Characterization of a rat liver factor that inhibits initiation of protein synthesis in rabbit reticulocyte lysates

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Abstract
Protein synthesis in rabbit reticulocytes and their lysates is regulated by heme. In heme-deficient reticulocyte lysates, protein synthesis proceeds at the initial rate for several minutes and then declines abruptly. Inhibition of protein synthesis is due to the activation of a heme-regulated translational inhibitor (HRI) which blocks the initiation of protein synthesis. Addition of the isolated HRI to hemin-supplemented lysates causes inhibition of initiation similar to that observed in heme-deficiency. HRI was shown to be a protein kinase that specifically phosphorylates the Met-tRNAf binding factor (eIF-2). An inhibitor (LI) of protein chain initiation was isolated from rat liver which displays properties similar to those of HRI: the chromatographic behavior of LI on DEAE-Sephadex, DEAE-cellulose and phosphocellulose is similar to that of HRI; both LI and HRI inhibit protein chain initiation in rabbit reticulocyte lysates with the same kinetics of inhibition.sbd.i.e., an initial period of synthesis for several minutes at the control rate followed by an abrupt decline in the rate of initiation; both inhibitions are prevented or reversed by eIF-2; GTP (2 mM) prevents, and ATP (2 mM) potentiates, the inhibition of protein synthesis induced by either inhibitor; LI is associated with a protein kinase that also phosphorylates the 38,000-dalton subunit of eIF-2. These findings indicate that a mechanism for the regulation of protein synthesis similar to that found in rabbit reticulocytes may be present in rat liver.