ABSTRACT
FABP4 delivers specific ligands from the cytosol to the nuclear receptor PPARgamma in the nucleus, thereby facilitating the ligation and enhancing the transcriptional activity of the receptor. Here, we delineate the structural features that underlie the nucleocytoplasmic transport of FABP4. The primary sequence of FABP4 does not harbor a readily identifiable nuclear localization signal (NLS). However, such a signal could be found in the three-dimensional structure of the protein and was mapped to three basic residues that form a functional NLS stabilized by the FABP4/PPARgamma ligand troglitazone. We show that FABP4 is also subject to active nuclear export. Similarly to the NLS, the nuclear export signal (NES) is not apparent in the primary sequence, but assembles in the tertiary structure from three nonadjacent leucine residues to form a motif reminiscent of established NES. The data demonstrate that both nuclear export and nuclear import are critical for enabling FABP4 to enhance the transcriptional activity of PPARgamma. Additionally, the observations provide insight into the fundamental question of how proteins are activated by ligands. Such an activation may be understood by the "induced-fit" model, which states that ligand-induced conformational changes precede activation of a protein. Alternatively, the "pre-existing equilibrium" hypothesis postulates that activated conformations exist within the repertoire of apoproteins, and that ligands do not induce these but merely stabilize them. Studies of the subcellular localization of FABP4 support the validity of the "pre-existing equilibrium" model for the ligand-controlled activation of the nuclear import of FABP4.
