Aggregation and calcium-induced fusion of phosphatidylcholine vesicle-tubulin complexes.

Insertion of tubulin into the bilayer of dipalmitoyl phosphatidylcholine vesicles at the phase transition results in the formation of stable vesicle-tubulin complexes (Klausner, R. D., Kumar, N., Weinstein, J. N., Blumenthal, R., and Flavin, M. (1981) J. Biol. Chem. 256, 5879-5885). These complexes aggregated when maintained below phase transition for 10-20 min. Addition of millimolar concentrations of Ca2+, Mn2+, Zn2+, and Co2+, but not Mg2+, caused the vesicle-tubulin complexes to fuse into larger structures as shown by (a) electron microscopy, (b) increased trapped volume, and (c) changes ion resonance energy transfer between two fluorescent lipid probes incorporated into the same vesicle. There was no loss of internal aqueous contents from the vesicle-tubulin complexes during Ca2+-induced fusion. Anti-tubulin drugs had no effect on the aggregation or fusion, and vesicle-bound tubulin did not associate with microtubules when tubulin was assembled in vitro. Trypsin-treated vesicle-tubulin complexes were incapable of supporting Ca2+-induced fusion. This system provides a model for Ca2+-induced and protein-mediated nonleaky fusion of uncharged lipid bilayers.