Formation of a Mimetic Biomembrane from the Hydrophobic Protein Zein and Phospholipids: Structure and Application

alpha-Zein, a storage protein in corn endosperm, could be purified easily and in large amounts. In this study, a-zein was incorporated into phospholipid cholesterol (PC Chol) liposomes. The maximal amount of alpha-zein incorporated in the liposome was 0.05% (mol/mol) and the PC:Zein molar ratio was near 2400. At this level of zein insertion, the phase transition temperature of the lipid bilayer was little affected, but the leakage of doxorubicin (DOX) from the PC-Chol liposome became obviously slower when alpha-zein was added at a higher temperature than the phase transition temperature. Cryogenic transmission electron micrographs of the PC-Chol-Zein liposome showed that adjacent membranes in multilamellar vesicles were often aligned at a regular interval of about 7 mn. Data from synchrotron small-angle X-ray scattering of the PC-Chol-Zein liposome indicated the formation of the multilamellar structure with an intermembrane interval of 7.2 nm, whereas no homogeneous membrane alignment was observed in the absence of zein. The present observation can be well explained by supposing that alpha-zein takes on such an elongated conformation that it penetrates through two adjacent membrane layers. This feature seems to be compatible with a recently proposed superhelical structural model of alpha-zein. Meanwhile, experiments with the fluorescent-labeled alpha-zein showed that the PC Chol Zein liposome could be uptaken by an intact cell and localized in some specialized area (possibly endosomes) within the cell instead of being diffusely distributed in the cell. Thus, the PC Chol Zein liposome seems to act as an interesting biomembrane model and may be applicable as a drug delivery system.