C23 Interacts with B23, A Putative Nucleolar‐Localization‐Signal‐Binding Protein

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Abstract
The human protein C23 (nucleolin) is a major nucleolar protein. Its interactions with other proteins were studied with the two‐hybrid system which identified nucleolar protein B23 (nucleophosmin) as being associated with C23. Both proteins were co‐immunoprecipitated from HeLa cell nuclear extract by either monoclonal anti‐C23 or monoclonal anti‐B23. Binding studies utilizing deletion mutants indicated that the binding of C23 and B23 involves specific motifs. In addition to an approximately 46‐amino‐acid‐binding domain in B23 (amino acids 194–239), amino acids 540–628 of C23 were required for binding; this region of C23 is required for the nucleolar localization. In addition, nucleolar protein p120 was also found to be co‐immunoprecipitated with B23. A fragment of p120 containing a functional nucleolar localization signal bound to the truncated binding domain of B23, as did C23. These results suggest that the interaction of C23 and B23 may represent a nucleolar‐targeting mechanism in which B23 acts as a nucleolar‐localization signal‐binding protein.