Amino Acid Stress Induces Non-AUG Initiation of c-myc Translation

Initiation of translation at non-AUG start codons can generate novel isoforms of important cellular regulators, with activities or localization distinct from their AUG-initiated counterparts. Ribosomes scanning the 5'UTR upstream of canonical AUG starts recognize near-cognate starts inefficiently, however, and the mechanisms by which non-canonical starts might be regulated physiologically are poorly understood. We show here that the restriction of utilization of individual amino acid induces a switch in expression of c-myc from its AUG-initiated isoform, c-myc-2, to an upstream CUG-initiated isoform, c-myc-1. This switch is enhanced in cells lacking GCN2, the protein kinase linking amino acid stress to phosphorylation and inhibition of eif2alpha;. Induction of upstream CUG start site usage in c-myc by restriction of proline utilization depends on the proximity of proline codons downstream of the canonical AUG start site. These observations support a model in which the stalling of an elongating ribosome near a canonical start leads to the accumulation of a ribosome queue that facilitates initiation at an upstream non-canonical start site. We find similar effects associated with deprivation of amino acids other than proline, and isoform changes with ATF-3 as well as c-myc. These findings suggest a broad mechanism by which amino acid stress can regulate translation initiation site use to modify the N-terminal proteome.