Encapsulation of "core" eIF3, regulatory components of eIF3 and mRNA into liposomes, and their subsequent uptake into myogenic cells in culture.

Abstract

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 associates with 40S ribosomal subunits and particles sedimenting at 80-1205 (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 3 H-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 .

Mammalian keratin gene families: organisation of genes coding for the B2 high-sulphur proteins of sheep wool

“…Regulation of protein synthesis in eukaryotes may involve differential rates of initiation of translation, and regulatory proteins that discriminate between different mRNAs would be attractive candidates for such a role. Indeed, there is some evidence for the existence and interaction of such proteins with myosin mRNA during muscle differentiation (54,55). The conserved sequence adjacent to the AUG in the 5' non-coding region of the 4 high-sulphur keratin genes may participate in a specific protein-mRNA interaction that accelerates keratin mRNA translation.…”