Deconvoluting the effect of the hydrophobic and hydrophilic domains of an amphiphilic integral membrane protein in lipid bicontinuous cubic mesophases

Lipidic bicontinuous cubic mesophases with encapsulated amphiphilic proteins are widely used in a range of biological and biomedical applications, including in meso crystallization, as drug delivery vehicles for therapeutic proteins, and as biosensors and biofuel cells. However, the effect of amphiphilic protein encapsulation on the cubic phase nanostructure is not well-understood. In this study, we illustrate the effect of incorporating the bacterial amphiphilic membrane protein Ag43, and its individual hydrophobic β43 and hydrophilic α43 domains, in bicontinuous cubic mesophases. For the monoolein, monoalmitolein, and phytantriol cubic phases with and without 8% w/w cholesterol, the effect of the full length amphiphilic protein Ag43 on the cubic phase nanostructure was more significant than the sum of the individual hydrophobic β43 and hydrophilic α43 domains. Several factors were found to potentially influence the impact of the hydrophobic β43 domain on the cubic phase internal nanostructure. These include the size of the hydrophobic β43 domain relative to the thickness of the lipid bilayer, as well as its charge and diameter. The size of the hydrophilic α43 domain relative to the water channel radius of the cubic mesophase was also found to be important. The secondary structure of the Ag43 proteins was affected by the hydrophobic thickness and physicochemical properties of the lipid bilayer and the water channel diameter of the cubic phase. Such structural changes may be small but could potentially affect membrane protein function.