Fig. 1.

Complexin isoforms differentially stimulate/inhibit membrane fusion. (A) Effect of complexin isoforms on liposome fusion kinetics. v-SNARE (VAMP2) liposomes labeled with rhodamine and NBD lipids were incubated in the absence or presence of the indicated complexin isoforms with unlabeled t-SNARE (Syx1/SNAP-25) liposomes for 1 h on ice. With the exception of Drosophila complexin (dm Cpx), all other complexin isoforms were human orthologues (CpxI–CpxIV). After warming up of the reaction to 37 °C, the increase in NBD fluorescence was monitored for 30 min at 37 °C. The results were normalized to the maximum NBD fluorescence signal after detergent lysis as described in Material and Methods. (B) Dose-dependent stimulation of fusion by CpxI. Constant amounts of v- and t-SNARE liposomes were incubated with increasing amounts of CpxI, and the fusion was monitored and analyzed as described in A. The gain of total NBD fluorescence 1 min after warming up to 37 °C was plotted against the CpxI amount.