Intersubunit and interprotein interactions of α- and β-subunits of human eIF2: Effect of phosphorylation

Purified recombinant human subunits of eukaryotic initiation factor 2 (eIF2) expressed in bacteria are found to interact with each other to form αβ, αγ, and βγ complexes in a pull-down experiment. Recombinant phosphorylated human eIF2α that cannot interact with purified eIF2B, the GDP/GTP exchange factor of eIF2, however interacts efficiently with eIF2B along with the β-subunit of eIF2 of the rabbit reticulocyte lysates and also with the purified recombinant β-subunit. These findings therefore suggest that the β-subunit of eIF2 mediates the productive and non-productive interactions between eIF2 and 2B. Recombinant α and β-subunits serve as substrates for not only kinases but also for caspase 3 and interestingly phosphorylated subunits resist caspase action. Phosphorylation also modifies the β-subunit’s interaction with Nck1, a cofactor of eIF2α phosphatase, but not with eIF5, the GTPase activating protein. These findings suggest that subunits of mammalian eIF2 interact with each other and the β-subunit plays a critical role both in the regulation and function of eIF2.