A ligand-activated nuclear localization signal in cellular retinoic acid binding protein-II

Primary sequences of proteins often contain motifs that serve as signatures for subcellular targeting, such as a nuclear localization signal (NLS). However, many nuclear proteins do not harbor a recognizable NILS, and the pathways that mediate their nuclear translocation are unknown. This work focuses on CRABP-II, a cytosolic protein that moves to the nucleus upon binding of retinoic acid. While CRABP-II does not contain an NILS in its primary sequence, such a motif could be recognized in the protein's tertiary structure. We map the retinoic acid-induced structural rearrangements that result in the presence of this NILS in holo- but not apo-CRABP-II. The signal, whose three-dimensional configuration aligns strikingly well with a classical NILS, mediates ligand-induced association of CRABP-II with importin a and is critical for nuclear localization of the protein. The ligand-controlled NILS switch of CRABP-II may represent a general mechanism for posttranslational regulation of the subcellular distribution of a protein.