THE ROLE OF MEMBRANES AND INTRACELLULAR BINDING-PROTEINS IN CYTOPLASMIC TRANSPORT OF HYDROPHOBIC MOLECULES - FATTY-ACID-BINDING PROTEINS

The path of a small hydrophobic molecule through the aqueous cytoplasm is not linear. Partition may favor membrane binding by several orders of magnitude; thus significant membrane association will markedly dec rease the cytosolic transport rate. The presence of high concentration s of soluble binding proteins for these hydrophobic molecules would co mpete with membrane association and thereby increase transport rate. F or long chain fatty acid molecules, a family of cytosolic binding prot eins collectively known as the fatty acid-binding proteins (FABP), are thought to act as intracellular transport proteins. This paper examin es the mechanism of transfer of fluorescent anthroyloxy-labeled fatty acids (AOFA) from purified FABP's to phospholipid vesicles. With the e xception of the liver FABP, AOFA is transferred from FABP by collision al interaction of the protein with an acceptor membrane. The rate of t ransfer increased markedly when membranes contain anionic phospholipid s; this suggests that positively charged residues on the surface pi th e FABP may interact with the membranes. Neutralization of surface lysi ne residues of adipocyte and heart-type FABPs decreased the AOFA trans fer rate, and transfer was then found to proceed via aqueous diffusion rather than collisional interaction. Site-specific mutagenesis has fu rther shown that the helix-turn-helix domain of the FABPs is critical for interaction with anionic acceptor membranes. In addition, direct i nteraction of adipocyte FABPs with anionic membranes has been demonstr ated. Thus ''cytosolic'' FABP may function in intracellular transport of fatty acids to decrease their membrane association, as well as to t arget fatty acids to specific subcellular sites of utilization. Copyri ght (C) 1996 Elsevier Science Inc.