Nuclear import of Upf3p is mediated by importin-alpha/-beta and export to the cytoplasm is required for a functional nonsense-mediated mRNA decay pathway in yeast.

Upf3p, which is required for nonsense-mediated mRNA decay (NMD) in yeast, is primarily cytoplasmic but accumulates inside the nucleus when UPF3 is overexpressed or when upf3 mutations prevent nuclear export. Upf3p physically interacts with Srp1p (importin-alpha). Upf3p fails to be imported into the nucleus in a temperature-sensitive srp1-31 strain, indicating that nuclear import is mediated by the importin-alpha/beta heterodimer.