The MTR3 subunit of eukaryotic exosome is a member of the RNase_PH family, named after the bacterial Ribonuclease PH, a 3'-5' exoribonuclease. Structurally all members of this family form hexameric rings (trimers of Rrp41-Rrp45, Rrp46-Rrp43, and Mtr3-Rrp42 dimers). The eukaryotic exosome core is composed of six individually encoded RNase PH-like subunits and three additional proteins (Rrp4, Csl4 and Rrp40) that form a stable cap and contain RNA-binding domains. The RNase PH-like subunits are no longer phosphorolytic enzymes, the exosome directly associates with Rrp44 and Rrp6, hydrolytic exoribonucleases related to bacterial RNase II/R and RNase D. The exosome plays an important role in RNA turnover. It plays a crucial role in the maturation of stable RNA species such as rRNA, snRNA and snoRNA, quality control of mRNA, and the degradation of RNA processing by-products and non-coding transcripts.
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