# Short internal open reading frames repress the translation of N-terminally truncated proteoforms

## TL;DR

Short internal open reading frames can control the production of proteins with missing N-termini by repressing their translation.

## Contribution

The study reveals a new regulatory mechanism where internal sORFs repress translation of N-terminally truncated proteoforms.

## Key findings

- nTRIP6 is generated by internal translation initiation and repressed by an upstream sORF.
- Translation of nTRIP6 is transiently upregulated during myoblast differentiation via mTORC1.
- Internal sORFs upstream of internal AUGs repress translation of N-terminally truncated proteoforms in other mRNAs.

## Abstract

Internal translation initiation sites, as revealed by ribosome profiling experiments can potentially drive the translation of many N-terminally truncated proteoforms. We report that internal short open reading frame (sORF) within coding sequences regulate their translation. nTRIP6 represents a short nuclear proteoform of the cytoplasmic protein TRIP6. We have previously reported that nTRIP6 regulates the dynamics of skeletal muscle progenitor differentiation. Here we show that nTRIP6 is generated by translation initiation at an internal AUG after leaky scanning at the canonical TRIP6 AUG. The translation of nTRIP6 is repressed by an internal sORF immediately upstream of the nTRIP6 AUG. Consistent with this representing a more general regulatory feature, we have identified other internal sORFs which repress the translation of N-terminally truncated proteoforms. In an in vitro model of myogenic differentiation, the expression of nTRIP6 is transiently upregulated through a mechanistic Target of Rapamycin Complex 1-dependent increase in translation initiation at the internal AUG. Thus, the translation of N-terminally truncated proteoforms can be regulated independently of the canonical ORF.

In frame translation initiation sites within coding sequences drive the expression of N-terminally truncated proteoforms. This study shows that internal, upstream short open reading frames repress their translation.

nTRIP6, the short nuclear proteoform of the cytoplasmic protein TRIP6 is translated from an internal AUG within Trip6 mRNA and its translation is repressed by a short open reading frame immediately upstream of the nTRIP6 AUG.During myoblast differentiation, nTRIP6 translation is transiently activated through mTORC1-mediated re-initiation downstream of the internal upstream open reading frame.The translation of N-terminally truncated proteoforms at internal AUGs within other mRNAs is also repressed by internal upstream open reading frames.

nTRIP6, the short nuclear proteoform of the cytoplasmic protein TRIP6 is translated from an internal AUG within Trip6 mRNA and its translation is repressed by a short open reading frame immediately upstream of the nTRIP6 AUG.

During myoblast differentiation, nTRIP6 translation is transiently activated through mTORC1-mediated re-initiation downstream of the internal upstream open reading frame.

The translation of N-terminally truncated proteoforms at internal AUGs within other mRNAs is also repressed by internal upstream open reading frames.

In frame translation initiation sites within coding sequences drive the expression of N-terminally truncated proteoforms. This study shows that internal, upstream short open reading frames repress their translation.