The translation initiation factor eIF2β is an interactor of protein phosphatase-1

Abstract

It is reasonably well understood how the initiation of translation is controlled by reversible phosphorylation of the eukaryotic translation initiation factors eIF2α, eIF2Bϵ and eIF4E. Other initiation factors, including eIF2β, 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 eIF2β contains a putative PP1-(protein phosphatase-1) binding RVxF-motif. The predicted eIF2β-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 eIF2β contains, in addition to an RVxF-motif, at least one other PP1-binding site in its C-terminal half. eIF2β functioned as an inhibitor for the dephosphorylation of glycogen phosphorylase and Ser51of eIF2α by PP1, but did not affect the dephosphorylation of Ser464 of eIF2Bϵ by this phosphatase. Strikingly, eIF2β emerged as an activator of its own dephosphorylation (Ser2, Ser67, Ser218) by associated PP1, since the substrate quality of eIF2β was decreased by the mere mutation of its RVxF-motif. These results make eIF2β an attractive candidate substrate for associated PP1 in vivo. The overexpression of wild-type eIF2β or eIF2β 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.