nuclear import of ribosomal proteins in yeast

Nuclear import of ribosomal proteins: evidence for a novel type of nuclear localization signal

Rogier Stuger, Antonius C.J. Timmers, Hendrik A. Raué, and Jan van 't Riet

in The ribosome: structure, function, antibiotics and cellular interactions. RA Garrett, SR Douthwaite, A Liljas, AT Matheson, PB Moore, HF Noller (eds). ASM Press, Washington DC (2000) pp 205-214

department of biochemistry and molecular biology, Free University, Amsterdam

The formation of eukaryotic ribosomes is a highly complex process which requires the coordinated expression of a large set of ribosomal genes, transcribed by three different RNA polymerases, to ensure production of equimolar amounts of the four rRNAs and the approximately 80 ribosomal protein (r-protein) species under all growth conditions.
A further level of complexity is added to eukaryotic ribosome biogenesis by the fact that it involves different cellular compartments. Transcription of the rRNA and r-protein genes take place in the nucleolus and nucleoplasm, respectively. The r-protein mRNAs have to be exported to the cytoplasm. After translation, the r-proteins must be imported into the nucle(ol)us, where they have to be present in equimolar amounts to be assembled with the rRNAs into ribosomal subunits. The subunits are then exported from the nucleus to take up their function in the cytosol. Thus, ribosome biogenesis in eukaryotic cells involves massive transport of macromolecules and macromolecular complexes in both directions across the nuclear envelope. This transport not only concerns the ribosomes themselves (or their components) but also a large number of accessory factors, varying from constituents of the transcription, translation, and splicing machinery to pre-rRNA-processing and ribosome assembly factors.
In recent years considerable insight has been gained into the mechanisms of nucleocytoplasmic transport. Interestingly, a number of strong indications were found that nuclear import of r-proteins uses a specialized import pathway different from that used by the majority of karyophilic proteins. This suggests that the nuclear localization signals or sequences (NLSs) of r-proteins may be structurally distinct from the classical NLSs present in the latter class of proteins. In this chapter we review present knowledge of the mechanism and signals responsible for the nuclear import of proteins, in particular r-proteins. A database search of the complete set of yeast r-proteins for putative NLSs by using a set of criteria derived from the comparison of all experimentally identified signals in yeast r-proteins indicates that the large majority of these proteins may indeed possess a novel type of NLS, characterized by the presence of a sequence motif consisting of three basic residues within a 4- to 7-amino acid-long sequence, of which the first, the last, or both are helix-breaking residues. [full-text pdf]

Nuclear and nucleolar localization of Saccharomyces cerevisiae ribosomal proteins S22 and S25

Antonius C.J. Timmers, Rogier Stuger, Peter J. Schaap, Jan van 't Riet, and Hendrik A. Raué

FEBS Letters 452 (1999) 335-340

department of biochemistry and molecular biology, Free University, Amsterdam

Nuclear import usually relies on the presence of nuclear localization sequences (NLSs). NLSs are recognized by NLS receptors (importins), which target their substrates to the nuclear pore. We identified the NLSs of the yeast RPs S22 and S25 and studied the former by mutational analysis. Furthermore, in S25 the nucleolar targeting information was found to overlap with its NLS. Comparison with previously published data on yeast RP NLSs and computer analysis indicates the existence of a novel type of RP-specific NLS that differs from the classical Chelsky and bipartite NLSs. The existence of such an RP-specific NLS is in accordance with the recent identification of RP-specific importins.
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Wikipedia on nuclear localization signals