Env (gene)
[2] The retroviral glycoproteins are oligomeric complexes that are composed of SU-TM heterodimers, which are made in the endoplasmic reticulum after the translation of the glycosylated Env precursor.
[8] Env can be modified by the addition of mannose-rich oligosaccharides, a process that takes place in the rough endoplasmic reticulum and is carried out by the enzymes of the host cell.
At the other end of the spectrum, MMTV (Mouse Mammary Tumor Virus has only 4 sites for oligosaccharide addition (two on gp52 and two on gp37).
The addition of oligosaccharides is believed to play a role in the proper folding of Env, presumably by stabilizing the protein structure.
[9] The env gene codes for the gp160 protein which forms a homotrimer, and is cleaved into gp120 and gp41 by the host cell protease, furin.
To form an active fusion protein, SU gp120 and TM gp41 polypeptides remain non-covalently bound together, but this interaction is often not stable, leading to shed, soluble gp120 and membrane-bound, gp41 'stumps'.
Prior to binding the host cell, gp120 remains effectively hidden from antibodies because it is buried in the protein and shielded by sugars.
It is originally buried within the viral envelope, but when gp120 binds to a CD4 receptor, gp120 changes its conformation causing gp41 to become exposed, where it can assist in fusion with the host cell.
[15][16] MMTV Env is of particular interest to researchers because of the discovery that it encodes an immunoreceptor tyrosine-based activation motif (ITAM) that has been shown to transform human and murine mammary cell in culture.
This ITAM depolarizes epithelial acinar structures, thereby changing the phenotype of the cells and causing them to become cancerous.
The envelope glycoprotein of subgroup A is called EnvA and its env gene codes for precursor protein known as Pr95.
[19] The retroviral protein env has been captured multiple times during mammalian evolution and is expressed in placental tissue, where it facilitates fusion of fetal and maternal cells.
