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Figure 1
Structural organization of RNase PH complexes. The top panel shows a side view of their ring arrangement, with the S1/KH domains, also called the cap region, on top. The middle panel illustrates side-by-side the evolutionary architectural conservation of the RNase PH complexes. In bacterial PNPase, one chain contains two RNase PH domains and one S1/KH region, forming a homotrimer with three phosphorolytic active sites. The archaeal exosome evolved into three distinct subunits, carrying RNase PH subunits, Rrp41 and Rrp42, and a cap protein, which could be either Rrp4 or Csl4. This complex comprises a homotrimer of three different proteins that, similarly to the bacterial PNPase, has three phosphorolytic sites. The eukaryotic exosome, however, is composed of nine different subunits that are still somewhat related in sequence to the archaeal Rrp41-like subunits (Rrp41, Rrp46 and Mtr3), the archaeal Rrp42-like subunits (Rrp45, Rrp43 and Rrp42) and the cap proteins (Rrp4, Csl4 and Rrp40). As a consequence of this increase in structural complexity, the eukaryotic exosome core is catalytically inactive. Its catalytic function arises from the association of a tenth subunit, Rrp44 (violet; bottom panel), a processive hydrolytic exoribonuclease. In the nucleus of yeast cells, an eleventh component, Rrp6 (red; bottom panel), binds to the exosome, providing a second exoribonucleolytic site to the entire complex.

Journal logoBIOLOGICAL
ISSN: 1399-0047
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