SU or surface protein is also known as gp120 and is derived from gp160 (encoded in the env gene) along with TM (gp41). It is the fusion protein and therefore participate directly in virus entry. The SU protein binds to CD4 antigen with high affinity (Kd ~4nM) and the parts that bind to the receptor are mapped to four separate conserved regions of SU. As stated in the main notes, CD4 is not sufficient for infection of the cell and chemokine co-receptors are necessary. Binding of CD4 to SU causes changes in the conformation of SU that allow binding to the co-receptor that is required for entry of the virus. A variable region (V3 loop) determined viral tropism.

For further information see: Alan Frankel and John A. T. Young, HIV-1: Fifteen proteins and an RNA Annual Review of Biochemistry 67: 1-25, 1998