Methionine deprivation regulates the S6K1 pathway and protein synthesis in avian QM7 myoblasts without activating the GCN2/eIF2 alpha cascade.

Amino acids modulate mRNA translation through the 70 kDa ribosomal protein S6 kinase (S6K1) and the general control nondepressible 2 protein kinase (GCN2)/eukaryotic initiation factor 2 alpha eIF2 alpha pathways. The aim of the present study was therefore to explore the signaling cascades potentially modulated by methionine availability in quail muscle QM7 myoblasts using media providing all other amino acids. Methionine deprivation caused a lower S6K1 phosphorylation compared with control (Ctl) cells. Supplying the methionine-deprived media with L- and DL-methionine isomers restored S6K1 phosphorylation to the levels observed in Ctl cells. Methionine also regulated downstream S6K1 targets (i.e. ribosomal protein S6 and eukaryotic elongation factor 2), modulated translation preinitiation complex (PIC) assembly, and stimulated protein synthesis. Replacing the lacking methionine with D-methionine or its hydroxyanalog [2-hydroxy-(4-methylthio) butanoic acid] did not restore S6K1 activation or protein synthesis. Conversely, the S6K1 pathway was activated by a methionine precursor, the ketoanalog of methionine. Methionine availability regulated the GCN2/eIF2 alpha pathway. However, our results indicate that methionine deprivation led to lower protein synthesis without activating eIF2 alpha phosphorylation, a process known to limit the formation of the 43S PIC. Using the amino acid alcohol methioninol did not decrease S6K1 phosphorylation or activity and did not alter the regulation of protein synthesis by methionine. These findings suggest that methionine exerts an effect on S6K1 signaling and protein synthesis in avian QM7 myoblasts through a mechanism partly independent of the global regulation via tRNA charging.