Amino acid sequence of a non-specific wheat phospholipid transfer protein and its conformation as revealed by infrared and Raman spectroscopy. Role of disulfide bridges and phospholipids in the stabilization of the alpha-helix structure.

A wheat non specific phospholipid transfer protein has been isolated from wheat seeds and its amino acid sequence reveals that it is composed of 90 residues for a molecular weight of 9607. From the comparison of its sequence with those of the eight known proteins of the same family, hypotheses on the role of some conserved residues in the transfer activity can be made. The conformation of this protein has been studied by Raman and Fourier transform infrared spectroscopy and this is the first report on the structure of non specific plant phospholipid transfer proteins. As opposed to previous studies on the structure prediction from the amino acid sequence, the results obtained show that plant non specific phospholipid transfer proteins are not almost entirely composed of beta-sheets. Instead, infrared results show that the wheat protein contains 41% alpha-helix and 19% beta-sheet structures, while 40% of the conformation is undefined or composed of turns. Raman spectroscopy shows that three disulfide bridges adopt a gauche-gauche-gauche conformation while the other exhibits a gauche-gauche-trans conformation, and that the two tyrosine residues are hydrogen bonded to water molecules. The cleavage of the disulfide bonds affects significantly the conformation of the protein, the extended confirmation being increased by 15% at the expense of the alpha-helix content. On the other hand, the binding of 1-palmitoyllysophosphatidylcholine to the protein leads to an increase of 8% of the alpha-helix content compared to the free protein. Secondary structure predictions from the amino acid sequence suggest that the binding of a phospholipid stabilizes helicity of the amphipathic helices while the reduction of disulfide bonds would affect the stability of the N-terminal helix. The extended structure located at the C-terminus is not affected. Finally, the wheat phospholipid transfer protein has no effect on the thermotropic behavior of large unilamellar vesicles of dimyristoylphosphatidylcholine while it increases the conformational order of the acyl chains of large unilamellar vesicles of dimyristoylphosphatidylglycerol in the liquid-crystalline state. No major conformational changes of the protein are observed when it is adsorbed to phospholipid vesicles. These results suggest that the helical structure is essential for the transfer activity without excluding a possible role of the C-terminal extended structure on the adsorption to phospholipid vesicles.