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 nucle olus and requires nuclear import of ribosomal proteins, We have studie d this import in a mammalian system and found that the classical nucle ar import pathway using the importin alpha/beta heterodimer apparently plays only a minor role. Instead, at least four importin beta-like tr ansport receptors, namely importin beta itself, transportin, RanBP5 an d RanBP7, directly bind and import ribosomal proteins, We found that t he ribosomal proteins L23a, S7 and L5 can each be imported alternative ly 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 recept ors bind avidly. This domain might be considered as an archetypal impo rt signal that evolved before import receptors diverged in evolution. The presence of distinct binding sites for rpL23a and the M9 import si gnal in transportin, and for rpL23a and importin a in importin beta mi ght explain how a single receptor can recognize very different import signals.