Fusion activity of pyrene-labeled WT and mutant RSPs with liposomes at acidic pH. (A) Kinetic fusion curves of RSP WT (red) and the mutant RSPs H323A (orange) and H248N-H287A (blue) at pH 5.4. (B) Extent of fusion after 60 s of mutant RSPs relative to that of the WT (set at 100%). The experiments with those mutants that were significantly different from the WT were performed at least twice and the error bars represent the standard errors of the means.