The 5′ Untranslated Region, Signal Peptide, and the Coding Sequence of the Carboxyl Terminus of IL-15 Participate in Its Multifaceted Translational Control

We previously reported that the AUG-burdened 5' untranslated region (UTR) of IL-15 mRNA impedes its translation. Here we demonstrate that the nucleotide or protein sequences of the IL-15 signal peptide and carboxyl terminus also contribute to the poor translation of IL-15 transcripts. In particular, the exchange of the IL-15 signal peptide coding sequence with that of IL-2 increased cellular and secreted levels of IL-15 protein 15- to 20-fold in COS cells, while IL-2 transcripts with the IL-15 signal peptide generated 30- to 50-fold less IL-2 protein than wild-type IL-2. Furthermore, the addition of an artificial epitope tag to the 3' coding sequence of IL-15 increased its protein production 5- to 10-fold. Combining these two IL-15 message modifications, in addition to removing the 5' UTR, increased IL-15 synthesis 250-fold compared with a wild-type construct with an intact 5' UTR. These data suggest that IL-15 mRNA, unlike IL-2 mRNA, may exist in translationally inactive pools. By storing translationally quiescent IL-15 mRNA, cells might respond to intracellular infections or other stimuli by rapidly transforming IL-15 message into one that can be efficiently translated.