Polyamines as modulators of membrane fusion: aggregation and fusion of liposomes

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Abstract
The effect of the polyamines (spermine, spermidine and putrescine) on the aggregation and fusion of large (.apprx. 100 nm in diameter) unilamellar liposomes was studied in the presence of 100 mM NaCl, pH 7.4. Liposome fusion was monitored by the Tb/dipicolinic acid fluorescence assay for the intermixing of internal aqueous contents, and the release of contents was followed by carboxyfluorescein fluorescence. Spermine and spermidine at physiological concentrations aggregated liposomes composed of pure phosphatidylserine (PS) or phosphatidate (PA) and mixtures of PA with phosphatidylcholine (PC) but did not induce any fusion. However, liposomes composed of mixtures of acidic phospholipids, cholesterol and a high mole fraction of phosphatidylethanolamine could be induced to fuse by spermine and spermidine in the absence of divalent cations. Putrescine alone in the physiological concentration range was ineffective for both aggregation and fusion of these liposomes. Liposomes made of pure PC did not aggregate in the presence of polyamines. Addition of aggregating concentrations of spermine caused a drastic increase in the rate of Ca2+-induced fusion of PA liposomes and a large decrease in the threshold Ca2+ concentration required for fusion. This effect was less pronounced in the case of PS or PA/PC vesicles. Preincubation of PA vesicles with spermine before the addition of Ca2+ resulted in a 30-fold increase in the initial rage of fusion. Polyamines may be involved in the regulation of membrane fusion phenomena accompanying cell growth, cell division, exocytosis and fertilization.