Crosstalk between cell-division and cell-growth by the nucleolar proteins with multifunction

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Ribosome biogenesis is strictly regulated according to the changes in cell proliferation mechanisms. However, only little is known about the linkage between ribosome synthesis and other cellular systems. To elucidate this linkage, we focused on two trans-acting factors, Ebp2 and Rrsl, which interact with each other and are required for maturation of 25S rRNA and assembly of the 60S ribosomal subunit.(1) Roles of Ebp2 and Rrs1 in ribosome biogenesis: Two-hybrid screening exhibited that Ebp2 interacts with trans-acting factors essential for biogenesis of both 60S and 40S ribosomal subunits. The data suggest that Ebp2 is involved in cooperatively regulating synthesis of both ribosomal subunits (JBC, 2004). TAP-purification and mass spectrometry revealed that Ebp2 and Rrsl are involved in different stage of pre-ribosomal particle. We constructed temperature sensitive rrsl mutants and suggested that Rrs1 is required for nuclear export of the 60S ribosomal subunit (FEBS Lett., 2004). We also suggested that the assembly of L5-5S rRNA on 60S ribosomal subunits coordinates with assembly of L11 via Rrsl (NAR 2005).(2) Roles of Ebp2 and Rrsl in other cellular systems: We constructed temperature sensitive ebp2 mutants exhibiting TBZ-sensitivity, small cell morphology, an aberrant-shaped nucleus and short spindle. EGFP-Ebp2p and EGFP-Rrs1p are localized at the nuclear periphery besides in the nucleolus. However, in the ebp2 mutant cell, Ebp2 was not localized at the nuclear periphery. We suggested that Ebp2p and Rrslp have two functions in yeast; maturation of 25S rRNA and assembly of the 60S ribosomal subunit in the nucleolus and the maintenance of nuclear shape.