The α-Helical Domain of Liver Fatty Acid Binding Protein Is Responsible for the Diffusion-Mediated Transfer of Fatty Acids to Phospholipid Membranes

Intestinal fatty acid binding protein (IFABP) and liver FABP (LFABP), homologous proteins expressed at high levels in intestinal absorptive cells, employ markedly different mechanisms for the transfer of fatty acids (FAs) to acceptor membranes. Transfer from IFABP occurs during protein−membrane collisional interactions, while for LFABP, transfer occurs by diffusion through the aqueous phase. Earlier, we had shown that the helical domain of IFABP is critical in determining its collisional FA transfer mechanism. In the study presented here, we have engineered a pair of chimeric proteins, one with the “body” (ligand binding domain) of IFABP and the α-helical region of LFABP (αLβIFABP) and the other with the ligand binding pocket of LFABP and the helical domain of IFABP (αIβLFABP). The objective of this work was to determine whether the change in the α-helical domain of each FABP would alter the rate and mechanism of transfer of FA from the chimeric proteins in comparison with those of the wild-type proteins. The fatty acid transfer properties of the FABP chimeras were examined using a fluorescence resonance transfer assay. The results showed a significant modification of the absolute rate of FA transfer from the chimeric proteins compared to that of the wild type, indicating that the slower rate of FA transfer observed for wild-type LFABP relative to that of wild-type IFABP is, in part, determined by the helical domain of the proteins. In addition to these quantitative changes, it was of great interest to observe that the apparent mechanism of FA transfer also changed when the α-helical domain was exchanged, with transfer from αLβIFABP occurring by aqueous diffusion and transfer from αIβLFABP occurring via protein−membrane collisional interactions. These results demonstrate that the α-helical region of LFABP is responsible for its diffusional mechanism of fatty acid transfer to membranes.