HIV Envelope Protein gp120 (gp120)

The HIV envelope protein gp120 is a glycoprotein found on the surface of the Human Immunodeficiency Virus (HIV) envelope. It plays a critical role in viral entry into host cells by mediating attachment to specific cell surface receptors. Gp120 binds primarily to CD4 receptors on helper T-cells but can also interact with DC-SIGN and heparan sulfate proteoglycans. This binding initiates conformational changes necessary for subsequent interaction with chemokine co-receptors (CCR5 or CXCR4), leading to membrane fusion mediated by gp41. Gp120 has an inner domain and an outer domain bridged by a four-stranded antiparallel β-sheet. Gp120 is heavily glycosylated with N-linked glycans that form a "glycan shield," protecting it from immune recognition. Three molecules each of gp120 associate noncovalently with three molecules each of transmembrane protein gp41 to form functional Env spikes on HIV's surface. Variable regions sterically hinder antibody access; conserved neutralizing epitopes are recessed or masked by glycans or variable loops. This structural plasticity allows evasion from broadly neutralizing antibodies despite being immunogenic itself.

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