MBE Advance Access originally published online on January 4, 2008
Molecular Biology and Evolution 2008 25(3):580-590; doi:10.1093/molbev/msn002
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Research Articles |
Nucleolar Binding Sequences of the Ribosomal Protein S6e Family Reside in Evolutionary Highly Conserved Peptide Clusters
Center of Experimental Medicine, Institute of Molecular Cell Biology, Hamburg University, D-20246 Hamburg, Germany
E-mail: kruppa{at}uke.uni-hamburg.de.
Accepted for publication December 28, 2007.
Proteomic analyses of the nucleolus have revealed almost 700 functionally diverse proteins implicated in ribosome biogenesis, nucleolar assembly, and regulation of vital cellular processes. However, this nucleolar inventory has not unveiled a specific consensus motif necessary for nucleolar binding. The ribosomal protein family characterized by their basic nature should exhibit distinct binding sequences that enable interactions with the rRNA precursor molecules facilitating subunit assembly. We succeeded in delineating 2 minimal nucleolar binding sequences of human ribosomal protein S6 by fusing S6 cDNA fragments to the 5' end of the LacZ gene and subsequently detecting the intracellular localization of the β-galactosidase fusion proteins. Nobis1 (nucleolar binding sequence 1), comprising of 4 highly conserved amino acid clusters separated by glycine or proline, functions independently of the 3 authentic nuclear localization signals (NLSs). Nobis2 consists of 2 conserved peptide clusters and requires the authentic NLS2 in its native context. Similarly, we deduced from previous publications that the single Nobis of ribosomal protein S25 is also highly conserved. The functional protein domain organization of the ribosomal protein S6e family consists of 3 modules: NLS, Nobis, and the C-terminal serine cluster of the phosphorylation sites. This modular structure is evolutionary conserved in vertebrates, invertebrates, and fungi. Remarkably, nucleolar binding sequences of small and large ribosomal proteins reside in peptide clusters conserved over millions of years.
Key Words: ribosomal protein • nuclear import • nucleolar binding sequence • nuclear localization signal • ribosome biogenesis
1 Present address: Department of Neurological Surgery, Center of Clinical Neurosciences, Hamburg University, D-20246 Hamburg, Germany.