FIG. 1.

Low-pH-dependent fusion of pyrene-labeled SIN with liposomes. Fusion was measured on-line at 37°C as a decrease of viral pyrene excimer fluorescence, as described in Materials and Methods. Final virus and liposome concentrations were 0.5 and 200 μM (membrane phospholipid), respectively. Liposomes consisted of PC/PE/SPM/Chol (molar ratio, 1:1:1:1.5). (A) Curves: a, pH 5.0; b, pH 5.75; c, pH 7.4; d, pH 5.0, without liposomes. (B) The initial rate of fusion (solid circles) was determined from the tangent to the first part of the curve. The extent of fusion (squares) was determined 60 s after acidification.