Hiroshi Yanagawa

Six Classes of Nuclear Localization Signals Specific to Different Binding Grooves of Importin α

The importin α/β pathway mediates nuclear import of proteins containing the classical nuclear localization signals (NLSs). Although the consensus sequences of the classical NLSs have been defined, there are still many NLSs that do not match the consensus rule and many nonfunctional sequences that match the consensus. We report here six different NLS classes that specifically bind to distinct binding pockets of importin α. By screening of random peptide libraries using an mRNA display, we selected peptides bound by importin α and identified six classes of NLSs, including three novel classes. Two noncanonical classes (class 3 and class 4) specifically bound the minor binding pocket of importin α, whereas the classical monopartite NLSs (class 1 and class 2) bound to the major binding pocket. Using a newly developed universal green fluorescent protein expression system, we found that these NLS classes, including plant-specific class 5 NLSs and bipartite NLSs, fundamentally require the regions outside the core basic residues for their activity and have specific residues or patterns that confer the activities differently between yeast, plants, and mammals. Furthermore, amino acid replacement analyses revealed that the consensus basic patterns of the classical NLSs are not essential for activity, thereby generating more unconventional patterns, including redox-sensitive NLSs. These results explain the causes of the NLS diversity. The defined consensus patterns and properties of importin α-dependent NLSs provide useful information for identifying NLSs.

Musashi Proteins in Neural Stem/Progenitor Cells

Many RNA-binding proteins are encoded in the genomes of various organisms and play a critical role in several life systems. The human genome contains thousands of RNA-binding proteins (Glisovic et al., 2008). The most important biological role for these proteins involves the post-transcriptional events in gene expression, e.g., splicing, export, stabilization, localization, and translation. Recent studies have shown that these post-transcriptional events are of similar importance to transcriptional and post-translational events and are highly orchestrated (Keene et al., 2007).