FIG 4.

RBDs of type D gammaretroviruses, syncytin-1, and syncytin-Mar1. (A) First, amino-terminal β-sheet of the type D REV RBD. (B) Second, carboxy-terminal REV RBD β-sheet. (C) Syncytin-1 RBD. (D) Syncytin-Mar1 RBD. The β-sheets are shown in the same orientation in all panels with the three sequential β-strands indicated by numbers. Loops linking β-strands and flanking the β-sheet region are shown as dotted lines for clarity. Disulfides are shown as sticks. The amino and carboxy termini of each section are indicated.