Chemoproteomic discovery of cysteine-containing human short open reading frames

The application of ribosome profiling and mass spectrometry technologies has recently revealed that the human proteome is larger than previously appreciated. Short open reading frames (sORFs), which are difficult to identify using traditional gene-finding algorithms, constitute a significant fraction of unknown protein-coding genes. Thus, experimental approaches to identify sORFs provide invaluable insight into the protein-coding potential of genomes. Here, we report an affinity-based approach to enrich and identify cysteine-containing human sORF-encoded polypeptides (ccSEPs) from cells. This approach revealed 16 novel ccSEPs, each derived from an uncharacterized sORF, demonstrating its potential for discovering new genes. We validated expression of a SEP from its endogenous RNA, and demonstrated the specificity of our labeling approach using synthetic SEP. The discovery of additional human SEPs and their conservation indicate the potential importance of these molecules in biology.