Abstract
Eukaryotic initiation factor 6 (eIF6), a highly conserved protein from yeast to mammals, is essential for 60 S ribosome biogenesis and assembly. Both yeast and mammalian eIF6 are phosphorylated at Ser-174 and Ser-175 by the nuclear isoform of casein kinase 1 (CK1). The molecular basis of eIF6 phosphorylation, however, remains elusive. In the present work, we show that subcellular distribution of eIF6 in the nuclei and the cytoplasm of mammalian cells is mediated by dephosphorylation and phosphorylation, respectively. This nucleo-cytoplasmic shuttling is dependent on the phosphorylation status at Ser-174 and Ser-175 of eIF6. We demonstrate that Ca(2+)-activated calcineurin phosphatase binds to and promotes nuclear localization of eIF6. Increase in intracellular concentration of Ca(2+) leads to rapid translocation of eIF6 from the cytoplasm to the nucleus, an event that is blocked by specific calcineurin inhibitors cyclosporin A or FK520. Nuclear export of eIF6 is regulated by phosphorylation at Ser-174 and Ser-175 by the nuclear isoform of CK1. Mutation of eIF6 at the phosphorylatable Ser-174 and Ser-175 to alanine or treatment of cells with the CK1 inhibitor, D4476 inhibits nuclear export of eIF6 and results in nuclear accumulation of eIF6. Together, these results establish eIF6 as a substrate for calcineurin and suggest a novel paradigm for calcineurin function in 60 S ribosome biogenesis via regulating the nuclear accumulation of eIF6.
Highlights
Eukaryotic translation initiation factor 63 is a highly conserved protein between yeast and mammals
Eukaryotic initiation factor 6, a highly conserved protein from yeast to mammals, is essential for 60 S ribosome biogenesis and assembly. Both yeast and mammalian Eukaryotic translation initiation factor 6 (eIF6) are phosphorylated at Ser-174 and Ser-175 by the nuclear isoform of casein kinase 1 (CK1)
Nuclear and cytoplasmic fractions were subjected to Western blot analysis using anti-His antibody to detect only the Myc-His-tagged eIF6 (Fig. 1, panel C)
Summary
Eukaryotic translation initiation factor 6 (eIF6) is a highly conserved protein between yeast and mammals. EIF6 was originally isolated and characterized from the postribosomal supernatant of both wheat germ [3, 4] and mammalian cell extracts including anucleated rabbit reticulocyte lysates [1, 5, 6], most of our knowledge of the functional properties of eIF6 have been derived from molecular genetic studies examining the yeast eIF6 ortholog Tif6p in the yeast Saccharomyces cerevisiae [2, 7] These studies have provided compelling evidence that, at least in yeast cells, Tif6p, encoded by a single copy essential gene, does not function as a canonical translation initiation factor [2]. Neither the nuclear import nor the recycling of eIF6 has so far been directly demonstrated either in yeast or mammalian cells
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