Subcellular distribution of ribosomal proteins S6 and eL12. Analysis by autoradiography and immunofluorescence of sections from oocytes of Xenopus laevis.

The process of ribosome assembly in eukaryotes was studied by injecting tritium-labeled ribosomal proteins S6 and eL12 into oocytes of Xenopus laevis. The subcellular distribution of the two proteins was visualized by means of autoradiography in sections of oocytes. Protein S6 but not eL12 was found in the nucleus where it accumulated at the nucleoli. In the presence of actinomycin D the accumulation of S6 at the nucleoli was reduced. In-situ immunofluorescence studies indicated that S6 is located at the nucleoli and eL12 exclusively in the cytoplasm. It appears that S6 is involved in the early ribosomal assembly process at the nucleoli, whereas eL12 is restricted to the cytoplasm where it is incorporated into 60S ribosomal subunits in a late assembly step.