Importin beta, transportin, RanBP5 and RanBP7 mediate nuclear import of ribosomal proteins in mammalian cells.
The assembly of eukaryotic ribosomal subunits takes place in the nucleolus and requires nuclear import of ribosomal proteins. We have studied this import in a mammalian system and found that the classical nuclear import pathway using the importin alpha/beta heterodimer apparently plays only a minor role. Instead, at least four importin beta-like transport receptors, namely importin beta itself, transportin, RanBP5 and RanBP7, directly bind and import ribosomal proteins. We found that the ribosomal proteins L23a, S7 and L5 can each be imported alternatively by any of the four receptors. We have studied rpL23a in detail and identified a very basic region to which each of the four import receptors bind avidly. This domain might be considered as an archetypal import signal that evolved before import receptors diverged in evolution. The presence of distinct binding sites for rpL23a and the M9 import signal in transportin, and for rpL23a and importin alpha in importin beta might explain how a single receptor can recognize very different import signals.
Zentrum für Molekulare Biologie der Universität Heidelberg, Germany.
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D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism*; D004285:Dogs; D019204:GTP-Binding Proteins / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D028884:Karyopherins; D019913:Nuclear Localization Signals / Q000502:physiology; D009687:Nuclear Proteins / Q000378:metabolism / Q000502:physiology*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism / Q000502:physiology*; D012269:Ribosomal Proteins / Q000378:metabolism* / Q000502:physiology; D028901:alpha Karyopherins; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein
C107940:IPO5 protein, human; C466996:IPO7 protein, human; D028884:Karyopherins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; C486191:RPL23a protein, human; D018160:Receptors, Cytoplasmic and Nuclear; D012269:Ribosomal Proteins; D028901:alpha Karyopherins; D028961:beta Karyopherins; C029742:ribosomal protein L5; C037442:ribosomal protein S7; D019204:GTP-Binding Proteins; D020931:ran GTP-Binding Protein