Mutations in the G-domain of Ski7 cause specific dysfunction in non-stop decay

Ski7 functions as a cofactor in both normal mRNA turnover and non-stop mRNA decay (NSD) mRNA surveillance in budding yeast. The N-terminal region of Ski7 (Ski7N) interacts with the ski-complex and the exosome. The C-terminal region of Ski7 (Ski7C) binds guanine nucleotides and shares overall sequence and structural homology with the proteins of the translational GTPase superfamily, especially the tRNA/tRNA-mimic carrier protein subfamilies such as EF1α, eRF3, and Hbs1. Previous reports showed that Ski7N polypeptide functions adequately in vivo, while Ski7C, if any, only slightly. Furthermore, Ski7C does not exhibit GTP-hydrolysing activities under normal conditions. Therefore, the physiological and functional significance of the conserved Ski7C is unclear. Here, we report strong genetic evidence suggesting differential roles for Ski7N and Ski7C in normal and specific mRNA turnover pathways by creating/isolating mutations in both Ski7N and Ski7C conserved motifs using indicator yeast strains. We concluded that Ski7C participates in mRNA surveillance as a regulatory module competitively with the Hbs1/Dom34 complex. Our results provide insights into the molecular regulatory mechanisms underlying mRNA surveillance.