Structure and orientation of puroindolines into wheat galactolipid monolayers

Puroindolines (PINs), basic and cysteine-rich proteins of wheat endosperm, are composed of two proteins, puroindoline-a (PIN-a) and puroindoline-b (PIN-b). Using a monolayer assay at the air/liquid interface, both PIN-a and PIN-b were studied in pure components and mixed with wheat galactolipids, 1,2-di-O-acyl-3-O-(beta-d-galactopyranosyl)- sn-glycerol (MGDG) and 2-di-O-acyl-3-O-(beta-d-galactopyranosyl-1,6-beta-d-galactopyranosyl)-sn-glycerol (DGDG). Following the adsorption of PINs at the air/liquid interface thanks to surface pressure increases, we concluded that PIN-a displays a more amphipathic character than PIN-b. Compression isotherms combined with ellipsometric measurements showed that the area per molecule is smaller and the protein film is more condensed for PIN-a than for PIN-b. According to the polarization modulation-infrared reflection-absorption spectroscopy data, both proteins display a highly alpha-helical structure and the alpha-helices are oriented rather parallel to the interface. By measuring the overpressure due to PIN adsorption into MGDG and DGDG monolayers, we observed that PIN-a interacts more strongly into lipid films than PIN-b. The observation by atomic force microscopy of mixed protein/lipid films showed that the nature of the lipid plays a significant role in the organization of PINs, particularly for PIN-a. The presence of galactolipids at the interface stabilizes the alpha-helical structure of PINs, but significant changes were observed concerning the orientation of the alpha-helices. They adopt a perfect parallel orientation to the interface in the MGDG monolayer, whereas the bundle of alpha-helices orients normal to the interface in the DGDG film.