The exosome,

plugged The correct processing, quality control and turnover of cellular RNA molecules is crucial to many aspects of cell physiology. The exo-some—a large molecular assembly with exoribonuclease activity— has emerged as a crucial component in many, if not most, stages of RNA metabolism, including degradation and maturation of ribo-somal RNA, RNA quality control and turnover of cytosolic messen-ger RNA (reviewed by Houseley et al, 2006). Exosomes are found in eukaryotes and archaea. The eukaryotic core exosome consists of six polypeptides with sequence similarity to the phosphate-dependent 3'→5 ' exoribonuclease RNase PH and three protein K homology (KH) and/or S1-domain-containing RNA-binding proteins (Fig 1A; Mitchell et al, 1997). Additional subunits include the RNase R homologue Rrp44/Dis3 (yeast exosome) and the RNase D homo-logue Rrp6 (nuclear isoform). The archaeal (a) exosome exhibits