RST1 and RIPR connect the cytosolic RNA exosome to the SKI complex in Arabidopsis

The RNA exosome is a key 3’-5’ exoribonuclease with evolutionary conserved structure and roles. Its cytosolic functions require the co-factors SKI7 and the Ski complex. Here we demonstrate by co-purification experiments that the ARM repeat protein RESURRECTION1 (RST1) and RST1 INTERACTING PROTEIN (RIPR) connect the cytosolic Arabidopsis RNA exosome to the Ski complex. rst1 and ripr mutants accumulate small RNAs many of which are quality control siRNAs (rqc-siRNAs) produced by the postranscriptional gene silencing (PTGS) machinery when mRNA degradation is compromised. Indeed, quasi identical small RNA populations are observed in mutants lacking the RRP45B/CER7 subunit of the core exosome. This biochemical and genetic evidence supports a physical and functional link between RST1, RIPR and the RNA exosome. Our data reveal the existence of additional cytosolic exosome co-factors besides the known SKI subunits. Interestingly, RST1 is not restricted to plants, as homologues with a similar domain architecture exist in animals, including humans.