Differential effect of hemin‐controlled eIF‐2α kinases from mouse erythroleukemia cells on protein synthesis

Abstract

Cultured mouse erythroleukemia (MEL) cells can be induced to erythroid differentiation by a variety of chemical agents. This differentiation process is marked by the onset of globin mRNA and hemoglobin synthesis. In rabbit reticulocytes, globin synthesis is regulated by a hemin-controlled translational inhibitor (HCI) which acts via phosphorylation of the .alpha. subunit of eukaryotic initiation factor 2 (eIF-2). From both uninduced and induced MEL cells, hemin-controlled eIF-2.alpha. kinases have been partially purified. They resemble HCI with respect to their chromatographic behaviour and their sensitivity towards physiological concentrations of hemin (5-10 .mu.M). Further purification on phosphocellulose, however, reveals that the eIF-2.alpha. kinase from uninduced. MEL cells is chromatographically distinct from HCI, whilst the eIF-2.alpha. kinase activity from induced MEL cells represents a mixture of the former and the HCI-type eIF-2.alpha. kinase. The latter inhibits protein synthesis in a fractionated system from rabbit reticulocytes which is free of, but sensitive to, HCI, whereas the eIF-2.alpha. kinase from uninduced MEL cells respond in vivo to iron depletion with a shut-off of protein synthesis (as do rabbit reticulocytes), whilst uninduced MEL cells do not.