In bacteria, protein synthesis can be coupled to transcription, but in eukaryotes it is believed to occur solely in the cytoplasm. Using pulses as short as 5 s, we find that three analogues - L-azidohomoalanine, puromycin (detected after attaching fluors using 'click' chemistry or immuno-labeling), and amino acids tagged with 'heavy' 15N and 13C (detected using secondary ion mass spectrometry) - are incorporated into the nucleus and cytoplasm in a process sensitive to translational inhibitors. The nuclear incorporation represents a significant fraction of the total, and labels in both compartments have half-lives of less than a minute; results are consistent with most newly-made peptides being destroyed soon after they are made. As nascent RNA bearing a premature termination codon (detected by fluorescence in situ hybridization) is also eliminated by a mechanism sensitive to a translational inhibitor, the nuclear turnover of peptides is probably a by-product of proof-reading the RNA for stop codons (a process known as nonsense-mediated decay). We speculate that the apparently-wasteful turnover of this previously-hidden ('dark-matter') world of peptide is involved in regulating protein production. © 2014 Baboo et al.
Baboo, S., Bhushan, B., Jiang, H., Grovenor, C. R. M., Pierre, P., Davis, B. G., & Cook, P. R. (2014). Most human proteins made in both nucleus and cytoplasm turn over within minutes. PLoS ONE, 9(6). https://doi.org/10.1371/journal.pone.0099346