Reconstitution of proteolipid protein: some properties and its role in interlamellar attachment

Proteolipid apoprotein (PLP) isolated from human brain was reconstituted in dioleoylphosphatidylcholine vesicles by dialysis from 2-chloroethanol, using a dialysis buffer of pH 5.0. Under these conditions, and in contrast with dialysis carried out at pH 7.4, well-defined unilamellar vesicles containing the protein were formed. As judged by electron microscopy and quasi-elastic light scattering, the size of the vesicles was determined by the initial protein/lipid ratio used for reconstitution. When the vesicles were incubated in a buffer at neutral pH, aggregation of the vesicles was observed, but their structure remained intact. Asymmetric aggregation occurred when the reconstituted vesicles were incubated with large unilamellar vesicles (LUVs) devoid of protein. This aggregation was accompanied by loss of membrane integrity, as revealed by extensive leakage of the LUVs, and by membrane lipid dilution, indicative of the occurrence of membrane fusion. Destabilization of the vesicles depended on the presence of negatively charged phosphatidylserine in the membrane of the LUVs. Similar effects, but to a lesser extent, were seen when the LUVs contained sulphatide, a negatively charged lipid prominently present in myelin. DM 20, a natural mutant of PLP, appeared to be far less potent in causing membrane lipid dilution than PLP. This could suggest that a distinct protein sequence of PLP, which is absent from DM 20, may be involved in triggering the observed membrane destabilization. Temperature-dependent experiments indicate that this sequence in PLP displays dynamic properties, its exposure being affected by conformational criteria. Exposure of this particular domain, in conjunction with its affinity for negatively charged lipid, could be related to a perturbation of the integrity of the myelin sheath, as will be discussed.