Alteration of translation and stability of mRNA for the poly(A)‐binding protein during myogenesis

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Abstract
The regulation of synthesis of various factors involved in mRNA translation during differentiation of muscle cells was examined. The steady‐state levels of mRNAs coding for eukaryotic initiation factor (eIF) 2α, 2β and elongation factor (eEF)‐1α were measured in both proliferating rat L6 myoblast and differentiated myotubes. The steady‐state levels of these mRNAs were not altered during myogenesis. Furthermore, the distribution of these mRNAs between repressed and translated populations remained unchanged. Recent studies suggest a role for poly(A)‐binding protein (PABP) in translation initiation. Therefore, we also examined the expression of PABP mRNA during myogenesis. The PABP mRNA was less abundant in myotubes compared to myoblasts. However, the synthesis of PABP remained unchanged. In myoblasts, approximately 50–60% of the total mRNA was associated with polyribosomes, whereas in myotubes more than 80% of the mRNA was associated with polyribosomes. These results, therefore, suggest that the PABP mRNA was more efficiently translated in differentiated myotubes than in the proliferating myoblasts. Measurement of the stability and transcription of PABP mRNA showed that, while transcription was not affected during myogenesis, the stability of the mRNA was reduced in differentiated cells. The t1/2 of PABP mRNA in myoblasts was 13 h compared to 7.5 h in myotubes. This observation suggests that the reduced steady‐state level of PABP mRNA in myotube were largely due to the change in stability of this mRNA during myogenesis.