The morphology and size of hydrated lipid dispersions of bis(monoacylglycero)phosphate (BMP) mixed with varying mole percentages of the ganglioside GM1 were investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM) Electron paramagnetic resonance (EPR) spectroscopy of these same mixtures, doped at 05 mol% with doxyl labeled lipids, was used to investigate acyl-chain packing Results show that for 20-30% GM1, hydrated BMP-GM1 mixtures spontaneously form small spherical vesicles with diameters 100 nm and a narrow size distribution profile For other concentrations of GM1, hydrated dispersions with BMP have non-spherical shapes and heterogeneous size profiles, with average vesicle diameters >400 nm All samples were prepared at pH 5 5 to mimic the lumen acidity of the late endosome where BMP is an essential component of intraendosomal vesicle budding, lipid sorting and trafficking. These findings indicate that GM1 and BMP under a limited concentration range spontaneously form small vesicles of homogeneous size in an energy Independent manner without the need of protein templating Because BMP is essential for intraendosomal vesicle formation, these results imply that lipid-lipid interactions may play a critical role in the endosomal process of lipid sorting and trafficking (C) 2010 Elsevier Inc All rights reserved