Mapping microproteins and ncRNA-encoded polypeptides in different mouse tissues

Small open reading frame encoded peptides (SEPs), also called microproteins, play a vital role in biological processes. Plenty of their open reading frames are located on the non-coding RNA (ncRNA) range. Recent research has demonstrated that ncRNA-encoded polypeptides have essential functions and exist ubiquitously in various tissues. To better understand the role of microproteins and ncRNA-encoded polypeptide expression in different tissues, we profiled the proteomic characterization of five mouse tissues by mass spectrometry, including bottom-up, top-down, and de novo sequencing strategies. Bottom-up and top-down with database-dependent searches identified 791 microproteins in the OpenProt database. De novo sequencing obtained 309 microproteins and ten ncRNA-encoded polypeptides that were not in the SEP database. In this study, we discovered 1100 microproteins in total, including 208 ncRNA-encoded polypeptides. From the annotation of these microproteins, we found that the brain contains the largest number of neuropeptides, while the spleen contains the most immunoassociated microproteins.