Identification of an unconventional nuclear localization signal in human ribosomal protein S2

Ribosomal proteins must be imported into the nucleus after being synthesized in the cytoplasm. Since the rpS2 amino acid sequence does not contain a typical nuclear localization signal, we used deletion mutant analysis and rpS2-β-galactosidase chimeric proteins to identify the nuclear targeting domains in rpS2. Nuclear rpS2 is strictly localized in the nucleoplasm and is not targeted to the nucleoli. Subcellular localization analysis of deletion mutants of rpS2-β-galactosidase chimeras identified a central domain comprising 72 amino acids which is necessary and sufficient to target the chimeric β-galactosidase to the nucleus. The nuclear targeting domain shares no significant similarity to already characterized nuclear localization signals in ribosomal proteins or other nuclear proteins. Although a Nup153 fragment containing the importinβ binding site fused to VP22 blocks nuclear import of rpS2-β-galactosidase fusion proteins, nuclear uptake of rpS2 could be mediated by several import receptors since it binds to importinα/β and transportin.