The N-terminal domain of the human eIF2β subunit and the CK2 phosphorylation sites are required for its function

Abstract

CK2 (protein kinase CK2) is known to phosphorylate eIF2 (eukaryotic translation initiation factor 2) in vitro; however, its implication in this process in living cells has remained to be confirmed. The combined use of chemical inhibitors (emodin and apigenin) of CK2 together with transfection experiments with the wild-type of the K68A kinase-dead mutant form of CK2α evidenced the direct involvement of this protein kinase in eIF2β phosphorylation in cultured HeLa cells. Transfection of HeLa cells with human wild-type eIF2β or its phosphorylation site mutants showed Ser2 as the main site for constitutive eIF2β phosphorylation, whereas phosphorylation at Ser67 seems more restricted. In vitro phosphorylation of eIF2β also pointed to Ser2 as a preferred site for CK2 phosphorylation. Overexpression of the eIF2β S2/67A mutant slowed down the rate of protein synthesis stimulated by serum, although less markedly than the overexpression of the Δ2–138 N-terminal-truncated form of eIF2β (eIF2β-CT). Mutation at Ser2 and Ser67 did not affect eIF2β integrating into the eIF2 trimer or being able to complex with eIF5 and CK2α. The eIF2β-CT form was also incorporated into the eIF2 trimer but did not bind to eIF5. Overexpression of eIF2β slightly decreased HeLa cell viability, an effect that was more evident when overexpressing the eIF2β S2/67A mutant. Cell death was particularly marked when overexpressing the eIF2β-CT form, being detectable at doses where eIF2β and eIF2β S2/67A were ineffective. These results suggest that Ser2 and Ser67 contribute to the important role of the N-terminal region of eIF2β for its function in mammals.