Intracellular lipid-binding proteins have evolved from a common ancestral gene with the appearance of mitochondrial oxidation, to guarantee, for example, transport of fatty acids through the aqueous cytosol to their site of utilization. The mammalian forms of these lipid carriers are structurally well-characterized and have been categorized, on the basis of sequence similarities and several typical ligand-binding features, into four subfamilies. Only a single complex structure of an invertebrate fatty-acid-binding protein (FABP) has been reported to date, which reveals a unique ligand-binding arrangement yet unknown in vertebrate FABPs. In the present study, the structure of a second invertebrate FABP (locust muscle) complexed with a fatty acid has been determined on the basis of intermolecular NOE connectivities between the protein and the uniformly 13C-enriched oleate ligand. The resulting ligand conformation, although resembling the closely related mammalian heart- and adipocyte-type FABPs, is characterized by certain binding features that differ significantly from the typical hairpin-turn ligand shapes of the latter forms. This is primarily due to an alanine-to-leucine substitution in locust FABPs that produces a steric hindrance for ligand binding, A comparison with an FABP from tobacco hornworm larvae furthermore demonstrates that certain amino acid substitutions that appear to be specific for invertebrates decidedly influence the binding arrangement inside the protein cavity. Hence, as a result of these evolutionary variations, invertebrate FABPs may display a much greater diversity in intracellular lipid binding than observed for the mammalian transport proteins, thus possibly providing new insights for the design of modified lipid carriers.
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