eIF2 (eukaryotic translation-initiation factor 2) is a substrate and an interacting partner for CK2 (protein kinase CK2). Co-immuno-precipitation of CK2 with eIF2β has now been observed in HeLa cells, overexpressing haemagglutinin-tagged human recombinant eIF2β. A direct association between His6-tagged human recombinant forms of eIF2β subunit and both the catalytic (CK2α) and the regulatory (CK2β) subunits of CK2 has also been shown by using different techniques. Surface plasmon resonance analysis indicated a high affinity in the interaction between eIF2β and CK2α, whereas the affinity for the association with CK2β is much lower. Free CK2α is unable to phosphorylate eIF2β, whereas up to 1.2 mol of phosphate/mol of eIF2β was incorporated by the reconstituted CK2 holoenzyme. The N-terminal third part of eIF2β is dispensable for binding to either CK2α or CK2β, although it contains the phosphorylation sites for CK2. The remaining central/C-terminal part of eIF2β is not phosphorylated by CK2, but is sufficient for binding to both CK2 subunits. The presence of eIF2β inhibited CK2α activity on calmodulin and β-casein, but it had a minor effect on that of the reconstituted CK2 holoenzyme. The truncated forms corresponding to the N-terminal or central/C-terminal regions of eIF2β were much less inhibitory than the intact subunit. The results demonstrate that the ability to associate with CK2 subunits and to serve as a CK2 substrate are confined to different regions in eIF2β and that it may act as an inhibitor on CK2α.
Abbreviations used: CK2, protein kinase CK2 (also known as casein kinase 2 or II); Ct, C-terminus; DRB, 5,6-dichlorobenzimidazole riboside; eIFs, eukaryotic translation-initiation factors; HA, haemagglutinin; HS1, haematopoietic lineage cell-specific protein 1; Ni-NTA, Ni2+-nitrilotriacetate; Nt, N-terminus; SPR, surface plasmon resonance.