Eukaryotic initiation factor 3 (eIF3), encapsulated in liposomes, is taken up by chick muscle cells in culture. The exogenously supplied factor (isolated from 14-d embryonic muscle) rapidly associated with 40S ribosomal subunits and particles sedimenting at 80-120S (the known sedimentation value of myosin heavy chain [MHC] mRNPs). In addition, exogenously supplied eIF3 has a specific stimulatory effect on myofibrillar protein synthesis. This stimulation is most apparent at the onset of cell fusion and after the accumulation of MHC-mRNPs. As previously reported (8), total eIF3 can be fractionated on an MHC-mRNA affinity column into a "core" eIF3 and a high affinity component (HAF) which dictates the discriminatory activity of core eIF3. Liposome-encapsulated core eIF3 delivered to cells is found predominantly in 40S ribosomal subunits and gives only a slight stimulation of total protein synthesis. When 3H-MHC-mRNA, preincubated with HAF, is introduced into myoblasts via liposomes, the mRNA is found in heavy polysomes. On the other hand, when the messenger alone or with core eIF3 is taken up by the cells, it is found only on small polysomes. Similar experiments, using viral RNA with the HAF, show no increase in the size class of polysomes. These results mimic the differences observed between myoblast and myotube utilization of MHC-mRNA previously observed (17). These results demonstrate the mRNA discriminatory activity of specific proteins associated with muscle eIF3 and suggest that these proteins play a role in mRNA activation and translation during muscle differentiation.

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