Organization of Env proteins. Schematic representations of Env proteins are drawn for examples from each retroviral genus. The sequences representing the mature proteins SU and TM are indicated, along with the alternative older naming of these proteins based on their approximate molecular weights. Vertical lines, separating the leader peptide (L) and SU and separating SU and TM, mark cleavage sites for cellular proteases. Vertical lines mark cleavage sites for the viral protease near the carboxyl terminus of TM in MLV and M-PMV. The hydrophobic fusion domain of TM is shown in color and the membrane anchor is shown in gray. The Ys indicate sites of predicted N-linked carbohydrate addition.