Eukaryotic translation initiation factor 6 (eIF6) and regulation of 60S ribosome biogenesis
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Eukaryotic translation initiation factor 6 (eIF6) can bind to the 60S ribosomal subunit and prevent its association with the 40S ribosomal subunit. In yeast cells, Tif6p, the yeast homologue of mammalian eIF6, is required for the biogenesis of 60S ribosomal subunits because the protein is necessary for the processing of 35S pre-rRNA in the nucleolus to form the mature 25S and 5.8S rRNAs that are the constituents of the 60S ribosomal subunit. In the present work, we have isolated a protein kinase from rabbit reticulocyte lysates that phosphorylates recombinant human eIF6. Mass spectrometric analysis identified it as casein kinase I. The site of phosphorylation, which is highly conserved from yeast to mammals, has been identified to be the serine residues at positions 174 (major site) and 175 (minor site). The same serine residues are phosphorylated by the nuclear isoform of yeast CK I (Hrr25p) in yeast cells. Failure to phosphorylate at these sites caused a loss of cell growth and viability. Additional experiments showed that while both the phosphorylated and unphosphorylated forms of Tif6p bound well to mature 60S and pre-60S ribosomal particles, failure to phosphorylate Tif6p resulted in inhibition of 35S pre-rRNA processing to mature 25S and 5.8S rRNAs. Furthermore, while wild-type Tif6p was distributed both in nuclei and the cytoplasm of yeast cells, the mutant Tif6p (with Ser174Ala and Ser175A1a) became a constitutively nuclear protein. Taken together, these results suggest that phosphorylatable Ser-174 and Ser-175 play a critical role in the nuclear export of Tif6p and indirectly regulates 60S ribosome biogenesis.