Effects of the endosomal lipid bis(monoacylglycero)phosphate on the thermotropic properties of DPPC: A 2H NMR and spin label EPR study

Bis(monoacylglycero)phosphate (BMP) is an endosomal lipid with a unique structure that is implicated in the formation of intraendosomal vesicular bodies. Here we have characterized the effects of dioleoyl-BMP (BMP18:1) at concentrations of 5, 10, 15 and 20 mol% on the thermotropic behavior of dipalmitoyl phosphatidylcholine (DPPC) vesicles, and compared them to those of equimolar concentrations of dioleoyl phosphatidylglycerol (DOPG), a structural isoform of BMP18:1. Because BMP is found in the acidic environments of the late endosome and intralysosomal vesicles, samples were prepared at pH 4.2 to mimic the pH of the lysosome. Both 2H NMR of perdeuterated DPPC and spin-labeled EPR with 16-doxyl phosphatidylcholine were utilized in these investigations. NMR and EPR results show that BMP18:1 induces a lowering in the main phase transition temperature of DPPC similar to that of DOPG. The EPR studies reveal that BMP18:1 induced more disorder in the Lβ phase when compared to equimolar concentrations of DOPG. Analysis from dePaked 2H NMR spectra in the Lα phase reveals that BMP18:1 induces less disorder than equal concentrations of DOPG. Additionally, the results demonstrate that BMP mixes with other phospholipids as a phospholipid and not as a detergent molecule as once speculated.