Amino Acid-dependent Control of p70s6k

Abstract

In human T-lymphoblastoid cells, downstream signal-ing events of mammalian target of rapamycin (mTOR), including the activity of p70 s6k and phosphorylation of eukaryotic initiation factor 4E-binding protein 1, were dependent on amino acid concentration in the culture media, whereas other growth-related protein kinases were not. Amino acid-induced p70 s6k activation was completely inhibited by rapamycin but only partially inhibited by wortmannin. Moreover, amino acid concentration similarly affected the p70 s6k activity, which was dependent on a rapamycin-resistant mutant (S2035I) of mTOR. These data indicate that mTOR is required for amino acid-dependent activation of p70 s6k. The mechanism by which amino acids regulate p70 s6k activity was further explored: 1) amino acid alcohols, which inhibit aminoacylation of tRNA by their competitive binding to tRNA synthetases, suppressed p70 s6k activity; 2) suppression of p70 s6k by amino acid depletion was blocked by cycloheximide or puromycin, which inhibit utilization of aminoacylated tRNA in cells; and 3) in cells having a temperature-sensitive mutant of histidyl tRNA synthetase, p70 s6k was suppressed by a transition of cells to a nonpermissible temperature, which was partially restored by addition of high concentrations of his-tidine. These results indicate that suppression of tRNA aminoacylation is able to inhibit p70 s6k activity. Deacyl-ated tRNA may be a factor negatively regulating p70 s6k .