Late ribosomal protein localization in Arabidopsis thaliana differs to that in Saccharomyces cerevisiae

Ribosome biogenesis is essential for cellular function and involves rRNA synthesis, rRNA processing and modification, and ribosomal protein assembly. Ribosome biogenesis factors and small nucleolar RNA assist these events. Ribosomal maturation takes place in the nucleolus, the nucleoplasm, and the cytosol in a coordinated and controlled manner. For example, some ribosomal proteins are thought to be assembled in the cytoplasm based on the observations in Saccharomyces cerevisiae. Here, we used cellular fractionation to demonstrate that cleavage of the 20S intermediate, the precursor to mature 18S rRNA, does not occur in the nucleoplasm of Arabidopsis thaliana. It most likely occurs in the cytoplasm. Further, we verified the proposed localization of RPS10e, RPS26e, and RPL24a/b in the nucleus and RPP1 in the nucleolus of A. thaliana by ribosome profiling, immunofluorescence, and analysis of the localization of GFP fusion proteins. Our results suggest that the order of events during ribosomal protein assembly in the ribosome biogenesis pathway differs between plants and yeast. Ribosomal biogenesis involves processes in the nucleolus, nucleus, and cytoplasm. Certain ribosomal proteins of the small (40S) and large ribosomal subunits (60S) are believed to be assembled into complexes in the cytoplasm in yeast. We show here that in plants, several of these ribosomal proteins are imported into the nucleus and associate with preribosomal complexes earlier than in yeast.