PARAMETERS AFFECTING FUSION BETWEEN LIPOSOMES AND SYNAPTOSOMES - ROLEOF PROTEINS, LIPID-PEROXIDATION, PH AND TEMPERATURE

We investigated the effect of several parameters, such as temperature, pH and proteins, on the fusion between synaptosomes, freshly isolated from rat brain cortex, and large unilamellar phosphatidylserine lipos omes. These studies were carried out in both peroxidized and nonperoxi dized synaptosomes. Mixing of membrane lipids was monitored using a fl uorescence resonance energy transfer assay. Ascorbate (0.8 mM)/Fe2+ (2 .5 mu M)-induced peroxidation of synaptosomes enhanced the fusion proc ess (twofold) which may reflect an increase in synaptosomal protein hy drophobicity and hence a facilitation of intermembrane aggregation. Th e fusion process was shown to be temperature sensitive, a reduction in the extent being observed (twofold) as the temperature was towered fr om 37 to 25 degrees C. This effect may be due to changes in membrane f luidity. The fusion process is pH dependent, an increase in both kinet ics and extent being observed when the pH was lowered from 7.4 to 5.5. A significant inhibition (92% at pH 7.4; 35% at pH 5.5) of the intera ction between synaptosomes and liposomes by trypsin pretreatment of sy naptosomes was found, thus indicating that the fusion reaction is a pr otein-mediated process. The inhibitory effect of trypsin at pH 5.5 is not so strong as that at physiological pH. These results suggest that, in addition to the involvement of proteins, nonspecific interactions between the synaptosomal and liposomal membranes under acidic conditio ns may also play a role in the fusion process. The investigation of bi nding of synaptosomes to liposomes under several experimental conditio ns provided evidence for the participation of proteins in membrane agg regation, as well as for the role of electrostatic forces in this proc ess, at mild acidic pH.