Signals and receptors for the nuclear transport of TFIIIA in Xenopus oocytes.

The transcription factor IIIA (TFIIIA) is a zinc finger protein that binds to both 5S genes and 5S ribosomal RNA. In Xenopus oocytes it is predominantly associated with 5S rRNA and retained as storage particle (7S RNP) in the cytoplasm. In this study, we have mapped the nuclear localization signal (NLS) activity in TFIIIA both in vivo and in vitro.

Nuclear export of 5S rRNA-containing ribonucleoprotein complexes requires CRM1 and the RanGTPase cycle.

In Xenopus oocytes, 5S rRNA is exported out of the nucleus in the context of two ribonucleoprotein complexes (RNPs): complexed with transcription factor IIIA as the 7S RNP or as the 5S RNP with ribosomal protein L5. 5S rRNA-containing RNP export takes place at a slow rate in comparison to that of nuclear export signal-containing proteins and the U1 snRNP. Using oocyte microinjection assays we found that the export of 5S RNPs requires nuclear RanGTP and RanGTP hydrolysis and is leptomycin B-sensitive, indicating the process is mediated by the export receptor CRM1.