The translation initiation factor elF2β is an interactor, of protein phosphatase-1

It is reasonably well understood how the initiation of translation is controlled by reversible phosphorylation of the eukaryotic translation initiation factors eIF2 alpha, eIF2B epsilon and eIF4E. Other initiation factors, including eIF2 beta, are also established phosphoproteins but the physiological impact of their phosphorylation is not known. Using a sequence homology search we found that the central region of eIF2 beta contains a putative PP1-(protein phosphatase-1) binding RVxF-motif. The predicted eIF2 beta-PP1 interaction was confirmed by PP1 binding and co-immunoprecipitation assays on cell lysates as well as with the purified components. Site-directed mutagenesis showed that eIF2 beta contains, in addition to an RVxF-motif, at least one other PP1-binding site in its C-terminal half. eIF2 beta functioned as an inhibitor for the dephosphorylation of glycogen phosphorylase and Ser(51) of eIF2 alpha by PP 1, but did not affect the dephosphorylation of Ser(464) of eIF2B epsilon by this phosphatase. Strikingly, eIF2 beta emerged as an activator of its own dephosphorylation (Set(2), Ser(67), Set(218)) by associated PP1, since the substrate quality of eIF7 beta was decreased by the mere mutation of its RVxF-motif. These results make eIF2 beta an attractive candidate substrate for associated PP I in vivo. The overexpression of wild-type eIF2 beta or e1F2 beta with a mutated RVxF-motif did not differentially affect the rate of translation, indicating that the binding of PP1 is not rate-limiting for translation under basal conditions.