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 both 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, exposing the transcript body for 5′‐to‐3′ degradation. An alternative 3′‐to‐5′ decay pathway also follows the deadenylation and requires two multi‐complexes: the exosome containing various 3′‐exonucleases and the Ski complex consisting of the RNA helicase Ski2p, Ski3p and Ski8p. In addition, Ski7p, which has an N‐terminal domain and a C‐terminal elongation factor 1α‐like GTP‐binding domain, is involved in the 3′‐to‐5′ decay. However, physical interaction between the exosome and the Ski complex, together with the function of Ski7p, has remained unknown. Here we report that the N domain of Ski7p is required and sufficient for the 3′‐to‐5′ decay. Furthermore, the exosome and the Ski complex interact with the different regions of Ski7p N domain, and both 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‐complexes operating in the 3′‐to‐5′ mRNA‐decay pathway.