Membrane curvature effects on glycolipid transfer protein activity

The glycolipid transfer protein (GLTP) is monomeric in aqueous solutions, and it binds weakly to membrane interfaces with or without glycolipids. GLTP is a surface-active protein and adsorbs to exert a maximal surface pressure value of 19 mN/m. The change in surface pressure following GLTP adsorption decreased linearly with initial surface pressure. The exclusion pressure for different phospholipids and sphingolipids was between 23 and 31 mN/m, being clearly highest for the negatively charged dipalmitoyl-phosphatidylserine. This can be explained by electrostatic forces when GLTP is positively charged at neutral pH (isoelectric point = 9.0) and by phosphatidylserine being negatively charged. If GLTP is injected under a palmitoyl-galactosylceramide monolayer above 30 mN/m, the presence of GLTP leads to a decrease in the surface pressure as a function of time. This suggests that GLTP is able to remove glycolipids from the monolayer without penetrating the monolayer. On the other hand, if phospholipid vesicles with or without glycolipids are also present in the subphase, no change in the surface pressure takes place. This suggests that GLTP in the presence of curved membranes is not able to transfer from or to planar membranes. We also show that transfer of fluorescently labeled galactosylceramide is faster from small highly curved palmitoyl-oleoyl-phosphatidylcholine and dipalmitoyl-phosphatidylcholine bilayer vesicles but not from palmitoyl-sphingomyelin vesicles regardless of the size.