Upf3p is one of three trans-acting factors
required for nonsense-mediated mRNA decay (NMD) in S. cerevisiae .
NMD refers to the accelerated decay of mRNAs that contain a premature
stop codon. Previously, we identified three separate regions in Upf3p
that resemble bipartite nuclear localization signal (NLS) sequences.
Recently, we have identified two regions in Upf3p that resemble nuclear
export signal (NES) sequences. The NLS and NES sequence motifs in Upf3p
prompted us to explore the possibility that Upf3p shuttles between the
nucleus and cytoplasm. To explore whether Upf3p enters the nucleus and
then exports into the cytoplasm, alanines were substituted for two
conserved leucines and one semi-conserved isoleucine in the NES of
Upf3p, singularly and in combination. One mutant Upf3p containing three
alanine substitutions in the NES (Upf3p-Triple) resulted in the
redistribution of the protein from a primarily cytoplasmic localization
to the nucleolus. Strains carrying Upf3p-Triple also displayed a NMD-phenotype. The cytoplasmic localization of Upf3p and the nucleolar
localization of Upf3p-Triple suggests that Upf3p enters the nucleus and
then exports into the cytoplasm. The NMD- phenotype of strains carrying
Upf3p-Triple suggests that export is required for NMD. In an attempt to
rescue the cytological and NMD- phenotypes of Upf3p-Triple, the HIV-1
Rev NES was fused to Upf3p-Triple. Only the nucleolar localization of
Upf3p-Triple, and not the NMD- phenotype, was rescued by the Rev NES.
Return to YGM 1998 Abstract Index