Effect of lipid-based nanostructure on protein encapsulation within the membrane bilayer mimetic lipidic cubic phase using transmembrane and lipo-proteins from the beta-barrel assembly machinery

A fundamental understanding of the effect of amphiphilic protein encapsulation on the nanostructure of the bicontinuous cubic phase is crucial to progressing biomedical and biological applications of these hybrid protein-lipid materials, including as drug delivery vehicles, as biosensors, biofuel cells and for in meso crystallization. The relationship between the lipid nanomaterial and the encapsulated protein, however, remains poorly understood. In this study, we investigated the effect of incorporating the five transmembrane and lipo-proteins which make up the β-barrel assembly machinery from Gram-negative bacteria within a series of bicontinuous cubic phases. The transmembrane β-barrel BamA caused an increase in lattice parameter of the cubic phase upon encapsulation. By contrast, the mainly hydrophilic lipo-proteins BamB-E caused the cubic phase lattice parameters to decrease, despite their large size relative to the diameter of the cubic phase water channels. Analysis of the primary amino acid sequence was used to rationalize this effect, based on specific interactions between aromatic amino acids within the proteins and the polar-apolar interface. Other factors that were found to have an effect were lateral bilayer pressure and rigidity within the lipid bilayer, water channel diameter, and size and structure of the lipo-proteins. The data presented suggest that hydrophilic bioactive molecules can be selectively encapsulated within the cubic phase by using a lipid anchor or aromatic amino acids, for drug delivery or biosensing applications.