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| The EMBO Journal
(1998) 17, 4491–4502, doi:10.1093/emboj/17.15.4491 |
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Importin  , transportin, RanBP5 and RanBP7 mediate nuclear import of ribosomal proteins in mammalian cells |
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| Stefan Jäkel and Dirk Görlich |
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Zentrum für Molekulare Biologie der Universität Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany To whom correspondence should be addressed Dirk Görlich, dg@mail.zmbh.uni-heidelberg.de Received 30 April 1998; Revised 29 May 1998; Accepted 4 June 1998. |
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| Abstract |
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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  / heterodimer apparently plays only a minor role. Instead, at least four importin -like transport receptors, namely importin 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 in importin might explain how a single receptor can recognize very different import signals. |
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| Keywords: importin , nuclear transport, RanBP7, ribosomes, transportin |
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