Ski7p G protein interacts with the exosome and the Ski complex for 3 '-to-5 ' mRNA decay in yeast

Two cytoplasmic mRNA-decay pathways have been characterized in yeast, and b oth are initiated by shortening of the 3'-poly(A) tail. In the major 5'-to- 3' decay pathway, the deadenylation triggers removal of the 5'-cap, exposin g the transcript body for 5'-to-3' degradation. An alternative 3'-to-5' dec ay pathway also follows the deadenylation and requires two multi-complexes: the exosome containing various 3'-exonucleases and the Ski complex consist ing of the RNA helicase Ski2p, Ski3p and Ski8p. In addition, Ski7p, which h as an N-terminal domain and a C-terminal elongation factor la-like GTP-bind ing domain, is involved in the 3'-to-5' decay. However, physical interactio n between the exosome and the Ski complex, together with the function of Sk i7p, has remained unknown. Here we report that the N domain of Ski7p is req uired and sufficient for the 3'-to-5' decay. Furthermore, the exosome and t he Ski complex interact with the different regions of Ski7p N domain, and b oth interactions are required for the 3'-to-5' decay. Thus, Ski7p G protein appears to function as a signal-coupling factor between the two multi-comp lexes operating in the 3'-to-5' mRNA-decay pathway.