Quality Control
The cell places a high priority on ensuring that translation produces proteins that accurately reflect the corresponding genetic information. To this end, quality control can be seen at every step in translation where errors might accumulate. ~ Michael Ibba & Dieter Söll
In complex organisms, hundreds of thousands of different proteins are constantly being produced to replace degraded ones. A lot can go wrong in producing proteins, and regularly does. Preventing putting defective proteins on the job can be critical to health.
Protein production quality control is termed nonsense-mediated mRNA decay (NMD). As suggested by its name, NMD focuses on recognizing defective messenger RNA, and efficiently degrading them so that pathetic proteins are not produced.
Messenger RNAs exist in many different configurations in cells, including a stable closed-loop conformation. ~ Indian geneticist Srivathsan Adivarahan
For quality control, mRNA carry a specific protein, termed up-frameshift1 (UPF1). UPF1 is normally removed from the messenger RNA (mRNA) by the ribosome that processes the protein formula carried by the mRNA. But if a ribosome finds the mRNA suspicious, it lets UPF1 stick, thus tagging the mRNA as defective. The ribosome then recruits enzymes to break the bad mRNA down.
Quality control is also applied to ribosomes fresh off the assembly line in the cell nucleolus, before they are exported to the cytoplasm for production work. To ensure that a ribosome has been successfully assembled, a protein border guard does not let the ribosome pass until an enzyme acting as export inspector gives the go-ahead.
