Assembly and characterization of lipid–lipid binding protein particles

Lipid–protein complexes, lipoplexes, are currently of great interest because of their immunogenic, gene free, and low cost properties. For their applications as potential vaccines, it is critical to display a target protein on the surface of lipoplex particles to allow external interactions to take place. However, how to effectively assemble lipoplexes with target proteins externally accessible is a constant challenge. In this study, human liver fatty acid binding protein 1 (hl-FABP1) was used as a model protein in lipoplex assembly with a non-lipid binding protein, bovine serum albumin (BSA), serving as a comparison. The protein–lipid particles were assembled by four different processes and characterized by dynamic light scattering (DLS), transmission electron microscope (TEM), flow cytometry (FCM), and a modified ELISA. Results indicate that by incubating the target protein with pre-formed liposomes at a temperature higher than all transition temperatures (Tm) of the lipids used through an extended period of time, 1.48 × 10−6 nmol per lipoplex of incorporated proteins can be detected by ELISA and are externally accessible. Additional experiments showed that most of those externally accessible proteins are likely embedded in the lipid bilayer structure and are not subject to dissociation from the lipoplex particles at elevated salt concentrations.