Possesses 3'-5' exoribonuclease activity. Required for 3'-end trimming of AGO1-bound miRNAs, in particular multiple-isoform miRNAs, which represents a critical step in miRNA maturation. Interacts with AGO1; the interaction is not RNA dependent. Homozygous semilethal and sterile. Mutant flies show accumulation of the longest isoforms of multiple-isoform miRNAs including miR-3 and miR-34. Belongs to the mut-7 family.