LIPID SPECIFICITY AND LOCATION OF THE STEROL CARRIER PROTEIN-2 FATTY-ACID-BINDING SITE - A FLUORESCENCE DISPLACEMENT AND ENERGY-TRANSFER STUDY

Although it was recently recognized that sterol carrier protein-2 (SCP -2) interacts with fatty acids, little is known regarding the specific ity of SCP-2 for long-chain fatty acids or branched-chain fatty-acid-l ike molecules. Likewise the location of the fatty-acid binding site wi thin SCP-2 is unresolved A fluorescent cis-parinaric acid displacement assay was used to show that SCP-2 optimally interacted with 14-22 car bon chain lipidic molecules: polyunsaturated fatty acids > monounsatur ated, saturated > branched-chain isoprenoids > branched-chain phytol-d erived fatty acids. In contrast, the other major fatty-acid binding pr otein in liver, fatty-acid binding protein (L-FABP), displayed a much narrower carbon chain preference in general: polyunsaturated fatty aci ds > branched-chain phytol-derived fatty acids > 14- and 16-carbon sat urated > branched-chain isoprenoids. However, both SCP-2 and L-FABP di splayed a very similar unsaturated fatty-acid specificity profile. The presence and location of the SCP-2 lipid binding site were investigat ed by fluorescence energy transfer. The distance between the SCP-2 Trp (50) and bound cis-parinaric acid was determined to be 40 Angstrom. Th us, the SCP-2 fatty-acid binding site appeared to be located on the op posite side of the SCP-2 Trp(50) These findings not only contribute to our understanding of the SCP-2 ligand binding site but also provide e vidence suggesting a potential role for SCP-2 and/or L-FABP in metabol ism of branched-chain fatty acids and isoprenoids.