Binding and entry: gp120
From The HIV replication cycle a web-based interactive account
The HIV replication cycle begins with binding of a viral particle to a susceptible cell. The HIV gp120 on the viral particle binds to cellular CD4, explaining the tropism of HIV for CD4+ cells.
For review see: 
- Wilen CB, Tilton JC, Doms RW. 2012. HIV: Cell Binding and Entry. in HIV From Biology to Prevention and Treatment, pp. 23 Cold Spring Harbor Laboratory Press, Cold Spring Harbor.