FIGURE 2.

Identification of a minimal domain responsible for productive interaction of CDV F variants with MV H. A, quantification of cell-to-cell fusion activity of reciprocal chimeras between CDV F-ODP and Lederle upon co-transfection with MV H (black bars) or CDV H (gray bars) using the luciferase reporter-based fusion assay as outlined in Fig. 1B. The values were normalized for fusion activity observed upon co-transfection of cells with unmodified F-ODP and MV H or CDV H, respectively. The mean values ± S.D. of three independent experiments are shown, and the extent of differential triggering (based on the percentage of homotypic activity/percentage of heterotypic activity) is specified (see text for details). F constructs are schematized below the graph. The black boxes represent regions derived from F-Lederle, and the gray boxes represent F-ODP. The location of characteristic F protein domains, the site of the disulfide bridge linking the F₁ and F₂ subunits (black line), and the position of natural (BlpI, PvuII, EcoRV, and SmaI) and engineered (BglII and KpnI, in gray) restriction sites used for chimera generation are shown on the left. PS, N-terminal precursor sequence of CDV F; FP, fusion peptide; HR-A, N-terminal heptad repeat; HR-B, C-terminal heptad repeat; TM, transmembrane domain; CT, cytosolic tail. B, focused chimeras based on engineered restriction sites identify a domain in the N-terminal part of F-ODP (between restriction sites KpnI and EcoRV, construct XIII) to determine productive interaction of F-ODP with MV H. Quantification of fusion activity, calculation of differential triggering and color coding as described in A.