Two recent studies have provided new insight into archaeal exosome structure and mechanism. Büttner et al. (Mol. Cell 20, 461–471, 2005) solved the structure of two nine-subunit Archaeoglobus fulgidus exosome isoforms, and Lorentzen et al. (Mol. Cell 20, 473–481, 2005) solved the structures of the hexameric Sulfolobus solfataricus exosome-processing domain bound to either RNA or ADP.
In the A. fulgidus exosome, the RNase PH–like subunits, Rrp41 and Rrp42, form a trimer of dimers (green and blue ribbons) and constitute the hexameric RNA-processing center. The other subunits, either Csl4 or Rrp4, constitute a homotrimeric cap (orange or red ribbons) that sits on one face of the hexameric ring. Both Csl4 and Rrp4 contain an S1 domain between their N- and C-terminal domains. In both isoforms, the S1 domains are positioned in the cap's interior and are involved in restricting access through a pore to the processing core of the hexamer. The caps have a positive surface charge, and the S1 domain is even more strongly positively charged, suggesting that the RNA is attracted to the cap and then directed toward the pore by S1-domain interactions. Each cap subunit interacts with both Rrp41 and Rrp42, although the specific interactions differ between Csl4 and Rrp4. Modeling with different stoichiometries of Csl4 and Rrp4 cap subunits, which are thought to occur in eukaryotic caps, shows that this might be possible structurally without steric clash.
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