Definition of a Consensus Transportin-specific Nucleocytoplasmic Transport Signal
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The low cytoplasmic and high nuclear concentration of the GTP-bound form of Ran provides directionality for both nuclear protein import and export. Both import and export factors bind RanGTP directly, yet this interaction produces opposite effects; in the former case, RanGTP binding induces nuclear cargo release, whereas in the latter, RanGTP binding induces nuclear cargo assembly. Therefore, nuclear import and export receptors and their protein recognition sites are predicted to be distinct. Nevertheless, the approximately 38-amino acid M9 sequence present in heterogeneous nuclear ribonucleoprotein A1 has been reported to serve as both a nuclear localization signal and a nuclear export signal, even though only one protein, the nuclear import factor transportin, has been shown to bind M9 directly. We have used a combination of mutational randomization followed by selection for transportin binding to exhaustively define amino acids in M9 that are critical for transportin binding in vivo. As expected, the resultant approximately 12-amino acid transportin-binding consensus sequence is also predictive of nuclear localization signal activity. Surprisingly, however, this extensive mutational analysis failed to dissect M9 nuclear localization signal and nuclear export signal function. Nevertheless, transportin appears unlikely to be the M9 export receptor, as RanGTP can be shown to block M9 binding by transportin not only in vitro, but also in the nucleus in vivo. This analysis therefore predicts the existence of a nuclear export receptor distinct from transportin that nevertheless shares a common protein-binding site on heterogeneous nuclear ribonucleoprotein A1.
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