DISTINCT DOMAINS IN RIBOSOMAL-PROTEIN L5 MEDIATE 5-S RIBOSOMAL-RNA BINDING AND NUCLEOLAR LOCALIZATION

Ribosomal protein L5, a 34-kDa large ribosomal subunit protein, binds to 5 S rRNA and has been implicated in the intracellular transport of 5 S rRNA. By immunofluorescence microscopy, L5 is detected mostly in t he nucleolus with a fainter signal in the nucleoplasm, and it is known to also be a component of large ribosomal subunits in the cytoplasm. 5 S rRNA is transcribed in the nucleoplasm, and L5 is thought to play an important role in delivering 5 S rRNA to the nucleolus. Using RNA-b inding assays and transfection experiments, we have delineated the dom ains within L5 that confer its 5 S rRNA binding activity and that loca lize it to the nucleolus. We found that the amino-terminal 93 amino ac ids are necessary and sufficient to bind 5 S rRNA in vitro, while the carboxyl-terminal half of the protein, comprising amino acids 151-296, serves to localize the protein to the nucleolus. L5, therefore, has a modular domain structure reminiscent of other RNA transport proteins where one region of the molecule serves to bind RNA while another dete rmines subcellular localization.