SUMMARY PARAGRAPH for MPP6
The exosome complex possesses 3'-5' exonuclease and endoribonucleolytic activities that are essential for diverse ribonucleolytic processes in both the nucleus and the cytoplasm (3, 4, 5). The nuclear exosome is associated with the TRAMP complex and is involved in RNA catabolic processes including RNA surveillance (6, 7 and references therein), pre-mRNA turnover (8) and the production of mature 3' ends for snoRNAs, snRNAs and rRNAs (4, 9 and references therein). The cytoplasmic exosome is associated with Ski7p and the SKI complex and is involved in RNA catabolic processes that include both the routine turnover of normal mRNA (10) as well as the degradation of aberrant mRNAs (11 and references therein). The 10-subunit core exosome complex (Csl4p, Rrp4p, Rrp40p, Ski6p, Rrp42p, Rrp43p, Rrp45p, Rrp46p, Mtr3p, Dis3p) is the same in both locations, but the nuclear exosome contains an additional subunit (Rrp6p) and two additional accessory factors (Lrp1p, Mpp6p) (5).
Although the exosome was originally described as a "complex of exonucleases," with multiple subunits proposed to have RNase activity (3), later work has shown that this mechanism is unlikely in yeast. With the exception of Ski6p, none of the yeast subunits that show homology to E. coli RNase PH retain the active site residues seen in the bacterial or archael enzymes. Further research has also demonstrated that most, if not all, detectable enzymatic activity resides in the Dis3p and Rrp6p subunits (12, 13).
Last updated: 2009-09-09