Interaction between Ski7p and Upf1p is required for nonsense‐mediated 3′‐to‐5′ mRNA decay in yeast

Aberrant mRNAs containing premature termination codons (PTC‐mRNAs) are degraded by a conserved surveillance system, referred to as the nonsense‐mediated decay (NMD) pathway. Although NMD is reported to operate on the decapping and 5′‐to‐3′ exonucleolytic decay of PTC‐mRNAs without affecting deadenylation, a role for an opposite 3′‐to‐5′ decay pathway remains largely unexplored. In this study, we have characterized the 3′‐to‐5′ directed mRNA degradation in the yeast NMD pathway. PTC‐mRNAs are stabilized in yeast cells lacking the components of 3′‐to‐5′ mRNA‐decay machinery. The 3′‐to‐5′ directed degradation of PTC‐mRNAs proceeds more rapidly than that of the PTC‐free transcript, in a manner dependent on the cytoplasmic exosome and Upf proteins. Moreover, Upf1p, but not Upf2p, interacts physically with an N‐terminal domain of Ski7p, although the interaction requires Upf2p. The efficiency of 3′‐to‐5′ directed degradation of PTC‐mRNAs is impaired by overexpression of Ski7p N‐domain fragments that contain a sequence of the Upf1p‐interaction region. These data suggest that the activation of 3′‐to‐5′ directed NMD is mediated through the interaction between Upf1p and the Ski7p N domain.