The exosome: a macromolecular cage for controlled RNA degradation.

The exosome, a large multisubunit complex with exoribonucleic activity, emerges as the central 3' RNA degradation and processing factor in eukaryotes and archaea. But how are the many RNA substrates of the exosome degraded in a processive, yet controlled manner? Recent functional and structural progress shows that the exosome is a macromolecular cage, where the nuclease active sites are situated in a central processing chamber. A narrow entry pore controls access to the active sites in the processing chamber and prevents uncontrolled RNA decay. The emerging mechanism of exosome function suggests a strikingly parallel architectural concept to protein degradation by proteasomes.