The RNA exosome co-factor CTEXT exists as an independent multiprotein complex in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the CTEXT (CBC-Tif4631-dependent exosomal targeting) complex, consisting of Cbc1/2, Tif4631, Upf3, and Dbp2, defines a novel cofactor of the nuclear RNA exosome promoting the degradation of two distinct classes of faulty transcripts, aberrantly long 3'-extended and export-defective messages. Interestingly, CTEXT/nuclear RNA exosome also determines the cellular abundance of a group of normal ('special') mRNAs via their preferential decay, thereby exerting regulatory control over a few key cellular processes, including the unfolded protein response and the cell's adaptation to nutrient stress. While previous genetic evidence indicates that CTEXT is composed of Cbc1/2, Tif4631, Upf3, and Dbp2, compelling biochemical evidence supporting that they form a multiprotein complex is currently lacking. Using systematic biochemical and genetic analyses, we show that all known CTEXT components interact with one another and with specific nuclear RNA exosome components. Furthermore, the CTEXT components were shown to co-sediment during sucrose gradient centrifugation, thereby affirming that CTEXT exists as a distinct multiprotein complex. We also demonstrate that Cbc1, Tif4631, and Dbp2 are involved in the initial binding of the RNA exosomal substrate mRNAs-a crucial event for subsequent targeting of aberrant mRNAs to the RNA exosome. Moreover, Tif4631 and Upf3 facilitate the interaction between CTEXT and the nuclear RNA exosome complex and promote the recruitment of the bound target mRNAs to the RNA exosome for degradation. Collectively, our findings provide evidence that CTEXT exists as a distinct multiprotein complex that aids the RNA exosome in the preferential nuclear degradation of specific classes of faulty messages.