Penetration of a GPI-anchored protein into phospholipid monolayers spread at the air/water interface

Abstract

Mammalian alkaline phosphatases (AP) belong to glycosylphosphatidyl inositol (GPI) anchored proteins family, which are localised and clustered on the outer layer of the plasma membranes forming microdomains. Using Langmuir film and polarisation modulation infrared reflection absorption spectroscopy (PMIRRAS) techniques, the penetration process of the protein into a phospholipid monolayer have been studied at the air–buffer interface. The penetration of AP-GPI in distearoylphosphatidylcholine monolayers (DSPC) induces a more important surface pressure increase than in dioleoylphosphatidylcholine (DOPC) monolayer. However, the exclusion surface pressure rather similar for both lipids, 20.5 and 22 mN m −1 for, respectively, DSPC and DOPC, indicates that the AP-GPI cannot, in similar conditions, insert by itself into bilayer membranes of either biological or mimetic origin. PMIRRAS suggests that the pure acyl chains perdeuterated DSPC (d 70-DSPC) interact with Mg 2+ present into the buffer. AP-GPI inserts progressively into the d 70-DSPC monolayer changing the environment of phospholipid molecules. Amide I band exhibits α helix and β-sheets components with a predominance of the α helix. The shapes, intensities and positions of the amide I and II bands suggest for the α helix an orientation perpendicular to the interface after a period of molecular reorganisation.