The lentiviral envelope--gp120
Lentiviral particles are bounded by a lipid bilayer derived from the host cell cytoplasmic membrane (green in Figure 1). Studding the lipid bilayer are virus-encoded envelope proteins (cyan and cherry), that are responsible for binding to cells and directing entry. The envelope spike is comprised of a trimer of two different proteins. One is gp120, which comprises the outer portion of the trimer (cyan; right). The inner portion is the gp41 protein, which is involved in membrane fusion.