Abstract
RETICULOCYTE lysates incubated in the absence of haemin stop synthesising proteins after a few minutes due to the accumulation of a translational inhibitor (TI) which blocks initiation of protein synthesis1–3. This block is characterised by a depletion of initiator tRNA (Met-tRNAf) bound to 40S ribosomal subunits4,5. It has been reported that prottein kinase activity is associated with TI which has been partially purified from reticulocyte lysates6. Addition of the initiation factor eIF-2 (previously designated IF-E2 or IF-MP), which binds Met-tRNAf and GTP, can overcome the block in initiation which develops in reticulocyte lysates incubated in the absence of haemin or with added TI7,8. Phosphorylation of eIF-2 by a protein kinase present in TI preparations has been demonstrated (refs 9–12 and P. Farrel, K. Balkow, R. Jackson and T. Hunt, personal communication), and it has been proposed that phosphorylated eIF-2 is inactive8,9. We have investigated the mechanism by which haemin stabilises initiation of protein synthesis by examining the ability of TI to inhibit binding of Met-tRNAf to ribosomes obtained from reticulocyte lysates supplemented with haemin or from unsupplemented lysates, and the corresponding pattern of ribosome-associated proteins phosphorylated by the protein kinase(s) present in the TI preparation. We have established a correlation between the phosphorylation of proteins associated with native 40S ribosomal subunits and the loss of binding activity of the ribosomes.
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LENZ, J., BAGLIONI, C. Haemin protects Met-tRNAf binding activity of isolated reticulocyte ribosomes from inactivation by protein kinase. Nature 266, 191–193 (1977). https://doi.org/10.1038/266191a0
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DOI: https://doi.org/10.1038/266191a0
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