The distribution of ionizable amino lipids (KC2) is critical in structure of lipid nanoparticles, siRNA entrapment and endosomal release. Neutral KC2 segregates from phospholipids (POPC) and forms an oily core in the bilayer interior.

The transport of hydrophobic molecules, including long-chain fatty acids, within cells is highly dynamic. Hydrophobic molecules are unable to freely diffuse through the aqueous cytoplasm without a transporter. Fatty acid binding proteins (FABP) transport these molecules to different cellular compartments. As part of their transport, FABPs often associate with cell membranes to acquire and deliver their bound cargo. Understanding the nature of this transport is becoming increasingly important because lipid signaling functions are associated with metabolic pathways impacting disease pathologies such as carcinomas, autism and schizophrenia. Herein, we focus on Brain fatty acid binding protein (FABP7), which demonstrates localization to the cytoplasm and nucleus, influencing transcription and fatty acid metabolism. We use a combined biophysical approach to elucidate the interaction between FABP7 and model membranes. Specifically, we use microscale thermophoresis to show that FABP7 can bind oleic acid (OA) and docosahexaenoic acid (DHA) micelles, while differential scanning fluorimetry experiments show binding lowers the melting temperature of FABP7. Structural data from NMR and multiscale molecular dynamics simulations reveals that the interaction between FABP7 and micelles is through FABP7 portal region residues. Our simulations also capture binding events where fatty acids dissociate from the model membrane and bind to FABP7. Overall, our data reveals a novel interaction between FABP7 and OA or DHA micelles and provides key structural insight into the transport of hydrophobic molecules.

Members of the fatty acid binding protein (FABP) family function as intracellular transporters of long chain fatty acids and other hydrophobic molecules to different cellular compartments. Brain fatty acid binding protein (FABP7) exhibits ligand-directed differences in cellular transport behavior. For example, when FABP7 binds to docosahexaenoic acid (DHA), the complex relocates to the nucleus and influences transcriptional activity, whereas FABP7 bound with monosaturated fatty acids remain in the cytosol. We used a variety of biophysical techniques to enhance understanding of ligand-directed transport. Specifically, we examine how FABP7 binds to fatty acids, including saturated stearic acid (SA), monounsaturated oleic acid (OA), and polyunsaturated DHA. We find that at 37°C FABP7 has near equivalent binding affinities for the fatty acids, while at lower temperatures, FABP7 exhibits a preference for the unsaturated fatty acids. Therefore, nuclear localization of the FABP7-DHA complex cannot be explained by binding preferences. Using NMR spectroscopy and molecular dynamics simulations, we observe that DHA uniquely affects the portal region of FABP7, which could enhance the complex's nuclear localization. Mutations to purported critical binding residues (R126L and Y128F) have little effect on fatty acid binding, with molecular dynamics simulations revealing that the bound fatty acid can adopt binding poses that can accommodate the mutations.