Nuclear and nucleolar targeting of human ribosomal protein S25: Common features shared with HIV-1 regulatory proteins

The nuclear and nucleolar targeting properties of human ribosomal protein S 25 (RPS25) were analysed by the expression of epitope-tagged RPS25 cDNAs in Cos-l cells. The tagged RPS25 was localized to the cell nucleus, with a st rong predominance in the nucleolus. At the amino terminus of RPS25, two str etches of highly basic residues juxtapose. This configuration shares common features with the nucleolar targeting signals (NOS) of lentiviral RNA-bind ing transactivators, including human immunodeficiency viruses' (HIV) Rev pr oteins. Deletion and site-directed mutational analyses demonstrated that th e first NOS-like stretch is dispensable for both nuclear and nucleolar loca lization of RPS25, and that the nuclear targeting signal is located within the second NOS-like stretch. It has also been suggested that a set of conti nuous basic residues and the total number of basic residues should be requi red for nucleolar targeting. Signal-mediated nuclear/nucleolar targeting wa s further characterized by the construction and expression of a variety of chimeric constructs, utilizing three different backbones with RPS25 cDNA fr agments. Immunofluorescence analyses demonstrated a 17 residue peptide of R PS25 as a potential nuclear/nucleolar targeting signal. The identified pept ide signal may belong to a putative subclass of NOS, characterized by compa ct structure, together with lentiviral RNA binding transactivators.