INHIBITION OF PROTEIN-SYNTHESIS BY THE HEME-CONTROLLED EIF-2-ALPHA KINASE LEADS TO THE APPEARANCE OF MESSENGER-RNA-CONTAINING 48S COMPLEXESTHAT CONTAIN EIF-4E BUT LACK METHIONYL-TRNA(F)

Phosphorylation of the initiation factor eIF-2 by the heme-regulated e IF-2 alpha kinase (HCR) results in pronounced inhibition of protein sy nthesis and of binding of Met-tRNA(f) to 40S subunits in reticulocyte lysates. This inhibition is associated with the appearance of a more r apidly sedimenting 48S complex; this contains mRNA detectable by poly( U) hybridization, but not Met-tRNA(f). In contrast, 48S complexes that accumulate in the presence of the initiation inhibitor edeine contain both Met-tRNA(f) and mRNA. We have compared the composition of the pa rticles that accumulate in the presence of HCR with those seen in the presence of edeine and find that both particles contain the cap bindin g protein, eIF-4E. Moreover, both particles exhibit a buoyant density of 1.40 g/ml in CsCl equilibrium density gradients. This is consistent with the presence of 500-700 kDa of protein additional to ribosomal s tructural protein, and suggests the presence of eIF-3 on both types of 48S complex. Lysates pre-treated with HCR and then treated with edein e show the ability to accumulate 48S complexes containing Met-tRNA(f), though at a slower rate than control lysates. These observations are discussed in the light of mechanisms previously suggested for the appe arance of 48S particles following HCR treatment. In addition, we have observed association of eIF-4E with polysomes and 80S monosomes in ret iculocyte lysates, suggesting that this factor may not be released imm ediately following the binding of the 40S ribosomal subunit to the 5' end of the mRNA.