Nucleolar proteins that bind NLSs: a role in nuclear import or ribosome biogenesis?

In a search for proteins that bind nuclear localization sequences (NLSs), a number of nucleolar proteins with diverse functions were found. It is thought that the assay fortuitously uncovered a novel domain that mediates the interaction between these nucleolar proteins and ribosomal proteins containing NLS-like sequences. The domain is highly acidic and contains a number of serines forming putative casein kinase II sites. Here, we propose a model in which the nucleolar proteins catalyse the assembly of ribosomal proteins with pre-rRNA.