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Regulating ribosomes

by iansmith
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Dec8-03, 09:00 PM
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Evolutionarily ancient mechanism controls ribosomal subunit association | By Rossella Lorenzi

The beta4 integrin interactor p27(BBP/eIF6) is an evolutionarily highly conserved protein and, with the exception of bacteria, has been isolated from almost all organisms from yeast to mammals. The protein keeps the 40S and 60S ribosomal subunits dissociated in vitro, but the factors that lead to their association into 80S ribosomes have been unclear. In the December 4 Nature, Marcello Ceci and colleagues at the San Raffaele Scientific Institute in Milan, who isolated p27(BBP/eIF6) in 1997, show that release of this protein from the 60S subunit allows 80S ribosome assembly (Nature, 426:579-584 December 4, 2003).

Ceci et al. analyzed profiles of soluble cell extracts and observed that endogenous eIF6—mainly located in the cytoplasm—was bound to free 60S subunits but not to 80S. To identify factors directly interacting with the protein, the authors then performed yeast two-hybrid screening and isolated RACK1, a protein kinase C (PKC) scaffolding protein. RACK1 is a major component of translating ribosomes, which harbor significant amounts of PKC. Loading 60S subunits with eIF6 led to a dose-dependent block in translation and impairment of 80S formation, which were reversed by expression of RACK1 and stimulation of PKC in vivo and in vitro. PKC stimulation led to eIF6 phosphorylation, and mutation of a serine residue in the carboxy terminus of eIF6 impaired RACK1/PKC-mediated translational rescue.

“We propose that eIF6 release regulates subunit joining, and that RACK1 provides a physical and functional link between PKC signaling and ribosome activation,” conclude the authors.
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