Biological properties of poly-L-lysine-DNA complexes generated by cooperative binding of the polycation

We have evaluated the effect of NaCl concentration on the mode of binding O f poly-L-lysine to DNA and the resulting structural and functional features of the condensed DNA particles using DNA precipitation, DNase I resistance , electron microscopy, and receptor-mediated gene transfer assays. At a hig h concentration of NaCl and in the presence of excess DNA, poly-L-lysine in teracted with DNA cooperatively, fully condensing some of the DNA and leavi ng the rest of the DNA unbound. At low NaCl concentrations, poly-L-lysine m olecules interacted with DNA in a noncooperative fashion, i.e. they bind ra ndomly to the whole population of DNA molecules. Cooperative binding Of pol y-L-lysine to DNA occurred over a narrow range of NaCl concentrations, and the specific salt concentration depended on the length of the poly-L-lysine . The ability of condensed DNA to withstand digestion by DNase I was correl ated with the structural features of the condensed DNA as determined by ele ctron microscopy. Using our condensation procedure, cooperative binding Of poly-L-lysine to DNA is a necessary prerequisite for the preparation of con densed DNA having a spherical shape and a diameter of 15-30 nm. Condensed D NA, containing galactosylated poly-L-lysine, was evaluated further for the extent and specificity of receptor-mediated gene transfer into HuH-7 human hepatoma cells via the asialoglycoprotein receptor. Efficient receptor-medi ated transfection. occurred only when condensed DNA complexes had a spheric al shape with a diameter of 15-30 nm; asialofetuin, a natural ligand for th e asialoglycoprotein receptor, inhibited this process by up to 90%. Our res ults support the importance of appropriate DNA condensation for the uptake and ultimate expression of DNA in hepatic cells.