Interaction of the wheat endosperm lipid-binding protein puroindoline-a with phospholipids

Puroindoline-a is the main component of a new family of proteins that has been suggested to exert an antimicrobial activity in plant seeds through an interaction with lipid membranes. Here the interaction of puroindoline-a with model phospholipid membranes and micelles has been studied using intrinsic tryptophan fluorescence, fluorescence polarization of diphenyl hexatriene, and proteolysis experiments. The protein appears to interact with both zwitterionic and negative phospholipids. The interaction with phosphatidylcholine is characterized by low-affinity surface binding with very limited penetration into the hydrophobic membrane interior. On the other hand, the interaction with phosphatidylglycerol displays a high affinity and involves a partial penetration of the protein into the bilayer interior that disrupts acyl chain packing. The specificity appears to be due to the presence of a stretch of positively charged residues in the protein sequence. In all, the lipid-binding properties of puroindoline-a resemble those of cardiotoxins, another family of proteins for which a disruptive effect on the membrane structure has been involved to explain their biological function.