Which HIV protein mediates fusion and entry into host cells?

• A. P24
• B. gp41
• C. gp120
• D. Reverse Transcriptase

Answer: (B)

HIV entry into cells is driven by the envelope glycoprotein complex. gp120 binds the CD4 receptor and a coreceptor (CCR5 or CXCR4) on the host cell, which triggers changes that activate gp41. The gp41 subunit then inserts a fusion peptide into the host membrane and refolds to pull the viral and cellular membranes together, creating a fusion pore for the viral genome to enter. The other proteins have different roles: P24 is a capsid protein inside the virion, and Reverse Transcriptase is the enzyme that converts viral RNA into DNA after entry, not involved in the fusion process. Therefore, gp41 is the mediator of fusion and entry.