FABP7 Binds to Fatty Acid Micelles: Implications for Lipid Transport

Abstract

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.

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“…Fatty acids can form micelles or micelle-like structures despite their long hydrophobic moiety due to the carboxyl functional group that improves water solubility. 44 The formation of aggregations reduces the system's free energy by removing hydrophobic fragments from the aqueous environment. The concentration at which micelle formation is first seen in the solution is called the critical micelle concentration (CMC).…”

“…The Nile Red method relies on a sharp change in the fluorescence intensity in a lipid-rich environment when the dye binds to micelles. 44 The Nile Red fluorescence intensities of the samples were measured after exposure to various concentrations of free fatty acids in aqueous solutions such as DMEM culture medium and Ca5 buffer (Fig. 2), the latter used during insulin secretion experiments.…”