Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
As you can see from the diagram, gp41 is the other important component of the spikes.
gp120's gene is around 1500 nucleotides long - and since each amino acid in a protein is encoded by 3 nucleotides in DNA, this means gp120 is made of around 500 amino acid residues. The CD4 binding site of gp120 - the specific region of the molecule which attaches to CD4 via intermolecular attractions - has been found to include the amino acid residues numbered 400-430.
One half of the molecular weight of gp120 is due to the carbohydrate side chains (the "glyco-" in "glycoprotein"). These are sugar residues which form something almost like a sugar "dome" over the gp120 spikes. This dome prevents gp120 from being recognised by the human immune response. As the HIV virus and the human CD4 cell come together, the gp120 binding site "snaps open" at the last minute.