Nucleolar targeting signal of Rex protein of human T-cell leukemia virus type I specifically binds to nucleolar shuttle protein B-23.

Rex protein, the post-transcriptional regulator of human T-cell leukemia virus type I, is located predominantly in the cell nucleolus and is associated with the cytoplasmic accumulation of unspliced and singly spliced viral mRNAs. The N-terminal 19-amino acid segment of Rex has been identified as the nucleolar targeting signal (NOS) and shown to be important for Rex function. To study the molecular interaction between the NOS region of Rex and its binding host protein(s) in the nucleolus, we chemically synthesized a functional NOS peptide (wild type) and mutant NOS peptides. Fluorescein isothiocyanate-conjugated functional NOS peptide was rapidly taken up by human cells and was transported to the nucleolus. Using affinity chromatography, we identified nucleolar protein B-23 as the major protein that binds to NOS. We also identified two highly acidic regions of B-23 (amino acids 120-132 and 161-188) as acceptor regions for NOS. Previous experiments have suggested that B-23 functions as a shuttle protein for the nucleolar transport of ribosomal components. Our results suggest that B-23 may also serve as a shuttle for the import of Rex from the cytoplasm to the nucleolus coupled to the export of viral mRNAs containing the Rex-responsive element.