Although evidence from various eukaryotes has indicated that RNAi-related mechanisms play important roles in the nucleus, it is poorly understood how RNAi machinery is transported into the nucleus. 
The ciliated protozoan Tetrahymena provides an extreme example of a nuclear-acting RNAi machinery: the Argonaute protein Twi1p is sequentially localized in the parental and newly developed macronuclei and plays an essential role in programmed DNA elimination, which is evolutionary related to small RNA-directed heterochromatin formation in other eukaryotes.
 
In this manuscript, we show: 1) the nuclear localization of Twi1p requires the endoribonuclease (Slicer)-activity of Twi1p; 2) the Slicer-activity plays a key role in the removal of one of two strands (passenger strand) of Twi1p-associated siRNAs; 3) the novel Twi1p-binding protein Giw1p is essential for nuclear localization of Twi1p; 4) the Slicer-activity is essential for the interaction between Giw1p and Twi1p. These results suggest that Giw1p senses the state of Twi1p-associated siRNAs and selectively transports the mature Twi1p-siRNA complex into the nucleus. 
This is the first report describing that 1) localization of an Argonaute protein is regulated according to state of its bound siRNAs; 2) the existence of a protein sensor of the state of Argonaute-associated siRNAs.

> link to abstract on Pubmed