Further studies on the mode of action of the heme-controlled translational inhibitor.

We have isolated [de Haro, C. & Ochoa, S. (1978) Proc. Natl. Acad. Sci. USA 75, 2713-2716] a protein factor (eIF-2 stimulating protein, ESP) that is essential for formation of ternary and 40S initiation complexes by the eukaryotic polypeptide chain initiation factor 2 (eIF-2) at the low concentrations of eIF-2 present in reticulocyte lysates. The fact that stimulation of complex formation by ESP is virtually abolished when the small (38,000 daltons) subunit of eIF-2 is phosphorylated by ATP in the presence of eIF-2 kinase (heme-controlled inhibitor, HCI) is consistent with the notion that HCI inhibits translation in lysates by blocking the interaction of eIF-2 with ESP. Our present work, with highly purified eIF-2 and ESP, has additionally established that, unlike phosphorylation of the small subunit, phosphorylation of the middle (52,000 daltons) subunit of eIF-2, which does not lead to translational inhibition in lysates, does not affect eIF-2-ESP interaction. This provides further support for our model of translational inhibition by HCI.