FIG. 2.

Low-pH-dependent fusion of SIN with pyrPC-labeled liposomes. Fusion was measured on-line at 37°C as a decrease of liposomal excimer fluorescence, as described in Materials and Methods. Fusion of pyrPC-labeled liposomes (2 μM liposomal phospholipid) with SIN (10 μM viral phospholipid) was measured at pH 5.0, unless indicated otherwise. Liposomes consisted of PC/pyrPC/PE/SPM/Chol (molar ratio, 0.85:0.15:1.0:1.0:1.5). (A) Curves: a, pH 5.0; b, pH 7.4; c, pH 5.0, but in the absence of SIN. (B) The final extent of excimer fluorescence decrease (squares) was determined, 60 s after acidification, as a function of the viral phospholipid concentration.