Role of the Helical Domain in Fatty Acid Transfer from Adipocyte and Heart Fatty Acid-binding Proteins to Membranes

Abstract

The adipocyte and heart fatty acid-binding proteins (Aand HFABP) are members of a lipid-binding protein family with a ␤-barrel body capped by a small helix-turn-helix motif. Both proteins are hypothesized to transport fatty acid (FA) to phospholipid membranes through a collisional process. Previously, we suggested that the helical domain is particularly important for the electrostatic interactions involved in this transfer mechanism (Herr, Despite their using qualitatively similar FA transfer mechanisms, differences in absolute transfer rates as well as regulation of transfer from AFABP versus HFABP, prompted us to consider the structural determinants that underlie these functional disparities. To determine the specific elements underlying the functional differences between AFABP and HFABP in FA transfer, two pairs of chimeric proteins were generated. The first and second pairs had the entire helical domain and the first ␣-helix exchanged between A-and HFABP, respectively. The transfer rates of anthroyloxy-labeled fatty acid from proteins to small unilamellar vesicles were compared with the wild type AFABP and HFABP. The results suggest that the ␣II-helix is important in determining the absolute FA transfer rates. Furthermore, the ␣I-helix appears to be particularly important in regulating protein sensitivity to the negative charge of membranes. The ␣I-helix of HFABP and the ␣II-helix of AFABP increased the sensitivity to anionic vesicles; the ␣I-helix of AFABP and ␣II-helix of HFABP decreased the sensitivity. The differential sensitivities to negative charge, as well as differential absolute rates of FA transfer, may help these two proteins to function uniquely in their respective cell types.FFatty acids (FA) 1 are major substrates for the synthesis of complex lipids and for energy production. Due to their limited solubility, specific carriers, known as fatty acid-binding proteins (FABP), are expressed in various tissues that use FA. Structural analyses of several FABPs have revealed markedly similar three-dimensional folds consisting of ten antiparallel ␤-strands that form a ␤-barrel, which is capped by two short ␣-helices arranged as a helix-turn-helix segment (1-4). It is believed this ␣-helical domain, along with the ␤ C-D and D-E turns, functions as a "dynamic portal" that regulates FA entry and exit from the internal ligand binding cavity (5-7).Heart FABP (HFABP) and adipocyte FABP (AFABP) are homologous proteins sharing Ͼ60% amino acid sequence identity (8). Both proteins bind one FA in the binding pocket (1, 2), although x-ray crystallographic studies show that the FA adopts an entirely different conformation in their respective binding sites (9, 10). AFABP has generally lower binding affinities for FA than HFABP (11), and it appears that HFABP shows greater affinity for saturated versus unsaturated fatty acids, whereas AFABP does not show such a preference (9). Aand HFABP also exhibit unique patterns of tissue distribution. AFABP is found in adipose tissue and monocytes/macrophages (12, 13), whereas ...

The integrity of the α-helical domain of intestinal fatty acid binding protein is essential for the collision-mediated transfer of fatty acids to phospholipid membranes

“…The construction of chimeric proteins has been used to reveal the importance and function of isolated domains in several proteins [39,40]. The highly similar tertiary structures of I-and LFABP, despite their low primary sequence homology, indicated that this pair of proteins were very good candidates for the construction of distinct chimeric proteins that would reveal the specific functions of different domains in the transfer of AOFA to membranes.…”

Structural determinants of ligand binding in the ternary complex of human ileal bile acid binding protein with glycocholate and glycochenodeoxycholate obtained from solution NMR

“…It is important to note that the slower cluster overlaps with the C/D loop and the helical segments as well, which in FABPs are thought to be part of a dynamic portal region mediating ligand entry [26,31]. The helical region is additionally thought to be a key determinant in the interactions of iLBPs with membranes [32,33]. An important difference though is that, while in FABPs [26] and some of the reported CRBP structures [34,35] the helical region shows a considerable disorder in the apo state, in human I-BABP it is well defined in both the free [9] and the bound forms.…”

Tissue-specific Functions in the Fatty Acid-binding Protein Family

“…The HFABP ␣-helical domain is involved in the collision-mediated transfer of LCFA from HFABP to acidic membranes, and similar interactions could take place between HFABP and acidic peptide domains to facilitate protein-protein interactions (20). HFABP may also interact with PPAR␣, a nuclear receptor, to induce the expression of mitochondrial and peroxisomal ␤-oxidation pathways (12).…”