The First Contact: Receptor Binding and Tropism | AI Learning Guide

The key distinction here is between molecules that merely hold the virus and molecules that actually open the door. Attachment factors like heparan sulfate or sialic acid are everywhere on the cell surface; they concentrate virions but do not commit the virus to entry. The entry receptor is the one whose engagement triggers fusion or penetration. HIV-1 shows why this matters: CD4 is the primary receptor, but a co-receptor, CCR5 or CXCR4, must also be present. A cell with CD4 but no co-receptor binds the virus and stops there. That is the molecular reason tropism is narrower than receptor expression alone would suggest, and the same logic explains why a virus adapted to one species often cannot enter another.

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Attachment factors versus entry receptors

Attachment factor

Abundant surface molecule (e.g., heparan sulfate, sialic acid)
Concentrates virions on the membrane
Binding alone does not trigger entry
Removing it reduces efficiency but may not abolish infection

Entry receptor

Specific surface molecule engaged by the viral attachment protein
Engagement triggers the entry step (fusion or penetration)
Its presence is required for productive infection
Its tissue distribution largely defines tropism

HIV-1: receptor plus co-receptor

CD4 is the primary receptor, but binding CD4 is not enough. A co-receptor — CCR5 or CXCR4 — must also be engaged. Cells that carry CD4 but neither co-receptor bind virions without fusing, so they are not productively infected. This is why tropism is narrower than CD4 expression alone would predict.

Why host range can shift

The receptor-binding domain of the viral attachment protein is under strong selection. A few substitutions can let it recognize an orthologous receptor from another species, widening host range. Receptor compatibility is the first filter; intracellular permissiveness is the second.

References

[1]Molecular Biology of the Cell — Virus Entry and Receptorsncbi.nlm.nih.gov