Analysis of nucleo-cytoplasmic transport in a thermosensitive mutant of nuclear pore protein NSP1.

NSP1 is related to a group of nuclear pore proteins termed 'nucleoporins'. We observe that in thermosensitive nsp1 mutants lacZ fusion proteins which contain the nuclear targeting sequence of Mat alpha 2 or Pho2 are located inside the nucleus at the permissive temperature (23 degrees C), but are mislocalized in the cytoplasm at the restrictive temperature (37 degrees C). No evidence was obtained that the large lacZ reporter protein leaks out from the nucleus. Another nuclear passenger protein consisting of the NLS of ribosomal protein L25 and cytosolic dihydrofolate reductase (DHFR) is also accumulating in the cytoplasm after shifting ts nsp1 cells to 37 degrees C. In the latter case, this could be attributed to an increased leakage of the reporter protein from the nucleus into the cytoplasm. These data suggest that NSP1 mutation affects nuclear transport and nuclear retention, but the effects depend on the used NLS and the reporter protein.