Structural organization of proteocubosome carriers involving medium- and large-size proteins

Proteocubosome assemblies present unique structural advantages for development of novel lipid-based carriers of proteins. With this objective, the mechanism of protein uptake as well as the protein location with respect to the nanochannel network structures created by bicontinuous lipid cubic lattices needs to be determined. Here, the three-dimensional (3D) organization of porous architectures obtained by self-assembly of monoolein (MO) in aqueous phase containing the protein transferrin, and the uptake of transferrin in preformed lipid cubic phase structures, is investigated by high-resolution synchrotron X-ray diffraction. Our structural analysis revealed that the incorporated transferrin induces a cubic (DLarge) → cubic (DNormal) structural transition at temperatures around 35 °C. We found that this structural transition is characterized by a sharp alteration of the size of the aqueous nanochannels (from 5.9 nm in DLarge at T = 20 °C to 3.6 nm in DNomal at 52 °C ) and the thickness of the lipid bilayer (from 3.1 nm at 20 °C to 2.4 nm at 52 °C) that constitutes the 3D lattice. In contrast, we established that non-loaded MO (hydrated in a saline buffer solution without protein) displays a lamellar-to-cubic (DNormal) structural transition at temperatures above 21 °C.