Abstract
Heme-deficiency and double-stranded RNA (dsRNA) activate distinct cyclic 3′:5′-AMP independent protein kinases (HRI and dsI, respectively) in rabbit reticulocyte lysates. These kinases inhibit protein synthesis by phosphorylating the 38,000 daltons (38K) subunit of the initiation factor eIF-2 (eIF-2α). Using separation techniques to obtain a reticulocyte enriched fraction and reticulocyte-free erythrocytes, we have prepared lysates of these fractions from normal human whole blood. Human reticulocyte-enriched lysates contain the hemin-regulated and dsRNA-dependent protein kinases which inhibit protein synthesis and which phosphorylate rabbit eIF-2α. An endogenous 38K polypeptide which co-migrates with rabbit eIF-2α is also phosphorylated. In contrast, human mature erythrocytes contain little or no heme-regulated or dsRNA-dependent eIF-2α kinase activities which are inhibitory of protein synthesis.
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