The adipocyte lipid-binding protein at 1.6-A resolution. Crystal structures of the apoprotein and with bound saturated and unsaturated fatty acids.

Abstract

Crystals of the adipocyte lipid-binding protein which diffract to near atomic resolution have been obtained in Na/K phosphate bufferlprecipitant system.The structures of the apo-form and the protein with bound oleic acid and Ftearic acid have been determined and refined to 1.6-A resolution with R-factor around 18%.The conformations of the bound fatty acids are nearly the same.In both cases, the carboxylate group of the ligand interacts directly with Arg"" and TyrlZ8, indirectly with Arg"" through a water molecule.The hydrocarbon tail sticks out of the protein surface through a hydrophobic patch.Saturated and unsaturated fatty acids bind in essentially the same conformation.The remaining space of the binding pocket is filled with well ordered water molecules interacting with most of the polar side chains.Comparisons between the holoand apostructures reveal that the hydrophobic patch on the protein surface formed by a helix and several tight turns might serve as a portal for lipid binding.Since the adipocyte lipid-binding protein is phosphorylated at Tyr" by the insulin receptor kinase, the position of this side chain has been re-evaluated using the coordinates of the holo-forms.It appears that the position of Tyr" does not change significantly upon the binding of either of the fatty acids.Trafficking of fatty acids and retinoid compounds within the cytoplasm appears to be mediated by a family of low molecular weight, lipid-binding proteins (Glatz and Van der Vusse, 1990).These proteins have been found in a variety of lipid-active tissues.They are markedly abundant and can constitute as much as 2-8% of the total cytosolic protein (Clarke and Armstrong, 1989).Based on their gene structures and amino acid sequence similarity (Matarese et al., 1989), they are considered to form a multigene family.The family of intracellular lipid-binding proteins contains 10 members to date.For a review of their properties, see Sweetser et al. (1987) and Matarese et al. (1989).High resolution crystal structures of five fatty acid-binding proteins have been reported.They include, bovine myelin P2 protein (Jones et al., 1988), rat intestine fatty acid-binding