The induction of large unilamellar vesicle fusion by cationic polypeptides: the effects of mannitol,size, charge density and hydrophobicity of the cationic polypeptides

Polylysine and lysine-based copolymers induced fusion of large unilamellar vesicles only in media containing at least 0.4 M mannitol. In the absence of mannitol, polylysine and certain lysine-based copolymers also containing acidic amino acids were not able to induce fusion. Fusion, in the presence of mannitol, was induced at nanomolar concentrations of the polycations. Excess polymer caused reduced rate and extent of fusion. In the presence of 100 mM NaCl the effective concentration range of the polycations was narrower. Kinetic analysis determined that salt increased the aggregation constant C₁₁ while reducing the fusion constant f₁₁. Addition of polylysine in excess resulted in smaller C₁₁. Short polylysine (3500) was less effective on a molar basis than a long one (37 000). Copolymers were more effective than polylysine due to higher aggregation potential. Copolymers were also more effective in promoting Ca²⁺-induced fusion in the absence of mannitol, their greater efficiency resulting from substantially larger fusion potential, without a greater rate of leakage. Preincubation of the vesicles with the polycations for less than 20 s resulted in faster fusion rates, while longer preincubations caused slower fusion rates. Addition of polycations to the preincubated mixture enhanced the fusion rates, indicating that the polycations were not available, rather than the vesicles being not susceptible to fusion. The effect of preincubation suggests two phases in the binding of the polycations to the vesicles; a fast phase of partial binding and a slower phase resulting in complete binding. The addition of millimolar concentrations of pyrophosphate or sulphate provided a fine control of the effective polycation concentration and its effect on fusion.