SUMMARY PARAGRAPH for MTR3
The exosome complex possesses 3'-5' exonuclease and endoribonucleolytic activities that are essential for diverse ribonucleolytic processes in both the nucleus and the cytoplasm (7, 8, 9). The nuclear exosome is associated with the TRAMP complex and is involved in RNA catabolic processes including RNA surveillance (10, 11 and references therein), pre-mRNA turnover (12) and the production of mature 3' ends for snoRNAs, snRNAs and rRNAs (8, 13 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 (14) as well as the degradation of aberrant mRNAs (15 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) (9).
Although the exosome was originally described as a "complex of exonucleases," with multiple subunits proposed to have RNase activity (7), 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 (4, 5).
MTR3 encodes a core subunit of the exosome and is predicted to be a member of the RNase PH class of RNases (2, 3 and references therein). Like most exosome components, Mtr3p is highly conserved among eukaryotes, including humans (hMtr3p (EXOSC6)) (4 and references therein). MTR3 is an essential gene, but a temperature sensitive mutant (at the restrictive temperature) accumulates aberrant forms of snoRNA, snRNA (8), and rRNA (2, 13, 16). This same mutant also accumulates nucleolar polyA+ mRNA, a phenotype with led to the original description of MTR3 as a gene involved in mRNA transport (1).
Last updated: 2009-09-09