Identification and functional characterization of the nuclear and nucleolar localization signals in the intrinsically disordered region of nucleomethylin.

The nucleolar localization of proteins is regulated by specific signals directing their trafficking to nucleus and nucleolus. Here, we elucidate the mechanism underlying the nuclear and nucleolar localization of the nucleomethylin (NML) protein, focusing on its nuclear localization signals (NLSs) and nucleolar localization signal (NoLS). Using a combination of bioinformatic analysis and experimental validation, we identified two monopartite and one bipartite NLS motifs within NML. The combined presence of both monopartite NLSs significantly enhances nuclear localization of the protein, while specific basic amino acid clusters within the bipartite NLS are crucial for their functionality. We also reveal the functional role of the NLS-coupled NoLS motif in driving nucleolar localization of NML, which contains an arginine-rich motif essential for its function. The basic residues of the arginine-rich motif of NoLS of NML interacts with nucleophosmin 1 (NPM1), allowing the possible liquid-liquid phase separation and retention of NML in the nucleolus. Remarkably, the strong NoLS of NML can direct the nucleolar localization of a cytosolic protein, aldolase, emphasizing its potency. Overall, our findings provide insights into the combinatorial functioning of NLSs and NoLS in regulating the subcellular localization of NML, highlighting the intricate regulatory mechanisms governing its localization within the nucleus and nucleolus.

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