Translation in amino acid-poor environments is limited by tRNAGln charging

An inadequate supply of amino acids leads to accumulation of uncharged tRNAs, which can bind and activate GCN2 kinase to reduce translation. Here, we show that glutamine-specific tRNAs selectively become uncharged when extracellular amino acid availability is compromised. In contrast, all other tRNAs retain charging of their cognate amino acids in a manner that is dependent upon intact lysosomal function. In addition to GCN2 activation and reduced total translation, the reduced charging of tRNAGln in amino acid-deprived cells also leads to specific depletion of proteins containing polyglutamine tracts including core binding factor α1, mediator subunit 12, transcriptional coactivator CBP and TATA-box binding protein. Treating amino acid-deprived cells with exogenous glutamine or glutaminase inhibitors restores tRNAGln charging and the levels of polyglutamine-containing proteins. Together, these results demonstrate that the activation of GCN2 and the translation of polyglutamine-encoding transcripts serve as the key sensors of glutamine availability in mammalian cells. Mouse embryonic fibroblasts were cultured for 6h with or without amino acids in the presence of glutaminase inhibitor (CB-839) or vehicle (DMSO). RNA was extracted from cells and treated with either 10mM sodium periodite ("oxidized") or sodium chloride ("non-oxidized) prior to sequencing library preparation to calculate tRNA charging ratios.