To investigate the mechanism for the transfer of ribosomal proteins from cytoplasm into nuclei and nucleoli, the uptake of 3H-labelled ribosomal proteins by the isolated rat liver nuclei was investigated in the system containing ATP, GTP, CTP, UTP, and an energy-regenerating system.1. In the presences of cell sap, the transfer became temperature-dependent. The concentration of ribosomal proteins was also very important for their specific transfer and 10-15 pg of ribosomal proteins/ml were suitable in the presence of lo7 nuclei.2. Removal of one of the four nucleoside triphosphates from the complete system containing cell sap, especially that of CTP or UTP, resulted in a marked decrease of the uptake. A low concentration of actinomycin D inhibited significantly the transfer of ribosomal proteins, while a-amanitin to a less extent. The results indicate that the uptake of ribosomal proteins by liver nuclei is largely dependent on RNA synthesis especially rRNA synthesis.3. The transfer was enhanced to some extent by ATP alone. Other nucleoside triphosphates were less effective. Non-hydrolyzable ATP analogues, adenosine 5'-[p,pmethylene]triphosphate and adenosine 5'-[a,P-methylene]-triphosphate were only slightly stimulative. Although ATP enhanced the transfer into the extranucleolar fraction to some extent, the maximal transfer not only into nucleoli but also into the extranucleolar fraction was dependent on the rRNA synthesis.4. Sedimentation analyses of the nucleolar fraction of rat liver nuclei incubated with [3H]GTP or 3H-labelled ribosomal proteins, showed that small but distinct amounts of the both labelled compounds were incorporated into 60s preribosomal particles although most of them were found in ribonucleoprotein particles below 30s which were previously shown to be degraded products containing larger rRNA precursors [Tsurugi, K., Morita, T., and Ogata, K. (1972) Ear. J. Biochem. 25, 117-1281.There have been many reports, including ours [l -31, showing that in eukaryotic cells ribosomal proteins are synthesized on cytoplasmic ribosomes, preferentially on free polysomes and most of them are assembled with newly synthesized rRNA in nucleoli, forming preribosomal particles (for review see [4, 51). Only a few reports, however, have been published on the transport of ribosomal proteins from the cytoplasm into the nuclei. Employing a cell-free system containing HeLa cell nuclei, Roth et al. [6] reported that ribosomal proteins were transferred more rapidly than cytosol proteins and that they were associated with nucleoli. They suggested, however, that an active uptake may not be involved in their system because the nuclei used were stripped of their nuclear membranes and that ribosomal proteins were taken up even at 0°C. Bolla et al. [7] showed that ribosomal proteins were taken up by rat liver nuclei in the system which permitted RNA synthesis, and transferred ribosomal proteins were associated with preribosomal particles. They did not describe, however, the relationship between the uptake of ribosomal prot...
