Preventing faulty cell protein production

Production of proteins in a cell is fraught with potential for error and there are many mechanisms to minimise this. When information from DNA is transcribed to messenger RNA (mRNA), which is then used to produce a specific protein, one such 'failsafe' is nonsense-mediated mRNA decay (NMD). Detecting and destroying any mRNAs that stop short and produce a truncated protein, the NMD mechanism prevents potential harm wreaked by faulty proteins.

Due to its essential role in embryo development, genetic disorders and tumour development, NMD has considerable therapeutic potential. For this to be realised, more knowledge is needed on the mechanisms involved. The 'Functional and structural analysis of the mammalian nonsense-mediated mRNA decay pathway' (NMDPATHS) project has investigated the molecular mechanics of NMD.

Key to its function, NMD requires action by the Smg5–Smg7 protein complex that connects recognition of the faulty mRNA target to the degradation machinery. NMDPATHS scientists found that Smg5–Smg7 must be fully intact to bind with the effector protein Upf1.

There are several mechanisms, including deadenylation in addition to endonucleolytic cleavage and decapping, to ensure robust and efficient decay of the target mRNAs. The researchers investigated the molecular requirements of deadenylation, usually the first step in mRNA degradation.

Results showed that two proteins in the human CCR4-NOT subcomplex crucial for docking platform co-fold for correct complex formation and so mediate contacts between proteins in CCR4-NOT. The structure of these two proteins, CNOT2 and CNOT3, were previously thought to be flexible and unstructured.

Crystal structure of the PAN2–PAN3 complex revealed the unusual asymmetry of the complex as a result of binding between the two proteins. Apart from acting as downstream effectors, the PAN2–PAN3 complex is also important for regulation of mRNA abundance in all cells.

Project results promise applications in formulation of therapies to combat the many diseases that result from lack of quality control at the mRNA stage. Elucidation of pathways involved has also increased understanding of accurate production of proteins in all cells.