| Ago1, Dcr1 and Rdp1 are the core components of the RNA interference machinery in the fission yeast, Schizosaccaromyces pombe. In addition to their functions in two distinct RNAi-related pathways that direct homology-dependent degradation of mRNA and chromatin modification, Ago1 and Dcr1 are required for regulated tyrosine-15 phosphorylation of Cdc2 when encountering genotoxic insults. My results suggest that Cdc2 phosphorylation regulated by Ago1 and Dcr1 is independent of small interfering RNAs. In addition, hAgo2, a human homologue associated with the endonuclease activity of RISC, compensates for the loss of ago1+ function in S. pombe suggesting that PPD proteins may be important for regulation of cell cycle events in humans.; Functioning of the RNAi apparatus in these divergent pathways presumably requires differential localization of Ago1, Dcr1 and Rdp1. The localizations of chimeric HA- and GFP-Ago1, Dcr1 and Rdp1 were studied by fluorescence and immunoelectron microscopy. The results indicate that Rdp1 is predominantly localized to the nucleus where it is thought to function in siRNA generation. In contrast, Ago1 and Dcr1 are predominantly associated with large cytoplasmic complexes. However, in the absence of Ago1, or when cells are arrested in S- or M-phases, nuclear localization of Dcr1 was evident. In addition, conditions that arrested cell cycle progression in G1 resulted in nuclear accumulation of both Ago1 and Dcr1. These results suggest that the roles Ago1 and Dcr1 perform in genome stability require dynamic localizations of these proteins. |
