THE TAIL OF A DEGRADING ACTIVITY

The exosome is one of the cell's fundamental machines. It processes and trims precursor RNAs, making them ready for use by the cell. At the University of Edinburgh, Professor David Tollervey, a Wellcome Trust Principal Research Fellow, and colleagues have now identified a complex of proteins that speeds up the exosome's work.

Eukaryotic cells produce a huge amount of RNA, but this is seldom used 'as is'. In most cases, long precursor RNAs need to be trimmed and cut up into mature forms that can be used as templates for protein synthesis or as components of the protein factories themselves, the ribosomes.

In 1997, Professor Tollervey and colleagues identified the complex of ten exonuclease proteins – the exosome – that plays a key role in this processing, as well as in degrading defective RNAs that fail quality control checks. Two forms of the exosome were found: a cytoplasmic form that is involved in the turnover of messenger RNA (mRNA), and the version in the nucleus that processes nuclear pre-mRNAs, ribosomal RNAs, small nuclear RNAs and small nucleolar RNAs.

On its own, in a test-tube, the exosome works rather slowly, so it was thought that its activity in cells might be boosted by additional factors. Professor Tollervey's team has now found such an accelerating factor: the TRAMP complex (short for the Trf4p/Air2p/Mtr4p polyadenylation complex) adds a poly(A) tail to defective ribosomal RNAs and small nucleolar RNAs, which promotes their degradation in the nucleus by the exosome. This system shows notable similarities to RNA degradation in bacteria, suggesting that it is of ancient evolutionary origin.

External links

• LaCava J et al. RNA degradation by the exosome is promoted by a nuclear polyadenylation complex. Cell 2005;121(5):713–24.