Because the nuclear envelope of
Schizosaccharomyces pombe remains intact throughout the cell
cycle, the cDNA and integrase complexes of the LTR-retrotransposon Tf1
must be transported through an intact nuclear envelope to enter the
nucleus. S. Pombe genes with mutations that could potentially cause
defects in the nuclear import of Tf1 cDNA were identified with a
recombination assay. One strain with a 12-fold defect in transposition
exhibited a 42-fold reduction in homologous recombination of Tf1 cDNA.
Genetic analysis of this strain revealed that the defects in
transposition and recombination were due to a mutation in an essential
gene that we have named nup124. nup124 is related to a family of
nucleopore proteins that possess multiple copies of FXFG motifs that
bind nuclear import receptors to the nuclear pore. The prediction that
nup124 is indeed a component of the nuclear pore was supported by
immunofluorescence microscopy. We also used immunofluorescence
microscopy to ask whether the mutation in nup124 inhibited nuclear
import of any Tf1 complexes. Tf1 Gag produced a prominent signal from
74% of the wild-type nuclei. An equivalent signal was seen in only 8% of
the mutant nuclei. Because mutant cells grew with wild-type rates, and
because the localization of known nuclear proteins was normal in the
mutant strains, the mutation in nup124 appears to specifically inhibit
Tf1 import without significantly reducing bulk nuclear import of
cellular proteins. Purified Gag fused to GST interacted directly with
the N-terminal half of nup124. This interaction suggests a direct and
specific role for nup124 in the nuclear import of Tf1.
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