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In addition to the PIN domain, the Dis3 N-terminus (～300 NU7441 long) includes several domains that may contribute to the ribonuclease activities of the protein. All Dis3 homologs contain a C3 motif, or a putative iron-sulfur binding domain, and two OB folds, or oligonucleotide-binding folds [8] and [10]. These domains have been shown to be important for cell growth in yeast [8] as well as core exosome binding [9] and [12], and nuclear targeting [12]. A domain with homology to the mitotic cohesin STAG resides within the Drosophila Dis3 N-terminus, but its function is unclear [12]. A fundamental question is to determine how or whether these domains contribute to Dis3 ribonuclease activities.
Another challenge to understanding Dis3 ribonuclease activities is identification of their substrate specificities and reaction products. The full-length ScDis3 cleaves RNAs regardless of sequence or structure, indicating that the protein is a non-specific ribonuclease [3], [5], [7], [8], [9], [10], [11] and [13]. The products of these reactions vary depending on substrate, but typically range in length from 2 to 5 nucleotides, resulting from either endo- or exoribonuclease activity. ScDis3 activity also releases 5′NMP products, a characteristic of exoribonuclease activity, and RNB point mutations eliminate phospholipids ability [5].