RESUMO
The fatty acid-binding proteins (FABPs) are a very well known protein family which includes the liver basic FABPs (Lb-FABPs), a subgroup so far characterized in several vertebrates but not in mammals. The most important difference recognized between the proteins in this subgroup and the better known mammalian liver FABPs (L-FABPs) is the stoichiometry of ligand binding: two fatty acid molecules in L-FABPs compared with one in Lb-FABPs. The only Lb-FABP with a known three-dimensional structure is that of chicken Lb-FABP, but the details of ligand binding are still unresolved as the crystals of the protein are grown at an acidic pH and the protein has been shown to lose its ligand under these conditions. The two proteins whose crystallizations are reported here are the second and third members of this subfamily to be crystallized. The crystals of axolotl Lb-FABP belong to either space group P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 65.38, c = 60.90 A, and diffract to a resolution of 2.0 A on a conventional source at room temperature. The crystals of toad Lb-FABP belong to either space group P4(1)22 or P4(3)22, with unit-cell parameters a = b = 48.14, c = 135.23 A, and diffract to 2.5 A resolution under the same conditions. It is expected that the solution of these two structures will help to clarify the structural differences between Lb-FABPs and L-FABPs and will possibly explain the different binding stoichiometries observed in these otherwise so similar protein subfamilies.