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. Nuclear export of Upf3p is mediated by a leucine-rich nuclear export sequence (NES-A), but export is not dependent on the Crm1p exportin. Mutations identified in NES-A prevent nuclear export and confer an Nmd(-) phenotype. The addition of a functional NES element to an export-defective upf(-) allele restores export and partially restores an Nmd(+) phenotype. Our findings support a model in which the movement of Upf3p between the nucleus and the cytoplasm is required for a fully functional NMD pathway. We also found that overexpression of Upf2p suppresses the Nmd(-) phenotype in mutant strains carrying nes-A alleles but has no effect on the localization of Upf3p. To explain these results, we suggest that the mutations in NES-A that impair nuclear export cause additional defects in the function of Upf3p that are not rectified by restoration of export alone.
Shirley
Renee L
RL
Laboratories of Genetics and Molecular Biology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Ford
Amanda S
AS
Richards
M Rachel
MR
Albertini
Markus
M
Culbertson
Michael R
MR
D048868:Adaptor Proteins, Signal Transducing; D002467:Cell Nucleus / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism*; D028884:Karyopherins / Q000378:metabolism*; D009154:Mutation; D021382:Protein Sorting Signals; D012333:RNA, Messenger / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D020798:Two-Hybrid System Techniques
D048868:Adaptor Proteins, Signal Transducing; D028884:Karyopherins; C092470:NMD2 protein, S cerevisiae; D021382:Protein Sorting Signals; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; D015534:Trans-Activators; C096360:UPF3 protein, S cerevisiae