EVIDENCE FOR TERNARY COMPLEX-FORMATION BY HISTONE H1, DNA, AND LIPOSOMES

Using three different donor-acceptor pairs for resonance energy transfer, interactions in systems composed of histone H1, liposomes, and DNA were investigated. While weak attachment of H1 to phosphatidylcholine (PC) liposomes was observed, the inclusion of phosphatidylserine (PS), phosphatidylglycerol (PG), or phosphatidic acid (PA) strongly enhanced the membrane association of H1, the extent of binding increasing with the content of the acidic lipid. Increasing the content of the negatively charged lipid also made the membrane attachment of H1 less susceptible to dissociation by NaCl, thus indicating, in keeping with our previous studies, that protonation of the acidic lipid is an important factor. Whereas DNA binds to sphingosine-containing cationic liposomes, these vesicles did not bind H1. Instead, H1 effectively competed with sphingosine for binding with DNA. In systems comprising DNA, liposomes, and H1, the interactions were clearly dependent on the liposome composition. While moderately acidic liposomes (PS content <30 mol %) seemed to form ternary complexes with DNA and I-Il, strongly acidic liposomes (PS content >30 mol %) competed with DNA for binding H1, partly removing the histone from the nucleic acid. The tendency to form ternary complexes also seemed to depend on the type of the acidic lipid. Possible physiological consequences of the interactions detected in these simple model systems are discussed.