Nuclear Import of Upf3p Is Mediated by Importin alpha/beta and Export
of Upf3p is Required for a Functional NMD Pathway in Yeast.
Amanda Ford (1), Renee Shirley (1), M. Rachel Richards (1),
Markus Albertini (2), Michael Culbertson (1)
(1) Molecular Biology
and Genetics, University of Wisconsin, 1525 Linden Drive, Madison, WI
53706, USA; (2) Laboratory of Cell Biology, Howard Hughes Medical
Institute, The Rockefeller University, New York, New York 10021.
Upf3p, which is required for nonsense-mediated mRNA decay (NMD) in
yeast, is primarily cytoplasmic but accumulates inside the nucleus when
UPF3 is over-expressed or when upf3 - mutations prevent
nuclear export. Upf3p physically interacts with Srp1p (importin-alpha)
and Upf3p fails to import into the nucleus in a temperature-sensitive
srp1-31 strain. These data indicate 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 a Nmd- phenotype. The
addition of a functional NES element to an export-defective upf3- allele restores export and partially restores a 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 over-expression of Upf2p suppresses the
Nmd - phenotype in mutant strains carrying nes-A
alleles but has no effect on the localization of Upf3p, indicating 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. Our results suggest a model in which Upf3p functions in
one of the initital steps leading to the rapid decay of mRNAs that
cannot be translated full length during the first round of
translation.
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