The archaeal exosome.

The archaeal exosome is a protein complex with structural similarities to the eukaryotic exosome and bacterial PNPase. Its catalytic core is formed by alternating Rrp41 and Rrp42 polypeptides, arranged in a hexameric ring. A flexible RNA binding cap composed of the evolutionarily conserved proteins Rrp4 and/or Csl4 is bound at the top of the ring and seems to be involved in recruitment of specific substrates and their unwinding. Additionally, the protein complex contains an archaea-specific subunit annotated as DnaG, the function of which is still unknown. The archaeal exosome degrades RNA phosphorolytically in 3' to 5' direction. In a reverse reaction, it synthesizes heteropolymeric RNA tails using nucleoside diphosphates. The functional similarity between the archaeal exosome and PNPase shows that important processes of RNA degradation and posttranscriptional modification in Archaea are similar to the processes in Bacteria and organelles.