CCR5-delta 32 deletion), some strains of HIV cannot complete the entry process after binding to CD4, rendering the individual highly resistant to infection by those viral strains.

How to Approach the Question

First, identify the core of the question: it asks for the specific HIV protein that first binds to the CD4 receptor.
Recognize the keywords 'glycoprotein' and 'HIV'. Glycoproteins (gp) are typically found on the outer surface of a virus and are involved in attachment.
Differentiate between the 'gp' (glycoprotein) and 'p' (protein) prefixes. 'gp' proteins are on the envelope, while 'p' proteins (like p24 and p17) are typically internal (capsid/matrix), so you can likely eliminate options C and D.
Between the two glycoproteins, gp120 and gp41, recall their roles. One is for docking (attachment) and the other is for fusion. The question asks for the first binding event, which is attachment.
Conclude that gp120 is the docking protein that binds first, while gp41 is the fusion protein that acts subsequently.

Concept Tested & Keywords

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