envelope glycoprotein GP120
gp120 is a glycoprotein exposed on the surface of the HIV envelope. The 120 in its name comes from its molecular weight of 120 kilodaltons.
The structure of gp120 involves an outer domain, an inner domain and a bridging sheet. The gp120 gene is around 1.5Kb long coding for around 500 amino acids. gp120 forms a sort of cap over the end of gp41 to make a gp120/gp41 subunit. This cap prevents the human immune response from recognising the virus via binding to gp41. However, when the virus needs to bind to a cell, the gp120 can change confirmation very quickly, exposing gp41.
The Human Immunodeficiency Virus (HIV) can mutate frequently to stay ahead of the immune system. There is however a highly conserved region in the virus genome that codes for structures to allow the virus to bind to a human cell and enter. The glycoprotein gp120 is anchored to the viral membrane through non-covalent bonds along with gp41, both coming from a cleaved protein, gp160. It infects any target cell with a CD4 receptor, particularly the helper T-cell, by binding to that receptor. Binding to CD4 is mainly electrostatic although there are van der Waals interactions and hydrogen bonds.
The exact mechanism of virus entry into a cell is unknown although the gp120 protein is thought to have at least responsibilities. It seeks out viable receptors on cells for virus entry, fixes the virus to the receptor on the cell and helps in passing the viral genome into the cell.
gp120 vaccines
Since CD4 receptor binding is the most obvious step in HIV infection, gp120 was among the first targets of HIV vaccine research. These efforts have been hampered by the chemical and structural properties of gp120, which make it difficult for antibodies to bind to it; also, it can easily be shed from the virus' surface and captured by T-cells due to its loose binding with gp41. This has meant interest in using gp120 based HIV vaccine has been lost.