Two models for the packaging of retroviral Env glycoproteins. (A) Passive packaging. Cellular proteins on the surface of the cell interact with specific proteins beneath the plasma membrane, and this interaction prevents them from being packaged into the emerging particle. In contrast, viral Env glycoproteins do not interact with cellular proteins and are free to move into the sites of budding. (B) Active packaging. A specific interaction between the cytoplasmic tail of Env and the MA sequence of Gag results in specific packaging of viral glycoproteins. (Adapted from Hunter 1994.)