Rat intestinal fatty acid binding protein. A model system for analyzing the forces that can bind fatty acids to proteins.

Rat intestinal fatty acid binding protein (I-FABPF is produced in the columnar absorptive epithelial cells (enterocytes) of the intestine (1). Its precise function in the absorption of long chain fatty acids and their subsequent rafficking to intracellular sites of metabolic processing remain unclear. Nonetheless, there are several features of this fatty acid “receptor” that contribute to its attractiveness as a model for studying the manner in which fatty acids interact with proteins. It is a small monomeric protein composed of 13 1 residues that binds a single molecule of long chain fatty acid in a noncovalent fashion (2-5). It has no known coor post-translational modifications and can be efficiently produced in bacteria with full preservation of function (7). Escherichia coli-derived rat I-FABP can be readily crystallized with and without bound fatty acid. The structure of the apoprotein has been refined to 1.2-A resolution (6). The structures of I-FABP with bound tetradecanoate (myristate, C14:0), hexadecanoate (palmitate, C16:0), and 92-octadecanoate (oleate, C18:l) have been refined to 1.5, 1.9, and 1.75 A, respectively, so that an assessment can be made of how the protein accommodates changes in the chain lengthkonformation of its bound ligand (7, 8L2 Moreover, I-FABP is a member of a family of lipid binding proteins. This family includes other FABPs that are expressed in different cell lineages as well as proteins that have evolved to bind different types of ligands, e.g. retinoids (cellular retinol binding protein (CRBP), CRBPII, cellular retinoic acid binding protein, CRABP) or bile acids (ileal lipid binding protein). The structures of several of these other family members are known: mouse adipocyte lipid binding protein with bound octadecanoate and 92-octadecanoate to 1.6 A (9, lo), bovine peripheral nerve myelin P2 protein with endogenous fatty acids and with 92-octadecanoate to 2.4 and 2.7 A (11, 12), human muscle FABP with a population fatty acid to 2.1 A (13), bovine heart FABP with endogenous fatty acid ligands to 3.2 A (141, chicken liver FABP to 3.0 A (15),oplus CRBP and CRBPII with bound all-trans-retinol t o 2.1 and 1.9 A, respectively (12, 16). All these proteins have similar conformations even though multiple sequence alignments reveal that they have as little as 20% amino acid sequence identity (11, 14). Sequence similarities and ifferences between these proteins have provided insights about the structural basis for their overlapping and distinct binding specificities. In this minireview, we focus on how x-ray crystallography combined with mutagenesis has been used to analyze the forces that bind fatty acids t o I-FABP. This “mutagenesis” has involved both the protein and its ligand.