Fusion and fragmentation of phospholipid vesicles by apohemoglobin at low pH

Human apohemoglobin in acidic media was found to induce fusion of phosphatidylcholine/phosphatidylserine (1:1) vesicles at low protein concentration but to fragment the same vesicles to form micellar complex at high protein concentration. The fusion was demonstrated by size increase, vesicle content mixing, lipid mixing, and electron microscopy. The micellization of phospholipid vesicles was observed by light scattering, gel filtration, and electron microscopy. The hydrophobic labeling of the apohemoglobin/vesicle complex followed by CNBr cleavage of apohemoglobin showed that an N-terminal segment of the beta subunit with a molecular weight of approximately 6,000 seems to be mainly involved in the fusion process, but the whole sequences of both alpha and beta chains participate in the micellization process.