| Abstract: | In heme-deficient reticulocyte lysates, protein synthesis initiation is inhibited due to the activation of a heme-regulated protein kinase which blocks protein synthesis by the specific phosphorylation of the alpha-sub-unit of eukaryotic initiation factor 2 (eIF-2 alpha). The restoration of synthesis requires both hemin and glucose-6-P (Ernst, V., Levin, D. H., and London, I. M. (1978) J. Biol. Chem. 253, 7163-7172). The sugar phosphate fulfills two functions in initiation: (i) the generation of NADPH, and (ii) an effector function in some step in initiation. This latter effect is readily demonstrated in lysates depleted of low molecular weight components by filtration in dextran gels. In gel-filtered lysates, linear protein synthesis is sustained only by the addition of both hemin (20 microM) and glucose-6-P (or 2-deoxyglucose-6-P) (50-500 microM). The omission of either component gives rise to inhibitions which are characterized by the activation of heme-regulated eIF-2 alpha kinase and the concomitant phosphorylation of both endogenous heme-regulated eIF-2 alpha kinase and endogenous eIF-2 alpha, indicating that glucose-6-P is involved in the regulation of heme-regulated eIF-2 alpha kinase. In support of this, we find (a) that gel-filtered lysates incubated with hemin but depleted of glucose-6-P produce sufficient heme-regulated eIF-2 alpha kinase to inhibit protein synthesis when mixed with normal hemin-supplemented lysates; (b) the inhibitions of protein synthesis produced by heme-regulated eIF-2 alpha kinase generated either in glucose-6-P-depleted lysates or heme-deficient lysates are reversed by added eIF-2; and (c) the eIF-2 alpha kinase activities formed in the absence of either hemin or glucose-6-P are both neutralized by an anti-heme-regulated eIF-2 alpha kinase antiserum. We conclude that the physiological activation of heme-regulated eIF-2 alpha kinase is controlled by both hemin and glucose-6-P. |
