Yeast U1 snRNA is exported from the nucleus.
Brian L.
Olson, Paul G. Siliciano
Biochem.,Mol.Bio.,& Biophysics,
University of Minnesota, 321 Church St. SE, Minneapolis, MN 55455,
USA
Small nuclear ribonucleoprotein particles (snRNPs) are required
for eukaryotic pre-mRNA splicing. In metazoans, snRNPs assemble in the
cytoplasm after Xpo1p-mediated nuclear export of their respective snRNA.
It is proposed that his compartmentalized assembly does not occur in
yeast because they lack an obvious homologue for the export adapter and
snRNP importer. We have found that mutations in Xpo1p and the yeast U1
snRNP-component, Prp40p, are synthetically lethal. Interestingly, Prp40p
contains consensus nuclear export signals that cause fragments of Prp40p
to shuttle between the nucleus and the cytoplasm in a leptomycin B-sensitive manner. These findings suggest that yeast U1 snRNPs may indeed
exit the nucleus similar to their metazoan counterparts. Here we have
developed a novel assay to localize U1 snRNA movements by in situ
hybridization in heterokaryon yeast cells. We form heterkaryons by
mating two strains, one of which contains a mutation in the
kar1gene that prevents nuclear fusion. One parental strain
contains an aphenotypic deletion in the gene encoding U1 snRNA. We can
follow the full length U1 snRNA produced in the other parental nucleus
using a fluorescent probe homologous to the deleted region. In these
heterokaryons, we observe time-dependent, leptomycin B-sensitive
transfer of U1 snRNA from one parental nucleus to the other. Currently
we are investigating transfer of other snRNAs and the effect of
prp40mutations on U1 snRNA transfer.
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